NS Q's of the Day

A 10-cubic-cm cork floats in water with 1/2 of its volume submerged. Approximately what fraction of the cork's volume will be submerged when in mercury (specific gravity = 13)?

??????? A floating object will displace a mass of fluid equal to its own mass. For example, if a 10-ton ship is floating in water, it must displace 10 tons of water. Or if a water strider weighing just 0.2 grams is skimming along the surface of the water, it must be displacing 0.2 grams of water. Since the cork is floating, it must be displacing water (and mercury!) equal to its own mass. That lets us write the following: mwater = mmercury Since ρ = m / V, we can rearrange the density equation to get: m = ρV Thus: ρwaterVwater = ρmercuryVmercury We're told that the cork floats with half of its volume submerged. So the cork is displacing 5 cm3 of water. (1 g/cm3)(5cm3) = (13 g/cm3)(Vmercury) Vmercury = 5/13 cm3 The cork is 10 cm3, which means that 5/13 cm3 represents a tiny fraction of the cork's volume. Among the answer choices, only (a) is anywhere close. Alternatively, you can solve this question using the ratio of the specific gravity of the cork to the specific gravity of mercury. Since water has a specific gravity of 1 and half of the cork's volume is submerged when in water, the cork must have a specific gravity of 0.5. The ratio of this value to the S.G. of mercury (13) gives us the fraction of the cork's volume that is submerged: 0.5/13 = 1/26, closest to choice A. A. 1/20th, correct

Sodium nitrate and ammonium sulfate react at 230ºC to yield sodium sulfate, nitrous oxide, and water vapor. If the total pressure of the products is 1.8 atm, what is the partial pressure of the nitrous oxide? The reaction is as follows: 2 NaNO3 + (NH4)2SO4 → Na2SO4 (aq) + 2 N2O (g)+ 4H2O (g) (230—300° С)

A. 0.5 atm, incorrect, this would indicate that 2/7 of the pressure comes from 2 moles of nitrous oxide in the presence of 4 additional moles of water vapor and 1 mole of aqueous sodium sulfate (liquid). B. 0.6 atm, correct, 2 moles nitrous oxide and 4 moles of water vapor in the products indicate that 1/3 of the pressure is from the nitrous oxide C. 0.9 atm, incorrect, this answer incorrectly shows that 1/2 the pressure is from nitrous oxide, but the correct ratio is determined by the molar ratio of gas products D.1.8 atm, incorrect, this would indicate that all pressure is from the nitrous oxide -partial pressure only accounts for gases!!!!!!!!

All all of the following are true regarding the function of neurons EXCEPT:

A. Hyperpolarization at the end of an action potential is one mechanism by which neurons limit the rate at which action potentials may fire. B. The flow of sodium into the neuron depolarizes the membrane in the first phase of an action potential. C. The transmitting neuron secretes neurotransmitters into the synaptic cleft from its dendrites. FALSE D. An action potential is initiated when the axon hillock reaches the threshold potential. Retrograde neurotranmission (the secretion of neurotransmitters from dendrites) is rare, occurring mostly with nitric oxide and cannabinoid neurotransmitters.

Mg(OH)2 is slowly dissolved in 500 mL of 25oC water until the solution becomes fully saturated. Which of the following occurs when 10.0 mL of 0.1 M HCl is added? A. MgCl2 precipitates B. Mg(OH)2 precipitates C. Ksp for Mg(OH)2 increases D. [H2O] increases

ANS: C) [H2O] increases This question is testing your understanding of solution chemistry, equilibrium, and solubility rules. In a saturated solution of Mg(OH)2, the maximum concentrations of Mg2+(aq) and OH-(aq) ions have been reached and are in equilibrium with the solubility product, as follows: Mg(OH)2(s) ó Mg2+(aq) + 2OH-(aq) When HCl is added to the solution, the pH of the solution decreases. The additional H+ ions react with the dissolved OH- ions forming water. Thus, D is the correct answer. Magnesium chloride is soluble in water, eliminating choice A. When HCl is added, the hydroxide concentration decreases shifting the equilibrium to the right making Choice B false. The solubility product, Ksp, is a constant that varies only with temperature making choice C incorrect.

A person suffering from severe dehydration and starvation would NOT be expected to have elevated plasma concentrations for which of the following hormones? A. ADH B. Cortisol C. Aldosterone D. Insulin

ANS: D) Insulin This question asks you to determine the body's physiological response to the extreme conditions of dehydration and starvation. In such cases, the person would be expected to have low blood pressure and low nutrients (i.e. proteins, carbohydrates, and lipids). As a result, the body will respond by producing hormones that increase blood pressure and mobilize stored nutrients (glycogen, fat, etc.). ADH and aldosterone both increase blood pressure so they would be expected to appear in high plasma concentrations in response to the low blood pressure, eliminating choices A and C. Cortisol would be expected in high plasma to increase carbohydrate, protein, and fat availability, making B false. That leaves answer choice D. Insulin lowers blood glucose levels, which is expected to appear in the blood after a sugary meal. In the case of starvation, the body would respond by increasing blood glucose levels via the production of glucagon.

