Question of the day 2

Provide the IUPAC name for the following compound: (see picture in answer) A. 1-cyclopropyl-4-isopropyl-1,3-dimethylcyclohexane B. 4-cyclopropyl-1-isopropyl-2-methylcyclohexane C. 1-cyclopropyl-4-isopropyl-2,5-dimethylcyclohexane D. 4-cyclopropyl-1-isopropyl-2,4-dimethylcyclohexane

1. First, identify the (longest) parent chain or cycle -> six carbon ring = "cyclohex" 2. If several substituents are on the ring, they are listed in alphabetical order, and the first substituent is given #1 priority. The cyclohexane has the following substituents: cyclopropyl (#1), 2 methyl groups, iso-propyl (note that iso is not considered when placing in alpha order) 3. Substituents are numbered such one of them is at carbon #1 and the other locations have the lowest possible numbers, going in either a clockwise or counter-clockwise direction, giving the prefix di-, tri-, tetra-, to groups of the same kind (prefixes are not considered when alphabetizing). 1-cyclopropyl-4-isopropyl-1,3-dimethylcyclohexane

Which of the following is the best example of a case-control observational study? A Children from rich and poor backgrounds are tracked over time for key health indicators. B A large manufacturing workplace is studied for a correlation between smoking and productivity. C A drug is tested for effectiveness by administering it to one group of patients with a disease while another group of patients does not receive the drug. D Patients of a rare cancer are surveyed for common past exposures.

A case-control observation study is one which looks retrospectively for links between an exposure and an outcome (or lack thereof). This is best described by option (D). Option (A) is a description of a cohort study; (B) is an example of a cross-sectional study, and (C) is an example of a treatment study.

What process is described by an animal's ability to involuntarily contract a skeletal muscle without receiving a direct signal from the brain? A. Depolarization B. Reflex Arc C. Metabolism D. Redirection

A reflex arc is a nervous system process whereby the stimulus causes a motor movement that is coordinated by the spinal cord and not the brain. Sensory neurons are able to capture the environmental stimuli and send the afferent signal to the spinal cord, which is able to send an efferent (motor) signal directly to the effector, a muscle, which then moves. Plasticity refers to the brain's ability to repair itself following damage. Metabolism is the general term defining the cell's use and production of energy. Redirection is not an accurate term and is not used in biology. Depolarization is the process of changing the cell membrane's to become more positive, commonly observed when in the nervous system with the propagation of an action potential. Therefore the correct answer is [B].

How many liters of 0.3M HCl can be made from a 2L stock solution of 1.5M HCl? A. 10L B. 12L C. 6L D. 20L

A. 10L M1V1 = M2V2 (0.3)(V1) = (1.5)(2) V1 = 10 L

Which of the following occurs when the heart is in systole? A. Blood is pumped from the ventricles to the arteries. B. Blood is pumped from the veins to the heart. C. Blood is collected in both veins and arteries. D. Blood is collected in the right atria and right ventricle.

A. Blood is pumped from the ventricles to the arteries. Systole is the state of contraction of the heart muscles. The heart contracts to pump blood out of the ventricles and into the arteries to move the blood to the body. During diastole or when the heart is resting, blood collects in the chambers of the heart. Arteries carry blood away from the heart and veins carry blood back to the heart. During systole, the blood is being pumped away from the heart, thereby the use of arteries is essential. The only answer option that aligns with the correct function of the heart during systole is [A].

Which of the following statements regarding electric current is true? A. Electric current will only flow from a higher potential to a lower potential. B. Electric current will only flow from a lower potential to a higher potential. C. Oscillate between potentials of higher and lower energy. D. Remain in equilibrium

A. Electric current will only flow from a high potential to a lower potential. Electric current moves in the direction positive charges move in. Therefore, the electric flow of current flows from the point of higher potential to lower potential, similar to how water falls from the top of the waterfall to the bottom.

Once a mammalian zygote has become a blastocyst and implants itself into the uterine wall, it will develop into? A. Gastrula B. Embyroblast C. Trophoblast D. Oocyte

A. Gastrula The correct answer is A. The oocyte and morula are both precursors to the blastula stage. The embryoblast and the trophoblast are both components of the blastula, but are not the next stage in embryonic development. Once the blastula undergoes gastrulation, it will become a gastrula with three distinct layers of tissue: endoderm, mesoderm, and ectoderm.

Penicillin exerts its effect on bacterial growth through inhibition of cell-wall synthesis. This antibiotic blocks the last step in the formation of the cell wall. Glycopeptide transpeptidase is a bacterial enzyme that catalyzes the cross-linking reaction between peptidoglycan molecules, which are the main constituents of the cell wall. Penicillin inhibits the enzyme by binding covalently to its active site. The penicillin-enzyme complex cannot react further. This is an example of: A. irreversible inhibition. B. competitive inhibition. C. noncompetitive inhibition. D. uncompetitive inhibition.

A. Irreversible competition. Penicillin covalently binds to the active site of the bacterial enzyme and permanently disables its action. This mode of action is inherent to irreversible inhibitors. Notice this is similar to competitive inhibitors, but competitive inhibitors are reversible and adding more substrate can overcome the competitive inhibitor. Here is an excellent explanation on enzyme mechanics.

The rate at which a chemical reaction occurs is dependent on many factors. Which of the following will increase the rate of reaction? A. Decreasing the temperature. B. Adding a catalyst. C. Decreasing the concentration of a reactant in solution. D. Decreasing the partial pressure of a reactant gas.

B. Adding a catalyst. The rate at which a chemical reaction occurs is increased by increasing the temperature, adding a catalyst, increasing the concentration of a reactant in solution, increasing the partial pressure of a reactant gas and increasing the surface area of a reactant solid. Catalysts increase the rate of reaction by providing an alternative way for the reaction to continue with a lower activation energy, thus making the reaction easy to occur under any given conditions.

What is the most correct definition of competitive inhibition? A. Another enzyme completes the same reaction and uses the available reactant B. An inhibitor binds directly to the active site and prevents the reactant from binding C. A metal cofactor prevents the binding of the reactant by binding at the active site D. An inhibitor binds to the enzyme at a binding site, and prevents the enzyme from catalyzing the reaction

B. An inhibitor binds directly to the active site and prevents the reactant from binding The correct answer is B. Metal cofactors and ATP are often used to help boost enzyme activity, and are not competitive inhibitors. Non-competitive inhibition(D) occurs when an inhibitor is able to prevent the enzyme from binding with the reactant by binding to the enzyme at a site away from the active site, and change the enzyme's conformation so it cannot bind to the reactant. Competitive inhibition occurs when the inhibitor competes directly with the reactant at the active site, and this substrate takes the place of the reactant and prevents the reaction from occurring.