In which of the following structures of a healthy human male would immotile sperm incapable of fertilization be found? Vas deferens Seminiferous tubules Urethra

ANS: Seminiferous tubules This question is testing your understanding of sperm development within the male reproductive system. To answer this question, you must know the pathway sperm travels from the testes to ejaculation: seminiferous tubules -> epididymis -> vas deferens -> ejaculatory duct -> urethra -> penis. Furthermore, you must also know that the epididymis is the location where spermatozoa gain motility and fertilization capabilities. Therefore, immotile sperm that are incapable of fertilization may be found in the parts of the male reproductive system preceding the epididymis.

The voltage of a galvanic cell, E, depends on the concentration of reactants and products. The Nernst equation, E = Eo - (0.0592/n)log(Q) can be used to calculate the cell potential. Eo is the standard cell potential, n is the number of moles of electrons exchanged in the reaction, and Q is the reaction quotient. Which of the following is true regarding the value of E? A. E is always less than Eo B. E is always greater than Eo C. E is always equal to Eo D. E can be less than, greater than, or equal to Eo

According to the Nernst equation, the cell potential E will vary based on the logarithm of the reactant quotient Q. The reactant quotient follows the same equation as the equilibrium constant K, but measures the ratio of the actual concentration of products to concentration of reactants at any time during the reaction. Therefore, when Q > 1, log(Q) is positive and E < Eo. If Q < 1, log(Q) is negative, then E > Eo. Lastly, E is equal to Eo when Q = 1 because the log of one is equal to zero. Thus, E can be less than, greater than, or equal to Eo making D the correct answer.

Which of the following pure compounds form ionic bonds? I. CaCl2 II. HCl III. NaF IV. Mg(OH)2 A. I only B. I and II only C. I, III, and IV D. I, II, III, and IV

An ionic bond is formed between two atoms when one atom loses one or more electrons and another atom gains one or more electrons. This results in a positively charged cation and a negatively charged anion. The electrostatic attraction between the oppositely charged ions creates the bond, which can be measured by the finding the difference in electronegativity of the two atoms. Generally, a metal and nonmetal atom form ionic bonds. Metals prefer to give up electrons to form cations whereas nonmetals prefer their reduced anion form. In the case of CaCl2, NaF, and Mg(OH)2, each atom contains a metal that is bonded to a nonmetal. These all represent ionic bonds. While hydrochloric acid tends to dissociate in water to form H+ and Cl-, a pure solution cannot. Pure HCl contains two nonmetals, which forms a polar covalent bond. Therefore, C is the correct answer.

A woman stands 1m in front of a plane mirror. What is the magnification and distance from the woman to her mirror-image? 1/2 and 2m 1 and 1m 2 and 1m 1 and 2m

Ans: 1 and 2m (HIGH YIELD STUDY) A flat plane mirror has a radius of curvature that is infinite (it doesn't curve!) and thus a focal point distance that is also infinite. Thus the equation 1/f = 1/object + 1/image can be simplified to 1/object = -1/image So a flat plane mirror will always have an image distance that is equal to the object distance. The negative sign tells us that the image is virtual. Using the equation m = -i/o we can also see that a flat plane mirror will always produce an object with a magnification of 1. That is, when you look at yourself in the bathroom mirror, your image is not magnified nor reduced. If a person is standing 1 m in front of their mirror, then the image will be 1 m away from the mirror as well ("inside" the mirror). So the total distance between the person and the image will be 2 m. 1/2 and 2m, incorrect, Uses image distance of 2m rather than 1m and inverts the magnification equation. 1 and 1m, incorrect, Does not sum object and image distance for distance between woman and image. 2 and 1m, incorrect, Uses image distance of 2m in magnification equation and distance from woman to image also incorrect. 1 and 2m, correct.

Thin-layer chromatography (TLC) is a laboratory technique used to separate non-volatile mixtures. Using a silica gel plate and a hexane solvent, which of the following compounds in a mixture containing CH3OH, HCOOH, CH2O, and C6H6 would have the highest Rf value? A. C6H6 B. HCOOH C. CH2O D. CH3OH

Ans: A - C6H6 - benzene This question is testing your familiarity with thin-layer chromatography (TLC), which is a technique used to separate mixtures. Silica gel is composed of a siloxane matrix (-O-Si-O-). The highly polar substance is capable of forming intermolecular bonds, including hydrogen bonding. As a result, polar compounds adhere to the silica gel plate and travel little from the initial starting position. Meanwhile, nonpolar compounds elute away from the starting position with the nonpolar hexane solvent via capillary action. In the mixture above, HCOOH and CH3OH hydrogen bond with the siloxane matrix. Therefore, B and D will have the lowest Rf values. The oxygen atom in CH2O makes formaldehyde slightly polar allowing for intermolecular interactions between the gel plates. Benzene is the most nonpolar compound of the choices and therefore travels the farthest from the starting position along with the hexane solvent. Thus, A is the correct answer.

Which of the following molecules will stop being produced first when oxygen is no longer supplied to the cell? A. None B. Pyruvate C. Water D. Adenosine triphosphate

Ans: A - none Oxygen is necessary to aerobic respiration in mitochondria, as oxygen is the final electron acceptor of the electron transport chain. However, each of the products listed can be produced via anaerobic respiration. a) Incorrect: Pyruvate is a product of glycolysis and can be converted into lactic acid via anaerobic pathways. b) Incorrect: Water is a product of glycolysis, as well as other metabolic pathways. c) Incorrect: Adenosine triphosphate (ATP) is a product of glycolysis d) Correct: None of the above will cease being produced.

A beam of light is traveling through air (n = 1) when it passes through a transparent glass (n = 1.5) medium. Which of following is true concerning the angles θ and the velocity V? A. θ1 > θ2 and V1 > V2 B. θ1 > θ2 and V1 < V2 C. θ1 < θ2 and V1 > V2 D. θ1 < θ2 and V1 < V2

Ans: A. θ1 > θ2 and V1 > V2 This question tests the examinees understanding of Snell's law of refraction, n1(sinθ 1) = n2(sinθ 2). The index of refraction (n) is the ratio of the speed of light to the speed of light through a specific medium. The index of refraction for air is 1, which means V1 is equal to the speed of light (3 x 10^8 m/s). The refraction index for glass is 1.5, which makes V2 = 2 x 10^8 m/s. Therefore, B and D are eliminated. To compare the angle of incidence (θ 1) to the angle of refraction (θ 2), use Snell's law to determine the ratio. Since (sinθ 1)/(sinθ 2) = n2/n1, θ1 must be greater than θ2. Thus, A is the correct answer. n=c/v

The production of ATP via cellular respiration is governed by the following balanced reaction: C6H12O6 + 9 O2 > 6 CO2 + 6 H2O + ATP If there are 360 grams of C6H12O6 (MW = 180 g/mole) in the reaction vessel initially and 440 grams of CO2 are produced, what is the limiting reagent? A. C6H12O6 B. O2 C. CO2 D. H2O

Ans: B !!!!!!! This question is asking the examinee to determine the limiting reagent of a chemical reaction. To solve this problem, you must calculate the theoretical amount of CO2 that is produced given 360 grams of C6H12O6. If all 360 grams (2 moles) of glucose initially present react, then 528 g (12 moles) of CO2 should theoretically be produced. 360 g C6H12O6 x (1 mol C6H12O6 / 180 g C6H12O6) x (6 mol CO2 / 1 mol C6H12O6) x (44 g CO2 / 1 mol CO2) = 528 g CO2 Since only 440 grams of carbon dioxide are actually produced, not all of the glucose combusts. Thus, the oxygen must be the limiting reagent in this problem, making answer choice B correct. Choices C and D can be eliminated because products are never limiting reagents. Since there will be leftover C6H12O6, it is not the limiting reagent, eliminating answer choice A.

Similarities in chemical behavior can be observed between elements within the same family in the periodic table. For instance, sodium and potassium react with oxygen to produce a metal oxide. Which of the following gives the balanced reaction for the oxidation of an alkali metal (M)? A. 2M(s) + O2(g) → 2MO(s) B. 4M(s) + O2(g) → 2M2O(s) C. 3M(s) + O2(g) → M3O2(s) D. M(s) + O2(g) → MO2(s)

Ans: B - 4M(s) + O2(g) → 2M2O(s) Sodium and potassium are congeners found in the first column of the periodic table. They are included in the alkali metal family and exhibit similar chemical reactivity. In the presence of oxygen, alkali metals are oxidized. To form a complete octet, alkali metals lose one valence electron and take on their preferred oxidation state of +1. Oxygen, on the other hand, completes its octet by gaining two electrons to form an anion with a charge of -2. Thus, in the reaction above, two metal (M) atoms must each lose one electron in order for each oxygen atom to gain two electrons. Of the reactions above, B is the only choice where the metal (M) has a charge of +1 and oxygen had a charge of -2 with a 2:1 coefficient ratio of metal to oxygen. Thus, B is the correct answer for the oxidation of alkali metals.

The electronegativity of elements increases going from left to right across a row in the periodic table. Which of the following best accompanies this trend in the periodic table? A. A decrease in ionization energy and an increase in atomic radii B. A decrease in atomic radii and a decrease in ionization energy C. An increase in ionization energy and a decrease in atomic radii D. An increase in ionization energy and an increase in atomic radiiThis question tests the examinees knowledge of trends within the periodic table. In general, as you move from left to right across a row in the periodic table, electronegativity and ionization energy increases while atomic radii decrease. Therefore, C is the correct answer.

Ans: C This question tests the examinees knowledge of trends within the periodic table. In general, as you move from left to right across a row in the periodic table, electronegativity and ionization energy increases while atomic radii decrease. Therefore, C is the correct answer.

If the half-life of carbon-14 is 5700 years, and some plant remains from an archeological dig site have 1/8 the carbon-14 found in the atmosphere, what is the approximate age of the remains in the dig site in years? 700 5700 17000 45000

Ans: C - 17,000 The half-life is the time it takes for 1/2 of a radioactive substance to decay. For example, 20 units of a substance with a half-life of 5 years will have 10 units remaining after 5 years. In this example, carbon-14 has undergone 3 half-lives: 1 → ½; ½ → ¼; ¼ → 1/8. 3 x 5700 years = 17100 years. 700, incorrect, This answer results from multiplying the amount of sample by the half life, 5700/8. 5700, incorrect, This is the half-life, not the amount of time elapsed during three half-lives. 17000, correct. 45000, incorrect, This answer results from multiplying the half-life by the remaining fraction: 5700 x 8.