A high intensity peak in the mass spectrum of 2,2-dimethylpentane is observed at m/z 57. What chemical species does this peak represent? A. methyl cation B. tert-butyl carbocation C. n-propyl carbocation D. ethyl carbocation

B. tert-butyl carbocation The most stable fragment likely formed from 2,2-dimethylpentane fragmentation is t-butyl species.

A patient is found to have a renal disease causing excess excretion of bicarbonate in the urine. This would be most likely to result in: A increased PTH release. B decreased pH of the blood. C increased numbers of protons released in the urine. D increased calcium reabsorption from the gut.

Bicarbonate ions serve as bases within the body, thus loss of these ions would presumably decrease the pH of the blood by making the blood more acidic. Thus (B) is the correct answer.

For a very weak acid, you would expect the pH to be: a. Higher than the pKa b. Equal to the pKa c. Lower than the pKa d. Equal to double the pKa

C. Lower than the pKA A weak acid will not dissociate much, meaning that A-/HA will be very low, and log(A-/HA) will be negative. Thus, a weak acid with a pKa of 10 will likely have a pH around 8 or 9

During the reaction to form ammonia from nitrogen and hydrogen according to the reaction: N2(g) + 3 H2(g) <-> 2 NH3(g), ammonia was being produced at the rate of 1.50 mol.L-1.s-1. What were the respective rates of consumption of nitrogen and hydrogen at this time? A. N2(g) 1.50 and H2(g) 1.50 mol.L-1.s-1 B. N2(g) 2.25 and H2(g) 0.75 mol.L-1.s-1 C. N2(g) 0.75 and H2(g) 2.25 mol.L-1.s-1 D. N2(g) 3.00 and H2(g) 1.00 mol.L-1.s-1

C. N2(g) 0.75 and H2(g) 2.25 mol.L-1.s-1 The rate of consumption of the reactants will be the product of the rate of formation of the product multiplied by the ratio of the (co-efficient of the reactant) / (co-efficient of the product). Rate N2(g) = 1.50 x 1/2 = 0.75 mole.L-1.s-1 and H2(g) 1.50 x 3/2 = 2.25 mole.L-1.s-1.

Which of the following accurately describes the complement system? A. Part of the innate immune system comprised of leukocytes from the hematopoietic system. B. Part of the innate immune system involving the inflammatory response of vascular tissues. C. Part of the innate immune system comprised of plasma proteins synthesized primarily by hepatocytes. D. Part of the adaptive immune system that involves Helper T-cells from the hematopoietic system.

C. Part of the innate immune system comprised of plasma proteins synthesized primarily by hepatocytes. The complement system is a clear example of innate immunity, being a form of host defense that is non-specific and does not adapt in response to antigens. However, there is evidence of cross-talk between the complement system and adaptive immunity. Similarly components of the complement system communicate with components of the inflammatory system and hematopoietic system. Nonetheless, complement system components are distinct from leukocytes and the components of the inflammatory system and are primarilysynthesized in the liver.

Which of the following is the most accurate statement regarding the theory of Natural Selection? A. Individuals who reproduce in greater numbers are more likely to pass on their genes B. Individuals who have favorable random genetic mutations are not likely pass on their genes C. The environment can only support a finite number of individuals D. Individuals with the most genetic mutations are more likely pass on their genes

C. The environment can only support a finite number of individuals The most accurate statement is C. The statement for A can be true when discussing particular reproductive strategies, however, is not true in all cases. Individuals that are best adapted to their environment will have the greatest chance to pass on their genes, not the ones that produce the most amount of offspring. If an animal produces 1000 offspring and 999 of them die, then the fitness level of that animal is very low, compared to another animal with high fitness that only produces 2 offspring and both live to pass on their genes. Statement B would be correct if it were stated oppositely, since favorable genetic mutations is what creates natural selection. Statement D is not at all accurate regarding the principles of natural selection.

The following statement is a description of which Gas Law? "As a gas is heated under constant pressure its volume will increase in proportion to the change in temperature." A. Dalton's law of Partial Pressures B. Boyle's gas law C. Charles's gas law D. The Ideal gas law

Charles's Law states: V T Dalton's law of Partial Pressures states: Ptotal = P1 + P2 + P3 + ... Boyle's gas law states: P 1/V The Ideal gas law states: PV = nRT

According to scientists, Oparin and Haldane, which of the following gases were present in the primitive atmosphere? A. CH4, NH3, H2, H2O B. CO2, NH3, H2, H2O C. N2, O2, H2, H2O D. CH4, NH3, O2, H2O

Correct Answer: A. CH4, NH3, H2, H2O In the primitive atmosphere, no O2 was present as most organisms used chemosynthesis to create organic compounds. The Oparin and Haldane hypothesis states that methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O) were the only gases present. It was after the Miller and Urey experiment that it was proven that these gases and lightning could have given rise to simple organic compounds that then later evolved into more complex forms of life.

Which statement below about recrystallization is NOT correct? A. The method relies on the fact that solids are more soluble in a cold solvent than they are in the same amount of hot solvent. B. Precipitation is initiated by adding a seed crystal of the compound being purified. C. Scratching the side of the vessel at the air/solution interface can induce crystal nucleation. D. Slowly cooling the solution promotes precipitation of the compound.

Correct Answer: A. The method relies on the fact that solids are more soluble in a cold solvent than they are in the same amount of hot solvent. This statement is reversed: The process of recrystallization involves dissolution of the solid in appropriate solvent at a high temperature and the subsequent re-formation of the crystals upon cooling, so that any impurities remain in solution.

Which of the following statements is NOT true regarding gastrin secretion and function? A. increased production of HCl by parietal cells stimulates gastrin secretion B. gastrin stimulates ECL cells to release histamine C. somatostatin is an inhibitor of gastrin secretion D. elevated gastric pH stimulates G cells to release gastrin.

Correct Answer: A. increased production of HCl by parietal cells acts stimulatory on gastrin secretion Gastrin stimulates parietal cells to release HCl. If A were true, then there would be a positive feedback loop where gastrin is stimulating more HCl production, and HCl is stimulating more gastrin production, causing us to burn a hole through our stomach! Increased production of HCl by parietal cells leads to somatostatin secretion, which inhibits the secretion of gastrin. Lastly, an elevated gastric pH (i.e. low amount of acid) would stimulate the G cells to release more gastrin in an attempt to stimulate the parietal cells to release more HCl to bring the acidity back to normal.