Which phase change has the same magnitude of energy to the heat of vaporization for water? A. sublimation B. freezing C. condensation D. deposition

Ans: C) condensation Phase changes require the same energy input or give off the same energy depending on the direction of phase change. Freezing and melting are paired, and the enthalpy change is called the heat of fusion. Evaporation and condensation are paired, and the enthalpy change is the heat of vaporization. Deposition and sublimation are paired, representing solid to gas change and there is no specific term for the enthalpy change. "Heat of sublimation" describes the enthalpy change. A. sublimation, incorrect, sublimation refers to a phase change from solid to gas while vaporization refers to a phase change from liquid to gas. B. freezing, incorrect, freezing refers to a phase change from liquid to solid. C. condensation, correct, heat of vaporization and condensation both refer to a phase change from gas to liquid and are equal in magnitude and opposite in sense. D. deposition, incorrect, deposition refers to a phase change from gas to solid. A deposition is also a term for legal testimony.

The hormones involved with menstruation associated with 1) the hypothalamus, 2) the anterior pituitary, and 3) the ovaries are (in order): A. 1) LH and FSH, 2) GnRH, 3) estrogen and progesterone B. 1) GnRH, 2) cortisone and aldosterone, 3) LH and FSH C. 1) GnRH, 2) LH and FSH, 3) estrogen and progesterone D. 1) LH and FSH, 2) GnRH, 3) cortisone and aldosterone

Ans: C. 1) GnRH, 2) LH and FSH, 3) estrogen and progesterone During the menstrual cycle: 1) the hypothalamus releases GnRH, 2) the anterior pituitary releases FSH and LH, and 3) the ovaries release estrogen and progesterone. A. 1) LH and FSH, 2) GnRH, 3) estrogen and progesterone, incorrect, The hypothalamus releases GnRH, and the anterior pituitary releases LH and FSH. B. 1) GnRH, 2) cortisone and aldosterone, 3) LH and FSH, incorrect, The anterior pituitary releases FSH and LH. The adrenal cortex releases cortisone and aldosterone. Cortisone increases gluconeogenesis and decreases protein synthesis. Aldosterone increases sodium reabsorption; increases to water retention and, thereby, blood pressure are secondary effects. C. 1) GnRH, 2) LH and FSH, 3) estrogen and progesterone, correct. D. 1) LH and FSH, 2) GnRH, 3) cortisone and aldosterone, incorrect, see explanations a and b.

One mole of an unknown compound is determined to have a molecular weight of 86 grams per mole. The mass percent of the compound is as follows: 55.81% carbon, 6.98% hydrogen, and 37.21% oxygen. What is the empirical formula and molecular formula, respectively, of the unknown compound. C2H3O and C4H6O2 C2H3O and C6H9O3 C5H8O and C5H8O C5H8O and C10H16O2

Ans: C2H3O and C4H6O2 To answer this question, you must determine the empirical formula of the unknown compound by treating the mass percent as the number of grams of each element out of a 100g sample. The number of moles of each element is determined by the following calculations: Moles of C = (55.81g C) x (1mol C/12g C) = 4.65 mol C Moles of H = (6.98g H) x (1 mol H/1g H) = 6.98 mol H Moles of O = (37.21g O) x (1 mol O/16g O) = 2.33 mol O Dividing each by the largest common denominator of 2.33 gives the empirical formula of the unknown compound: C2H3O (4.65/2.33) mol C = 2 mol C (6.98/2.33) mol H = 3 mol H (2.33/2.33) mol O = 1 mol O Since the molecular weight of the compound is 86 g/mol, the correct answer must be A. Note that if you were in a hurry on Test Day, or if you forgot how to solve this problem, you could instead use the periodic table to calculate the molar mass of the molecular formula given in each answer choice. If only one answer gave a molar mass of 86 g/mol (as is the case here, with choice A), then you could be certain that this option was correct. This is true of many MCAT questions: there may be multiple ways to solve the same problem!

The combination of the cornea and the crystalline lens shown below serves as a converging lens. In a perfectly functioning eye, an image is projected at a fixed distance on the retina, which is approximately 2cm from the lens. What is the power of the converging lens when a person without any visual impairment stares at a distant object? A. 0.5 diopters B. 10 diopters C. 25 diopters D. 50 diopters

Ans: D) 50 diopters To answer this question, you must use the thin lens equation 1/f = 1/do + 1/di, where f is the focal length, do is the object distance, and di is the image distance. Since the power (P) of a lens is equal to 1/f, you can set P = 1/do + 1/di. The question stem states that image distance is fixed and equal to a length of 2 cm. Since the question states that the observer stares at a distant object, we can assume that 1/do is negligible and equal to zero. Therefore, the equation simplifies to P = 1/di. Since power is measured in diopters, which has the units of m-1, you must convert 2 cm to 0.02 m and solve. Thus, the power of the lens is equal to 50 diopters, or answer choice D

Which of the following pairs of molecules have bond angles that are the most similar? A. CH4, CO2 B. BF3, NH3 C. H2O, SO2 D. CH4, H2O

Ans: D) CH4, H2O Similar bond angles generally - but not always - imply that the molecules have similar VSEPR geometries. However, we also need to consider factors including the presence of lone pairs and the overall Lewis structures. Let's look at the shapes of each pair individually to find the right answer. A. tetrahedral, linear. CH4, like a typical tetrahedral molecule, has approximate bond angles of 109.5 degrees. In contrast, a linear arrangement implies that bond angles are 180 degrees. These are too distant to be correct. B. trigonal planar, trigonal pyramidal. These may seem similar initially! However, trigonal planar bond angles are 120 degrees, while a trigonal pyramidal arrangement resembles a tetrahedral structure with one substituent replaced by a lone pair. As a result, the associated angles are slightly smaller than 109.5 (107.8, if you want to be accurate). C. bent, bent. This, too, it tempting! But H2O possesses two lone pairs on its central atom. As a result, its bond angles are even smaller than 107.8. In fact, they're around 104.5 degrees. This differs significantly from the 120-degree angles expected in SO2. D. tetrahedral, bent. Finally, here's our correct choice! Tetrahedral bond angles are typically 109.5 degrees, while that between the O-H bonds of water is 104.5. This is only a 5-degree difference (smaller, if you consult some textbooks, though we don't really care about the exact details). The key is that, although their shapes differ, the presence of different numbers of lone pairs cause these angles to be nearly identical.

Antidiuretic hormone (ADH) is responsible for the retention of water in the body. In the kidney, ADH increases reabsorption at the collecting duct. Which of the following physiological responses will be most likely in a patient suffering from severe dehydration? A. Decreased secretion of ADH resulting in decreased blood volume B. Increased secretion of ADH resulting in decreased blood volume C. Decreased secretion of ADH resulting in increased blood volume D. Increased secretion of ADH resulting in increased blood volume

Ans: D) Increased secretion of ADH resulting in increased blood volume This question is asking you to determine the body's hormonal response with respect to ADH in the case of a person suffering from dehydration. Dehydration is the result of a low blood volume, which leads to low blood pressure. To maintain homeostasis, the body will attempt to equilibrate by increasing blood volume, which in turn, increases blood pressure. Since ADH increases reabsorption of water from the filtrate to the bloodstream, the body is expected to secrete high levels of antidiuretic hormone, which will cause an increase in blood volume. Thus the correct answer is D.

Aldosterone acts on the kidney by: A. increasing water permeability of the collecting duct B. increasing water permeability of the ascending limb of the loop of Henle C. increasing water permeability in the proximal tubule D. increasing sodium absorption and potassium secretion in the distal tubule

Ans: D) increasing sodium absorption and potassium secretion in the distal tubule Filtrate pressure in the distal tubule is monitored by the juxtaglomerular apparatus. Special cells in the juxtaglomerular apparatus secrete an enzyme called renin either when stretch receptors in vessel walls detect a blood pressure drop (decreased renal perfusion) or when macula densa cells in the distal convoluted tubule detect a drop of sodium concentration in the tubular filtrate. Renin release triggers a cascade of angiotensin I, II, and III which in turn causes the adrenal cortex to secrete aldosterone. Aldosterone acts on the collecting tubule of the distal tubule (not the collecting duct) to increase membrane proteins absorbing sodium and secreting potassium. For the MCAT: decreased [Na+]filtrate or decreased blood pressure -> juxtaglomerular apparatus releases renin -> angiotensin I,II,III cascade -> aldosterone -> increased distal tubule Na+ absorption from and K+ secretion into the filtrate. Increased blood pressure is a secondary effect of the aldosterone. A. increasing water permeability of the collecting duct, incorrect, Antidiuretic Hormone (also known as ADH and vasopressin) causes increased water permeability in the collecting duct. B. increasing water permeability of the ascending limb of the loop of Henle, incorrect, The ascending limb of the loop of Henle is nearly impermeable to water. C. increasing water permeability in the proximal tubule, incorrect, Water reabsorption occurs in the descending limb of the loop of Henle, the collecting tubule, and the collecting duct. D. increasing sodium absorption and potassium secretion in the distal tubule, correct.

Aldosterone acts on the kidney by: A. increasing water permeability of the collecting duct B. increasing water permeability of the ascending limb of the loop of Henle C. increasing water permeability in the proximal tubule D. increasing sodium absorption and potassium secretion in the distal tubule

Ans: D, increasing sodium absorption and potassium secretion in the distal tubule Filtrate pressure in the distal tubule is monitored by the juxtaglomerular apparatus. Special cells in the juxtaglomerular apparatus secrete an enzyme called renin either when stretch receptors in vessel walls detect a blood pressure drop (decreased renal perfusion) or when macula densa cells in the distal convoluted tubule detect a drop of sodium concentration in the tubular filtrate. Renin release triggers a cascade of angiotensin I, II, and III which in turn causes the adrenal cortex to secrete aldosterone. Aldosterone acts on the collecting tubule of the distal tubule (not the collecting duct) to increase membrane proteins absorbing sodium and secreting potassium. For the MCAT: decreased [Na+]filtrate or decreased blood pressure -> juxtaglomerular apparatus releases renin -> angiotensin I,II,III cascade -> aldosterone -> increased distal tubule Na+ absorption from and K+ secretion into the filtrate. Increased blood pressure is a secondary effect of the aldosterone. A. Increasing water permeability of the collecting duct, incorrect, Antidiuretic Hormone (also known as ADH and vasopressin) causes increased water permeability in the collecting duct. B. Increasing water permeability of the ascending limb of the loop of Henle, incorrect, The ascending limb of the loop of Henle is nearly impermeable to water. C. Increasing water permeability in the proximal tubule, incorrect, Water reabsorption occurs in the descending limb of the loop of Henle, the collecting tubule, and the collecting duct. D. Increasing sodium absorption and potassium secretion in the distal tubule, correct.