Cerebrospinal fluid (CSF) is created by special capillary networks located in the brain. CSF bathes the hollow regions of the brain, spinal cord, and around the central nervous system, within its protective coverings. The areas that the CSF pathway includes are: A. the ventricles, spinal canal, and meninges. B. the ventricles, spinal canal, and cranial nerves. C. the spinal canal, ventricles, and spinal nerves. D. the spinal canal, meninges, and cranial nerves.

Correct Answer: A. the ventricles, spinal canal, and meninges. The organs of the central nervous system (CNS), the brain and spinal cord, have hollow regions (ventricles and spinal canal, respectively). The CNS also contains the meninges, which consists of three layers of protective covering. The question states that CSF circulates in these areas. Thus A is the correct answer. The spinal nerves and cranial nerves are not part of the CNS, they are part of the peripheral nervous system (with the exception of cranial nerve II, but that's a little advanced for the MCAT).

For the Henderson-Hasselbalch equation, under what conditions would you expect the pKa of the acid to equal the pH? a. When the acid has not dissociated at all b. When the amount of conjugate base is the same as the acid c. When the acid is completely dissociated d. When the acid is a weak acid

Correct Answer: B. When the amount of conjugate base is the same as the acid For the pH to equal the pKa, log(A-/HA) must be zero, which only occurs at log(1), meaning A-/HA = 1. Thus, the amounts of acid and conjugate base would be equal. This means that roughly half of the acid has dissociated.

Which of the following molecules has the same molecular shape as CO2? A. H2O B. XeF2 C. SO2 D. SCl2

Correct Answer: B. XeF2 Start by drawing the Lewis structure of CO2. We find that the molecular shape is linear. Here is a video to help visualize this process: CO2 Structure Next, find which of the answer choices is also linear by drawing all of the remaining structures. We find that only XeF2 has the same linear molecular shape, even though it has a different hybridization! Structure of H2O Structure of XeF2 Structure of SO2 Structure of SCl2

Which of the following can be concluded from the chemical equilibrium constant? NH3(aq)+H2O(l) <--> NH4(aq)+OH(aq) Ksp=1.81E-5 A. the reaction is slow to reach chemical equilibrium B. the equilibrium mixture has more reactants than products C. the equilibrium mixture has more products than reactants D. the equilibrium mixture evaporates quickly leaving only products

Correct Answer: B. the equilibrium mixture has more reactants than products The chemical equilibrium constant is calculated by dividing the concentration of the products by the concentration of the reactants at equilibrium. The magnitude of the chemical equilibrium constant can shed light on the composition of the mixture at equilibrium. Since the equilibrium constant given in the question is small, the equilibrium mixture would contain more reactants than products. If the equilibrium constant were larger than 1, this would indicate that the equilibrium mixture was made up of more products than reactants. Note: the chemical equilibrium constant is not an indicator of the rate of the reaction, only the concentrations of the equilibrium mixture.

What term accurately describes the structural relationship between (1R,2S)-1,2-dibromocyclopentane and (1R,2R)-1,2-dibromocyclopentane? A. Conformers B. Enantiomers C. Diastereomers D. Constitutional isomers

Correct Answer: C. Diastereomers When drawing out the structures, one can see that the basic skeletal arrangements are identical, i.e. stereoisomers (not constitutional isomers). We can dismiss conformers, since the structures cannot be interconverted exclusively by rotations about formally single bonds. The compounds are diastereomers, a class of stereoisomers that are non-superimposable, nonmirror images of one another. More specifically, the compounds are epimers: in chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center.

According to the Arrhenius equation (k = Ae-E/RT), if a reaction occurs at room temperature, what would you expect to happen if the temperature was increased by a factor of 4? A. The k value would increase by a factor of 4. B. The k value would not change. C. The k value would increase, but only slightly. D. The k value would decrease, but only slightly.

Correct Answer: C. The k value would increase, but only slightly. The correct answer is C. The Arrhenius equation is given by k = Ae-E/RT, where A is the pre-exponential factor, R is the gas constant, T is the temperature, and E is the activation energy. Thus, if the temperature were increased by a factor of 4, then the exponent would decrease by a factor of 4, since T is in the denominator. However, since it is e-x, the actual value won't change much. For example, exp(-1) = 0.36, but exp(-1/4) = 0.77. Thus, although the value of k will increase, it will not increase by a factor of 4, only slightly.

Which expression gives the acceleration of a 1kg mass on a frictionless inclined plane at 30o to the horizon, given that g = 9.8 m/s2? A. a = 9.8 m/s2 B. a = cos 30o x 9.8 m/s2 = 8.5 m/s2 C. a = sin 30o x 9.8 m/s2 = 4.9 m/s2 D. a = tan 30o x 9.8 m/s2 = 5.7 m/s2

Correct Answer: C. a = sin 30o x 9.8 m/s2 = 4.9 m/s2 Gravity is calculated from the vertical component of the force on the mass: sin theta = opposite/hypotenuse.

In a small lake, sea urchins graze upon small grasses that grow on the bottom. After the introduction of an exotic sea urchin, the small grasses became over grazed and absent. To solve the problem, a species of otter is introduced into the lake to predate upon the sea urchins, and the balance between the urchins and sea grasses is restored. What would be the appropriate term for the otter that was introduced? A. tertiary consumer B. top carnivore C. keystone predator D. primary consumer

Correct Answer: C. keystone predator The otter would be considered a SECONDARY consumer, the urchins are the primary consumers, and a hypothetical predator of the otter would be a tertiary consumer. Since the otter is responsible for preventing overgrazing of the small grasses by predating on the out of control urchin population, it is termed a keystone predator that exerts a "top-down" effect on the ecosystem.

Which of the following should be observed with the addition of a catalyst to a chemical reaction? A. the formation of a new product B. the consumption of the catalyst C. the activation energy is lowered D. the decrease in enthalpy

Correct Answer: C. the activation energy is lowered A catalyst speeds up a chemical reaction by lowering the activation energy of a reaction. The catalyst is never consumed, nor does it alter the reactants or products. The total enthalpy of the reaction is also not changed from what it would have been without the catalyst. Only the rate at which the reaction occurs and the activation energy is affected. The only answer option that is correct is [C].