The molar heat capacities (J/mole K) for zinc, copper, silver, and gold is 25.2, 24.5, 24.9, and 25.6, respectively. If 1 mole of each substance is heated until the temperature increases by 10K, which metal required the addition of the most heat? A. Zinc B. Copper C. Silver D. Gold

Ans: Gold This question asks you to apply information on molar heat capacities to determine the most energetically intensive material. Since the change in temperature and the quantity of each substance is constant, the difference in molar heat capacities dictates the energy requirements for each substance. Molar heat capacity tells you how much energy (J) is necessary to increase one mole of a substance by 1 K. Therefore, the higher the molar heat capacity, the more energy required. In the question above, gold has the highest molar heat capacity and therefore requires the greatest amount of heat. The calculations are given below. Energy = (molar heat capacity) x (# of moles) x (change in temperature) Zinc: (25.2 J/mole K) x (1 mole) x (10 K) = 252 JCopper: (24.5 J/mole K) x (1 mole) x (10 K) = 245 J Silver: (24.9 J/mole K) x (1 mole) x (10 K) = 249 J Gold: (25.6 J/mole K) x (1 mole) x (10 K) = 256 J Multiplying the molar heat capacity by the number of moles and the change in temperature gives you energy required. Gold requires the most heat (256 J) making answer choice D correct.

Myoclonic epilepsy and ragged-red fiber (MERRF) is an extremely rare disorder that affects neuromuscular systems. MERRF results from a mutation in mitochondrial DNA (mtDNA) that impairs protein synthesis, oxygen consumption, and energy production. When an affected male and a normal female reproduce, which of the following best predicts the expected phenotypic ratios of the offspring? None of the offspring will be affected All males and no females will be affected Half of males and half of females will be affected One-fourth of the offspring will be affected

Ans: None of the offspring will be affected This question is asking the examinee to determine offspring phenotypes for a gene that is located on mitochondrial DNA. Since mtDNA follows maternal inheritance, all the offspring receive their mitochondrial DNA from the mother. In this case, the MERRF-causing mutation is located on the father's mtDNA. Therefore, the mutations are not passed on to the offspring. Since all of the offspring will receive functioning mtDNA from the mother, none of the offspring will inherit mutated genes causing MERRF. Thus, answer choice A correct.

A group of men and women are going to be rated on their driving abilities. The role of gender is emphasized in the experiment and the women perform worse than the men. In another experiment the role of gender is not mentioned and the ratings are comparable between the two groups. Which principle do these results support? Institutional discrimination Stereotype threat Prejudice The just-world hypothesis

Ans: stereotype threat Stereotype threat refers to the concept of people being concerned or anxious about confirming a negative stereotype about their social group. Stereotype threat can hinder performance, creating a self-fulfilling prophecy.

Mono-nucleated muscle cells are a predominant muscle fiber found in all of the following EXCEPT: A. the stomach B. the pharynx C. the heart D. the lungs

Ans: the pharynx This question is testing your familiarity with the predominant muscle cell types found in various structures or organs within the body. To answer this question, you must know the characteristics of the three different muscle types: 1) skeletal, 2) cardiac, and 3) smooth. Skeletal muscle is striated, multi-nucleated, and under voluntary control and is typically attached to bones and cartilages. Cardiac muscle is involuntary, striated, and mono-nucleated and is found only in the heart. Smooth muscle is involuntary, mono-nucleated and non-striated is generally associated with viscera. Since skeletal muscles contain only multi-nucleated cells, the correct answer to this question must be a structure that is under voluntary control. The stomach and the lungs are internal organs found in the thoracic and abdominal cavities. These visceral organs are involuntary and consist primarily of smooth muscle fibers eliminating choices A and D. The heart is also involuntary and contains mostly cardiac muscle eliminating answer choice C. The pharynx is part of the digestive tract and is located above the esophagus. Both the pharynx and the upper third of the esophagus consist mostly of skeletal muscle, while the rest of the digestive system is controlled by smooth muscle. Since the pharynx contains primarily skeletal muscle, the cells are multi-nucleated. Thus the correct answer is B.

Prior to vaccinations for poliomyelitis, widespread polio infection created wards of patients at risk of death due to loss of control of respiratory muscles. In 1928, Drinker and Shaw invented the first "iron lung," a device that sealed the patient's body from the neck down in a chamber with controllable pressure. The pressure change in the "iron lung" aided respiration by:

Breathing occurs when the diaphragm and intercostal muscles expand the thoracic cavity and increase the space in the pleural cavity, thus lowering its pressure relative to atmospheric pressure. This forces air down into the lungs and expands the lungs, forming the basis of 'negative pressure' breathing. The iron lung mimics this by decreasing the surrounding pressure in the air around the person's chest and abdomen, generating negative pressure that causes the patient's lungs to expand -negative pressure within the iron lung causing the patient's lungs to expand.