The velocity of nerve impulse propagation could be increased by: A. the process of nerve myelination B. decreasing nerve diameter C. increasing nerve diameter D. A and C.

Correct Answer: D. A and C During the evolution of living systems, there were two ways the velocity of nerve impulse propagation could be increased. The first way was increasing nerve diameter, while the second way was the formation of the myelin sheath. The latter mechanism is less energetically expensive than the former.

During the fusion stage in human fertilization, which of the following would you NOT expect to occur? A. A fusion of chromosomes from the sperm and oocyte. B. Oocyte transfers itself into a mature ovum. C. The disintegration of the flagellum and mitochondria of the sperm during the creation of the male pronucleus. D. Blending of the pronucleus from the ovum and sperm into one.

Correct Answer: D. Blending of the pronucleus from the ovum and sperm into one. The blending or fusing of the pronuclei from the oocyte and the sperm cell does not occur, in that the two do not become one. What does occur is that the protective membranes on both dissolve, allowing for the genetic material to co-mingle. A mitotic spindle is formed and attaches itself to the chromosomes from each pronucleus before the chromosomes are fully dispersed throughout the cytoplasm of the oocyte. Mitosis then takes place creating daughter cells that are genetically identical. You may remember that all of a female's eggs are arrested at meiosis II. The eggs remain in this stage until sperm fertilizes them.

If the hormone aldosterone is released in the bloodstream, which of the following is most likely to occur? A. Bile and lipid production will increase. B. The amount of glucose in the blood will decrease. C. The amount of calcium ions in the blood will decrease. D. The amount of salt reabsorbed in the kidneys will increase.

Correct Answer: D. The amount of salt reabsorbed in the kidneys will increase. Aldosterone is a hormone released from the adrenal gland. Its presence in the blood helps the kidney's retain more sodium chloride (NaCl) in the nephrons, thereby desalinating the urine. Glucose regulation is controlled by the pancreas' release of insulin. Calcium ion levels are controlled by calcitonin and parathyroid hormone. The amount of water retained by the kidneys is controlled by the release of the antidiuretic hormone (ADH) or vasopressin. Aldosterone helps to retain water too, so the amount of water excreted by the body will decrease. Therefore, the correct answer is [D].

What happens to the kinetic energy of a race car if its speed is tripled? A. The kinetic energy increases 3 times its original value. B. The kinetic energy increases 4.5 times its original value. C. The kinetic energy increases 6 times its original value. D. The kinetic energy increases 9 times its original value.

Correct Answer: D. The kinetic energy increases to 9 times its original value. Kinetic energy is: KE = (1/2)mv2. The kinetic energy is directly proportional to the square of the velocity (or speed). If the speed is multiplied by 3 (tripled), the KE must be multiplied by 32 = 9. Sidenote, race car spelled backwards is still race car.

The density of an unknown gas under STP conditions is 3.17 g.L-1. What is the molecular mass of the gas? A. 3.17/24.5 = 0.13 B. 3.17/22.4 = 0.14 C. 3.17 x 24.5 = 77.7 D. 3.17 x 22.4 = 71.0

D. 3.17 x 22.4 = 71.0 STP is Standard Temperature and Pressure or 0oC and 1 atm pressure. At STP 1 mole of an ideal gas occupies 22.4 L. A good approximation of the molecular mass of a gas is obtained by multiplying its density in g.L-1 by 22.4 L to attain the mass of 1 mole of gas. Hence: 3.17 x 22.4 = 71.0 24.5 L is the molar volume of an ideal gas under LTP or Laboratory Temperature and Pressure, a balmy 25oC and 1 atm pressure.

An object of mass 3 kg floats motionless in a fluid of specific gravity 0.8. What is the magnitude of the buoyant force? (Use g = 10 m/s2). A. 8 N B. 16 N C. 20 N D. 30 N

D. 30N. Since the object is not moving, its acceleration must be zero, so the net force on it must also be zero. Therefore, the magnitude of the buoyant force upward must equal the magnitude of the weight downward, the specific gravity is extraneous information: B = w = mg = (3 kg)(10 N/kg) = 30 N

Benzoic acid can be extracted into water from an organic solvent using _________ and then removed from the water into an organic solvent by the addition of _________. A. Sodium bicarbonate / sodium hydroxide B. Hydrochloric acid / sodium hydroxide C. Hydrochloric acid / sulfuric acid D. Sodium hydroxide / hydrochloric acid

D. Benzoic acid, an organic compound, is insoluble in water. Extraction into water can be achieved with aqueous sodium hydroxide (deprotonation -> polar sodium benzoate, now soluble in water). Transfer of the benzoate back into organic solution is accomplished by treatment with hydrochloric acid (protonation -> less polar benzoic acid).

Genetic drift is the process responsible for: A. gaining a new allele within large populations. B. gaining a new allele within sufficiently small populations. C. losing an existing allele due to emigration. D. losing an existing allele due to random sampling.

D. Genetic drift represents the change in an allele frequency in a population due to random sampling. Influence from genetic drift is particularly important in small populations. The random changes in allele frequencies could result in the elimination of an existing allele from a population.

In the phrase "SN1 Reaction", what does the "1" indicate? A. The equilibrium constant. B. The number of reactants involved. C. The stereochemical outcome. D. The rate order.

D. The rate order. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular.

Drive reduction theory suggests that: A outside incentives are the primary drivers of society. B individuals work to minimize states of arousal. C individuals will first satisfy self-actualization prior to prioritizing physiologic needs. D individuals reduce physiologic needs by focusing on external reward systems.

Drive reduction theory postulates that individuals work to decrease states of arousal based on physiologic need (e.g., thirst, hunger). Thus (B) is the correct answer.

Which of the following is NOT needed to calculate the endpoint of a titration? A. Initial volume of titrant B. Final volume of titrant C. Concentration of the titrant D. pH of the titrant

During a titration procedure, a buret is lowered into a clean Erlenmeyer flask. The initial volume of titrant is recorded and a stream of titrant is delivered while constantly mixing the sample solution. Once the pH indicator changes colors we have reached the endpoint. The final volume is recorded and used in conjunction with the initial volume to find the volume of titrant used, and along with the concentration of the titrant to find how many moles of titrant were used. Finally, we can use stoichiometry of the titration reaction to calculate the number of moles of reactant in our solution. The pH of the titrant is not needed. D

An object has the density of 0.75 g/cm3 and is placed in a pool of water. Which of the following lists the fraction of the object that rests above the water at equilibrium? A 0 B 1/4 C 3/4 D 1

For any object, you can divide its density by the density of water to determine what fraction will be underwater at equilibrium. We can divide 0.75/1 to get 0.75, thus 3/4 is underwater and 1/4 will be above water. Thus (B) is the correct answer.

A rope of length L holds a weight of weight W. The tension at 1/2 the length of the rope is T. Which of the following is the tension at 3/4 the length of rope (i.e. 1/4L away from the weight)? A T B T/2 C T/3 D T/4

For purposes of the MCAT, tension along a rope is constant along the entire length of rope. Thus the tension is T at any given point along the rope. Thus (A) is the correct answer.

Which of the following would be expected to have the least first ionization energy? A Na B Mg C K D Ca

Ionization energy refers to the energy required to remove an electron from an atom. Ionization energy generally increases going from bottom to top and from left to right on the periodic table. Potassium, K, would have the lowest first ionization energy. Thus (C) is the correct answer.

Which of the following is NOT an assumption made in the ideal gas law as it pertains to kinetic molecular theory? A molecules within gases have negligible volume B collisions between two gas molecules is inelastic C gas particles move randomly with respect to each other D there are no attractive or repulsive forces between molecules

Kinetic molecular theory assumes that collisions between gas particles is elastic, thus all energy is conserved in the system. Thus (B) is the correct answer. All other options are correct assumptions made by the model. C. Charles's gas law

Which of the following disorders is a mistake that occurred during meiosis, which resulted in a trisomy of the sex chromosome? a) Kawasaki Disease b) Ketoacidosis c) Down Syndrome d) Klinefelter Syndrome

Klinefelter syndrome is a result of an extra X chromosome in men. The syndrome is normally not diagnosed until after puberty, and its outward symptoms are mild. Generally, men are diagnosed when they seek treatment for symptoms of other diseases or infertility.

The epiphyseal plate is found between the: A epiphysis and metaphysis. B diaphysis and epiphysis. C diaphysis and metaphysis. D diaphysis and osteophysis.

The epiphyseal plate is the location where bones are able to grow length-wise. This is located in between the epiphysis and metaphysis, thus (A) is the correct answer.

Q: Which of the following structures has the most reduced carbon atom? A methanol B methane C carbon dioxide D CH2O2

Methane, CH4, has a carbon atom with a -4 oxidative state, which is the lowest state among the molecules listed. Thus (B) is the correct answer.

Mirror neurons are hypothesized to be responsible for: A human's ability to recognize themselves among a large group of faces. B allowing for the full expression of emotion in humans. C the biological underpinning for classical conditioning. D allowing humans to learn and understand the actions of others.

Mirror neurons are noted to fire both when an individual is performing a specific action and when that individual sees someone else perform that action. These neurons are hypothesized to help humans learn new behaviors and in understanding the actions of others.

The parasympathetic nervous system is responsible for all of the following EXCEPT: A digestion. B dilation of the pupils. C decreasing blood pressure. D slowing the heart rate.

The parasymptathetic nervous system is responsible for controlling an organism during resting state, as opposed to the fight-and-flight of the sympathetic nervous system. Option (B) is the only incorrect option as pupillary constriction is controlled by the parasympathetic system.

Split peaks in NMR suggest: A the presence of a functional group (e.g., -OH) in a molecule. B the presence of chemically equivalent adjacent carbon atoms. C the presence of neighboring hydrogen atoms which are not chemically equivalent. D the presence of neighboring hydrogen atoms which are chemically equivalent.

NMR, or nuclear magnetic resonance, is a technique used to determine the structure of molecules with respect to hydrogen atoms. Splitting of peaks in NMR suggests chemically non-equivalent, though adjacent, hydrogen atoms. Thus (C) is the correct answer.

Which of the following salts would have the greatest freezing point depression capacity if 0.05 M were added to water? Assume total solubility of each. A CaCl2 B Al2(SO4)3 C MgSO4 D NaCl

None of the salts needs to be corrected for its mass since the molarity of each is the same. Freezing point depression is a colligative property, meaning it depends on the quantity of molecules present and not the physical property of those molecules. Since Al2(SO4)3 would dissociate into the greatest number of ions, it would yield the highest overall ion content and therefore have the strongest capacity at depressing the freezing point of a solution.

A particular reaction takes place where 2A → B + C is the slow step, and B + 2D → 3E is the fast step. Which of the following is the rate law for this set of reactions? A rate = k[A]2 B rate = k[B][C] C rate = k[B][D]2 D rate = k[E]3

The rate law of this set of reactions depends on the reaction 2A → B + C, the slow step. This reaction relies on two molecules of A reacting to form B and C. Thus the rate law is rate = k[A]2, thus (A) is the correct answer.

An individual has a mutation in their parathyroid which results in increased PTH production. This would result in: A hypercalcemia. B hypocalcemia. C decreased osteoclast activity. D increased thyroid hormone activity.

PTH hormone results in increased calcium absorption in the body from bone, thus increasing calcium levels in the blood. This is called hypercalcemia, thus (A) is the correct answer.

Pinocytosis refers to: A diffusion of fluid between cells. B fusion of two reproducing cells. C effusion of various molecules between cells. D cellular ingestion of liquid.

Pinocytosis is a subset of endocytosis and is a method for small vesicles filled with extracellular fluid to enter the cell. Thus (D) is the correct answer.

Which of the following molecules would be expected to be found in the metabolic pathway of the greatest number of living organisms? A pyruvate B hydrogen sulfide C histones D terpenes

Pyruvate is a product of glycolysis, an ancient pathway shared by the vast majority of known living organisms. On the other hand, hydrogen sulfide is largely only metabolically used by organisms in ecosystems with no sunlight (e.g. bottom of the ocean). Histones are only present on eukaryotes, whereas terpenes are only metabolically relevant for a relatively small number of organisms including plants and insects.

Observers are asked to determine the shape of a frisbee from various angles. Observers are able to correctly state that the frisbee is circular despite visualizing the frisbee at different angles and orientations. This serves as an example of: A parallel processing. B the Gestalt principle of similarity. C perceptual constancy. D perseverance.

The example listed is an example of perceptual constancy, which refers to the phenomenon where the size and shape of an object do not appear to change despite changing perspectives. This is the reason why an open door is still perceived as a rectangle, whereas its actual shape from the perspective of an observer is closer to a trapezoid. (C) is the correct answer.