What is the effect of adding KNO3 to a saturated aqueous solution of KCl? A. Ksp increases because more K+ ions are present. B. Ksp decreases because NO3- ions displace Cl- ions. C. The concentration of Cl- ions decreases because KNO3 addition drives KCl dissociation towards the solid, undissociated form. D. More KCl dissolves because more aqueous K+ ions require more Cl- ions.

C is correct. Group IA elements, Cl- salts, and nitrate (NO3-) salts are soluble. Ksp for a compound does not change, although the concentration of dissolved species may. When a product concentration is increased in an equilibrium reaction, reactant concentration increases according to Le Chatelier's principle. Here, addition of K+ ions to a saturated aqueous solution of KCl will drive KCl dissociation in the direction of the reactants and cause precipitation of solid KCl. The concentration of Cl- ions in solution will decrease such that the Ksp remains the same for the new product of Ksp=[K+][Cl-]. The only time Ksp changes is when the temperature is changed, which is not the case here. A. This choice states that Ksp increases because more K+ ions are present. Incorrect, Ksp does not change as a result of concentration. B. This answer states that Ksp decreases because NO3- ions displace Cl- ions. Incorrect, Ksp does not change as a result of concentration. C. This option correctly states that the concentration of Cl- ions decreases because KNO3 addition drives KCl dissociation towards the solid, undissociated form. D. In this answer choice, more KCl dissolves because more aqueous K+ ions require more Cl- ions. Incorrect, this contradicts Le Chatelier's principle. In an accurate description of this principle, higher product concentrations should drive the equilibrium towards the reactant, not the product, side. Remember, Le Chatelier's principle states that a reversible process will shift in the direction that relieves stress and re-establishes equilibrium.

One mole of a compound consists of approximately 72g carbon and 6g hydrogen. What is its empirical formula?

CH, correct. The empirical formula is the smallest whole-number ratio of the elements making up a molecule. The empirical formula expresses the smallest number of atoms making up a molecule in the correct integer ratio. In this question, converting to moles gives us six moles of carbon and six moles of hydrogen in one mole of the entire compound. This gives us our molecular formula, C6H6. However, remember that empirical formulas must consist of the smallest ratio of all atoms involved! This ratio is CH, which is our answer.

An individual is born with a mutation causing her to partially retain a form of fetal hemoglobin into adulthood. Compared to a normal individual, this person would exhibit: A. no differences from a normal adult. B. significantly reduced oxygen binding in the lungs. C. no symptoms, since retention of fetal hemoglobin would be fatal. D. increased oxygen binding to hemoglobin in the tissues.

D) increased oxygen binding to hemoglobin in the tissues Fetal hemoglobin has a higher affinity for oxygen than does adult hemoglobin. Thus, it binds and holds oxygen more tightly. An individual with fetal hemoglobin would show increased oxygen binding to hemoglobin as a result. (A) is false because fetal and adult hemoglobin are different. (B) is false because fetal hemoglobin binds more to oxygen, not less. And (C) is false - in fact, reactivating fetal hemoglobin can be used as a treatment for sickle-cell. Therefore, the answer is (D).

The pressure above a solid compound is slowly reduced. Which of the following is true? A. If the substance is H2O, it will transition from solid directly into gas. B. For all substances except H2O, the substance will transition into liquid phase and then the gas phase as the pressure is reduced. C. For all substances except H2O, the substance will transition from solid directly into gas. D. If the substance transitions from solid directly into gas, it must be H2O (sublimination)

For most substances, the solid phase is more dense than the liquid phase, so lowering the pressure will cause the solid phase to transition either to the liquid phase or the gas phase, depending on the temperature. Because this transition (melting vs. sublimation) is temperature-dependent, neither (B) nor (C) is always true. Similarly, both water and other substances are capable of subliming at certain temperature/pressure combinations, so (D) is not always true. You should also remember that although H2O is rare in having a less-dense solid phase, it is not unique. Thus (A) remains as the only option, and it is true. !Because reductions in pressure favor the less-dense phase!, ice will transition directly to gas as the pressure is reduced.

A holding tank at sea level (atmospheric pressure 1.01 x 105 Pa) containing water has a pressure of 2 x 105 Pa at 10 m depth. What is the pressure in an equivalent holding tank containing mercury (specific gravity = 13) at 1 m depth in Denver, CO (atmospheric pressure 8.4 x 104 Pa)?