Refractive index for an optical medium is defined as: A speed of light through a vacuum composed of this optical medium. B the velocity at which light would escape the medium. C ratio of speed of light in a vacuum and speed of light through the medium. D fractional wavelength decrease of a light traveling through this medium when compared to a vacuum.

Refractive index is defined as c/v, where c is the speed of light in a vacuum and v is the velocity of light in the medium. Thus (C) is the correct answer.

A liquid that has stronger cohesive forces than adhesive forces would have which type of meniscus? A. flat B. concave C. convex D. parabolic

The meniscus is the curved upper surface of liquid. This is formed because of unequal cohesion and adhesion attractions. For example, in water the adhesive forces are greater than the cohesive forces, therefore the water is able to "walk up the sides" of the surface. In the case of water, a U-shaped or concave meniscus is formed. Mercury, for example has stronger cohesive attractions between the mercury molecules than adhesive attractions between the mercury and the glass surface, therefore producing a convex meniscus, which looks like an inverted U.

Q: Signal detection theory mostly focuses on: A the study of absolute thresholds. B the study of how stimulus intensity affects an individual's perception of a stimulus. C the impact of Weber's law on perception. D the separation of important stimuli from nois

Signal detection theory concerns itself with how perceiver's separate signal from noise, e.g. how individuals maintain a conversation in a crowded and noisy room. Option (D) is the correct answer; the other options relate to Weber's laws or various theories about how the strength of stimuli affect perception, however signal detection theory specifically relates to signal/noise separation.

Decreasing one's effort given the efforts of others can be referred to as: A social facilitation. B deindividualization. C discrimination. D social loafing.

Social loafing refers to decreasing one's efforts given the efforts of others; for example, a player on a sports team playing with less fervor given the thought that the other individuals on the team will compensate. Thus (D) is the correct answer.

The P site of a ribosome is where: A the exit site from which tRNA leaves the ribosome. B the site where tRNA enters the ribosome. C the site on which the polypeptide is attached. D the site which error checks polypeptide formation.

The P site in a ribosome is the central site (between the A and E sites) at which the polypeptide is attached and grows with each new tRNA bound to amino acid molecule that enters. Thus (C) is the correct answer.

With respect to psychology, the biopsychosocial model states that: A psychological disorders are often due to either prevailing biological factors or social factors. B most psychiatric processes are typically a result of specific biological underpinnings. C biological elements of psychological disease irrelevant. D biological, psychological, and social processes often work in tandem with respect to mental health.

The biopsychosocial model works to describe how biological, psychological, and social processes ALL contribute to the psychological disorders. No one element can be used to describe a particular individual and their specific disorder. Thus (D) is the correct answer.

Which of the following correctly lists the combination of orbitals required to form a triple bond? A 3 sigma bonds B 3 pi bonds C 2 sigma bonds, 1 pi bond D 1 sigma bond, 2 pi bonds

The correct answer is (D); triple bonds are composed of a hybridization of a sigma bond as well as two pi bonds.

People who have high blood pressure often go on low sodium diets, in which they substitute salt (NaCl) for another compound. Which of the following would be an appropriate supplementary salt? A. MgCl2 B. KCl C. BeCl2 D. SrCl2

The correct answer is B. The best substitute is one that is the closest to Na on the periodic table. Remember, families in the periodic table have similar chemical properties. Thus, anything in the second group is unacceptable, eliminating answers A, C, and D.

Organisms within the Kingdom Plantae have which of the following cellular components? a) chlorosomes b) rigid cell membrane c) vacuole d) lysosome

The correct answer is C. Cells within the Kingdom Plantae have chlorophyll containing cells called CHLOROPLASTS, a rigid cell WALL outside of the plasma membrane, and vacuoles that serve to store carbohydrates. Plant cells do not contain lysosomes, as other eukaryotic cells, instead they have peroxisomes for digestion of foreign material.

The equilibrium constant, K, and the reaction quotient, Q, for a reaction relate to each other in the sense that: A K is equal to Q when the concentrations of reactants equal the concentrations of products. B K can fluctuate however Q is constant despite reactant and product concentrations. C Q is able to express a non-equilibrium state. D Q reflects equilibrium at a specific temperature and pressure for a system

The reaction quotient represents a ratio of product concentrations over reactant concentrations and can apply to a system that is not in equilibrium. Calculating Q gives one a sense of whether the reaction is at equilibrium, will tend to shift towards production of more product, or will shift towards production of more reactant. The reaction quotient can reflect non-equilibrium states, thus (C) is the correct answer.

Which of the following is the predominant site for the maturation of T-cells? A lymph nodes B spleen C bone marrow D thymus

The thymus serves as the site of T-cell maturation. Immune cells are generally produced in the bone marrow however the maturation of the T-cell lineage takes place in the thymus. Thus (D) is the correct answer.

C cells in the thyroid are responsible for: A increasing renal reabsorption of electrolytes. B inhibiting osteoclasts. C increasing levels of PTH in the blood. D sending downstream signals to the posterior pituitary.

Why? C cells in the thyroid are released in response to rising calcium, and function to try to get calcium levels back to homeostatic levels by inhibition of osteoclasts. Thus (B) is the correct answer.

Galvanizing is a process used to protect steel by applying a sacrificial coating? Which metal is used in galvanizing? A. Zinc B. Gold C. Copper D. Aluminum

To provide a sacrificial coating the coating metal must be more reactive than the base metal being protected. Thus gold and copper are not applicable as they are both less reactive than iron and can only provide a passive coating. Aluminum and zinc are more reactive than iron. Aluminum and zinc both form protective oxide layers that will stop corrosion of the iron, but aluminum is too reactive to be used in exposed weathering conditions. It is zinc that has just the right degree of reactivity that allows it to both protect the iron and not be corroded away, providing a strong and long lasting sacrificial coating.

Which of the following parameters of sound waves depends solely on the wave source, rather than the medium through which the wave travels? I. period II. wavelength III. velocity A I only B II only C I and III only D I and II only

Velocity and wavelength of a wave both depend on the medium through which the wave travels. For instance, sound travels faster through solids than through liquids or gases. The frequency of a wave is determined by the wave source; since frequency * wavelength = velocity, we can see that since velocity is medium-dependent, wavelength will similarly be medium dependent. Period is the only option which depends on the sound source rather than the medium. Thus (A) is the correct answer.

A particular weak acid molecule is found to have a pKa of 3.4. What is the ratio of this molecule's conjugated base to its acid form in the the blood, which has a pH of 7.4? A 0.003 B 4.77 C 400 D 10,000

We can use the Henderson-Hasselbalch equation to determine the answer this question. The equation states that pH = pKa + log(A-/HA). Thus 7.4 = 3.4 + log(A-/HA), thus log(A-/HA) = 4. This implies that the ratio A-/HA = 10,000. Thus (D) is the correct answer.

Which of the following particles has the highest mass? A neutrons B gamma particles C alpha particles D beta particles

Why? Alpha particles contain two protons and two neutrons, thus are the heaviest among the other particles. Gamma particles are essentially massless and beta particle is composed of a high speed electron. Thus (C) is the correct answer.

A reaction occurs in a vessel but it is unclear to researchers whether the reaction is driven by enzymes or simply proceeds under ambient conditions. Which of the following provide the best evidence that the reaction is NOT enzymatic? A When the vessel is heated to 85 °C, the reaction stops. B When the pH is dropped, the reaction proceeds. C When one of the reactants is removed, the reaction stops. D When the vessel is rapidly heated and then cooled back to its original temperature, the reaction no longer proceeds.

Why? Although an enzymatic reaction could stop at 85 °C, a chemical reaction without enzymes could also stop if any of the reactants are degraded or if the chemical equilibrium is shifted along with the temperature. However, if the reaction is chemical only, with no enzyme application, when the temperature is returned and the reaction does not proceed, it would suggest that enzymes are involved as they have denatured. This is the best evidence the reaction is enzymatic even though it is not entirely conclusive evidence.

Bile acids are formed by metabolism of: A cholesterol. B hypodrophilic steroid molecules. C phospholipids D saturated fatty acids.

Why? Bile acids are formed through the metabolism of cholesterol molecules. Various pathways add various molecules to cholesterol to form the broad diversity of bile acids. Thus (A) is the correct answer.

A closed system: A exchanges both mass and heat with its surroundings. B exchanges mass but not heat with its surroundings. C exchanges heat but not mass with its surroundings. D exchanges neither mass nor heat with its surroundings.

Why? Closed systems are systems which do not exchange matter with surroundings, however are able to exchange heat. For example, a closed can of soda may be thought of as a closed system. Heat can be exchanged (thus the can will warm up if taken out of the refrigerator) but mass within the can stays constant. Thus (C) is the correct answer.

The mechanism behind electron capture may be described as: A an inner shell electron combining with a proton in the nucleus. B addition of external electron to the inner shell of an atom. C addition of external electron to the outer shell of an atom. D expulsion of a proton in exchange for an external electron.

Why? Electron capture can be defined as the absorption of an inner shell electron into the nucleus. In the nucleus, the electron combines with a proton to form a neutron. Thus (A) is the correct answer.

The spreading of action potentials of adjacent cardiomyocytes takes place through: A desmosomes. B gap junctions. C tight junctions. D pinocytotic junctions.

Why? Gap junctions are responsible for the spreading of action potentials (i.e. spreading of charge) for myocytes, thus (B) is the correct answer.

An extraterrestrial genetic code is discovered which utilizes a set of five nucleotides and combines them to make codons of length 4. How many possible codons exist in this genetic code system? A 20 B 110 C 125 D 625

Why? If there are five nucleotides (thus 5 possibilities for the nucleotide in a codon string) and codons are of length 4, there are 5*5*5*5 = 625 possible codons. Thus (D) is the correct answer.

Which of the following is the rate-limiting enzyme in glycolysis? A glycogen synthase B lactate dehydrogenase C glucose-6-phosphate dehydrogenase D phosphofructokinase-1

Why? In glycolysis, phosphofructokinase (PFK1) is the rate limiting enzyme. It is responsible for phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate. Thus (D) is the correct answer.

Which of the following physiologic changes would result in a rightward shift of the hemoglobin oxygen saturation curve? A decreased temperature B increased pH C increased number of hemoglobin molecules in the blood D increased carbon dioxide load in the body

Why? Increased carbon dioxide functionally serves to decrease the pH of the blood, given that CO2 functions as an acid in the body. In more acidic environments (e.g., when exercising), the hemoglobin saturation curve shifts to the right, as this decreases the affinity of hemoglobin for oxygen and allows for oxygen to be more easily deposited in target tissues. Thus (D) is the correct answer.

Meso compounds: A are optically active. B can have a single stereocenter. C represent structural isomers of a particular molecular formula. D always have a stereoisomer which is optically active.

Why? Meso compounds are optically inactive stereoisomers within a set of stereoisomers for a particular compound. These compounds must have greater than or equal to 2 stereocenters. For instance, for 2,3-difluorobutane, the (R,S) and (S,R) configurations are superimposable (and are thus meso compounds), whereas the (R,R) and (S,S) are not superimposable and are each optically active. (D) is the correct answer.

What percentage 's' character do the orbitals of methane possess? A 25% B 33% C 50% D 100%

Why? Methane contains 4 sp3 bonds. Each bond contains 25% s-character and 75% p-character (given that one s-orbital and three p-orbitals functionally combine to make these sp3 orbitals). Thus (A) is the correct answer.

An individual pushes a 1kg apple 2 meters along a surface with friction. Which of the following holds true: A the force of pushing is nonconservative. B potential energy at the end of the experiment is nonzero. C work must be performed against gravitational forces. D work performed is less in this experiment than if the surface were frictionless.

Why? Nonconservative forces refer to forces where the work performed by that force is lost. In this experiment, the work is lost due to friction and there is no storage of the energy required to push the apple. Thus (A) is the correct answer.

Q: Which of the following is not a broad category of hormones? A peptide hormones B tyrosine derivatives C steroid hormones D N-acetylcysteine derivatives

Why? Peptide hormones, tyrosine derivatives, and steroid hormones are the three major classes of hormones that you should know for the MCAT. Option (D) is a fictitious hormone.

In DNA, adjacent nucleotides are held together by: A hydrogen bonds. B phosphodiester bonds. C ribose-ribose bonds. D ionic forces.

Why? Phosphodiester bonds hold adjacent DNA nucleotides together. Thus (B) is the correct answer. Nucleotides on complementary strands of DNA undergo hydrogen bonding.

Which of the following is the correct units for power as it relates to lenses? A m B focal length/wavelength C m-1 D nm2

Why? Power for a lens is calculated as 1/f, where f is the focal length. Focal length is a measure of distance, thus the units of 1/f will be m-1, thus (C) is the correct answer.

The Tollens' test will yield a shiny silver mirror on the glass vial containing the reaction if what type of sugar is added? A reducing sugar B non-reducing sugar C any disaccharide D any non-monosaccharide

Why? The Tollens' test involves a solution of silver oxide and ammonia in the presence of a carbohydrate. If the carbohydrate is a reducing sugar, it will reduce the silver to its solid state, resulting in the formation of a mirror deposit on the vial in which the reaction is taking place. Thus the correct answer is (A).

With respect to sound, which of the following would be expected to have the lowest decibel level for a particular individual? A absolute threshold B recognition threshold C differential threshold D terminal threshold

Why? The absolute threshold is the absolute lowest level that a stimulus can be heard. Thus, we would expect that this threshold would have the lowest decibel level, thus (A) is the correct answer.

The smallest functional unit of the lung is referred to as the: A trachea. B bronchiole. C alveolus. D capillary.

Why? The alveolus is the smallest functional unit of the lung and allow for gas exchange across thin membranes. Thus (C) is the correct answer.

Which of the following is an example of the anchoring heuristic? A A student more strongly defends the opinions of her family than the opinions of her teachers. B A person is more likely to buy a lottery ticket when a close friend has won the lottery. C A car dealer sells an overpriced car by first showing a car that is only slightly better but is vastly overpriced. D A child makes a decision based on the mood he woke up with in the morning.

Why? The anchoring heuristic describes how individuals are more likely to rely on the first information they come across in order to make a decision. (B) is the correct answer because first showing the vastly overpriced car anchors the buyer to the price and quality of the car, making the second car seem like a much better bargain compared to the first instead of compared to a more objective baseline. None of the other answer options are related to the order in which information is received.

Arabinose is an indigestible sugar and known inhibitor of sucrase which breaks down sucrose in the lumen of the small intestine. Assuming sucrose itself is not absorbed through the small intestine, which of the following effects would arabinose in the diet be expected to have? A Increase the caloric impact of a food. B Increase the production of insulin by the body. C Decrease the glycemic impact of foods with sucrose as a sweetener. D The presence of arabinose would not affect the overall impact of sucrose on the body.

Why? The breakdown of sucrose by sucrase would release glucose and fructose. Glucose would be absorbed by the small intestine and enter the bloodstream, increasing the glycemic response of a food with more sucrose. If the breakdown is stalled, then less glucose is able to enter the blood and the overall impact of the sucrose is minimized. Thus (C) is the correct answer. (B) is incorrect as the opposite would happen; less insulin would be required if less glucose were being absorbed. (A) is incorrect because if more sucrose passes through the small intestine and is not absorbed, the overall calories absorbed would be lower, not greater.

The bystander effect relies on: A incorrect assumptions about others. B immoral actions of an individual. C issues between an in group and an out group. D a conflict of social norms.

Why? The bystander effect occurs when individuals all believe that other individuals will act in a situation, e.g call the police regarding a robbery. This is a faulty assumption and may lead to complete inaction. Thus (A) is the correct answer.

A container at standard atmospheric pressure contains two gases, A and B. It contains 1/3 moles of gas A and 5/3 moles of gas B. Which of the following is the approximate partial pressure of gas A in the container? A 0.1 atm B 0.16 atm C 0.2 atm D 0.3 atm

Why? The entire container is at standard pressure, thus it is at 1 atm. There is a total of 6/3 = 2 moles of gas in the container. The partial pressure will equal 1 atm multiplied by the fraction of moles of gas A (gas A takes up 1/6 of the total moles in the container). Thus the answer is 1*1/6 = 1/6 atm which is approximately 0.16.

A magnetic field of 15T is applied to a stationary charge of 2C in a controlled testing environment. Which of the following equates to the amount of force experienced by the charge? A 0N B 3N C 30N

Why? The equation for force on a point charge in a magnetic field is F = qvB*sin(theta). In this example, the charge is stationary, thus v = 0, thus F = 0. Thus (A) is the correct answer.

An individual presents to their doctor given alternations in their sleep-wake cycle. They feel that tired around 5PM every evening and sleep until 2AM, feeling entirely rested. This individual may have a defect in their: A parallel processing centers. B hypothalamus. C temporal lobe. D rods and cone cells of the eye.

Why? The hypothalamus contains the SCN (suprachiasmatic nucleus) which is responsible for circadian rhythms and sleep-wake cycles. Thus (B) is the correct answer.

A proponent of the theory of multiple intelligences would be least likely to argue: A intelligence is composed of fluid and crystal intelligence, which are themselves indivisible intelligences. B emotional intelligence is a true intelligence. C a single number is unlikely to encapsulate the intelligence an individual possesses. D an individual may have competencies in a multitude of intelligences.

Why? The theory of multiple intelligences suggests that there are numerous types of intelligence (e.g., musical, mathematical, etc...) and that individuals may have varying degrees of intelligence in the various domains. Individuals use various intelligences in problem solving. Option (A) is most inconsistent with this view and is actually one of the opposing views which believes that intelligence is set, is not multifaceted, and is split into fluid and crystal. Thus (A) is the correct answer.

Two resistors of resistance 3 Ohm are placed in series with each other and are then placed in parallel to a resistor also of resistance 3 Ohm. What is the overall resistance of this circuit? A 2/3 Ohm B 2 Ohm C 5/3 Ohm D 15/2 Ohm

Why? We know that resistors in series add, thus this question is fundamentally the same as a resistor of 6 Ohm in parallel with a resistor of 3 Ohm. We can find the ultimate resistance with the equation 1/R = 1/3 + 1/6 = 1/2, thus R = 2 Ohm. Thus (B) is the correct answer.

A sample of DNA is found to have 10% T. What percent G would be expected? A 10% B 20% C 30% D 40%

Why? We know that the percentage of T and A are the same, thus 20% of the DNA is T+A. This leaves 80% for G+C, thus there must be 40% G, thus (D) is the correct answer.

van der Waals forces increase with: A increased size of a molcule. B decreased size of a molecule. C increased polarity of a molecule. D increased number of branch points in a molecule.

van der Waals forcces are weak intermolecular forces and involves the phenomenon of fluctuating polarity within molecules. In general, longer molecules (e.g., long hydrocarbons) have higher van der Waals forces; additionally, molecules with less branching have higher such forces because the long non-branching structure allows for higher surface area of interaction between neighboring molecules. Thus (A) is the correct answer.