Formula for hydrostatic pressure in a fluid: Ptot = P0 + ρgh Here, we can simply plug in the values provided: Ptot = (8.4 x 104) + (13,000 kg/m3 x 10m/s2 x 1m) = 2.14 x 105 Pa A. 2 x 105 Pa correct, matches our calculation B. 1 x 105 Pa, incorrect, this is the same pressure at sea level for 10m of water, ignores fluid change and atmospheric pressure change C. 1.4 x 106 Pa, incorrect, this answer fails to change the fluid depth to 1m for the mercury rather than the 10m depth used for water D. 8.4 x 104 Pa, incorrect, this answer ignores the fluid contribution to pressure and only uses the atmospheric contribution

DNA and RNA

Nucleotides are monomers, or subunits, that are the building blocks of DNA and RNA molecules. Each nucleotide consists of a phosphate group, a pentose ring, and a nitrogenous base. In the case of the guanosine monophosphate nucleotide shown, the 5' end corresponds to the phosphate group attached to the fifth carbon on the pentose ring, indicated by the arrow labeled IV. This phosphodiester bond represents the 5' of a nucleotide making choice D the best answer. The arrow labeled III corresponds to the third carbon in the pentose ring. The hydroxyl group attached to the third carbon represents the 3' end of a nucleotide. The arrow labeled II represents the first labeled carbon. Lastly, arrow I points to the nitrogenous base of guanine.

In the ZW sex-determination system for birds, males (ZZ) are the homogametic sex while females (ZW) are the heterogametic sex. What is the most likely regulatory mechanism for the potential imbalance in gene expression between male and female birds?

Opposite to male birds (ZZ), mammalian males (XY) are heterogametic and carry one X and one Y chromosome. Similarly, female birds (ZW) are heterogametic while mammalian females (XX) are homogametic. In the mammalian XY sex-determination system, females randomly inactivate one of their X chromosomes to prevent a redundancy in gene products (i.e. mRNA and proteins). These inactivated chromosomes are known as Barr bodies. In the case of bird species, the homogametic males (ZZ) must inactivate one of their Z chromosomes to achieve the appropriate reduction in gene products. Therefore, the correct answer is: Inactivation of one Z chromosome in males

In order to send pain signals to the brain, a nerve in the spinal cord would have to synapse with an:

Sensory neurons are afferent (from periphery to central) and enter dorsally within the spinal cord. Motor neurons are efferent (from central to periphery) and leave the spinal cord ventrally. An interneuron carrying pain sensation to the brain would synapse with an afferent, dorsal sensory neuron. afferent dorsal sensory neuron, correct.

What is the azimuthal quantum number (l) for the orbital to which a fluorine atom gains an electron to form an F- ion?

The azimuthal quantum number (also known as the orbital quantum number or angular momentum quantum number), l, designates the subshell of an atom and gives the "shape" of an orbital. The number also indicates the number of angular nodes, or regions of zero probability, within the orbital. The orbital shapes commonly referred to as s, p, d, and f, correspond to azimuthal quantum numbers of 0, 1, 2, and 3, respectively. The electron configuration for elemental fluorine is 1s2 2s2 2p5. When fluorine gains an electron to become F-, the electron adds to the p orbital. P orbitals are dumbbell shaped with one angular node and correspond to an azimuthal quantum number of 1

Two model trains are propelled along a straight, frictionless track by electric motors. Train X has a total mass of 100g and is moving at speed v. Train Y has a total mass of 10g and is moving at speed 3v. The ratio of the kinetic energies of Train X to Train Y is: A. 10:9 B. 9:10 C. 10:3 D. 1:9

The kinetic energy equation is Ekinetic = ½ mv2. We can plug in the values given and set up a ratio to answer the question. EX = ½ (100)(v)2 EY = ½ (10)(3v)2 = ½ (10)(9v2) EX / EY = ½ (100)(v)2 / ½ (10)9v2 EX / EY = (100)(v)2 / (10)9v2 EX / EY = (10)(v)2 / 9v2 EX / EY = 10 / 9 Thus choice (A), 10:9 is the correct answer.

The flow of blood through the aorta can be approximated with the volumetric flow rate equation. When the smooth muscle of the aorta contracts, the radius can decrease by as much as 10%. If the blood flow is to remain unchanged, what must happen to the velocity of blood flow in the aorta? A. Decrease by approximately 10% B. Increase by approximately 10% C. Increase by approximately 25% D. Remain unchanged

The volumetric flow rate equation is Q = A x v where A is the cross-sectional area of the pipe, Q is the flow rate, and v is the velocity of the fluid. The problem states that Q is to remain unchanged. That allows us to set up the following equation: Q = A1v1 = A2v2 Plugging in 1 as the original values, we get: 1 x 1 = 0.81 x v2 Because the radius was decreased to 90% of its original value, the cross-sectional area will be reduced to 81% of its original value (A is proportional to radius squared and 0.92 = 0.81) r2 = 0.9r1 A2 = 0.81A1 Solving for v2 we get 1.23. Thus the velocity increased by approximately 25%, making choice (C) correct.

The voltage drop across Rd is 8 V. What is the current across Re?

This problem requires 3 electrical circuit concepts: 1) V = IR, which relates voltage, current, and resistance 2) The fact that the voltage drop across resistors in parallel is equal in each resistor 3) Kirchoff's junction law, which states that the sum of currents entering a node equals the sum of currents leaving a node The voltage drop is equivalent for each resistor between red nodes A and B. The voltage drop across Rd is given as 8 V, and therefore the voltage drop across resistors Rb and Rc is also 8 V. Using V = IR for each resistor, the current in resistor Rb = 4 A, Rc = 8 A, and Rd = 2A. Using Kirchoff's current law at node B: 4 A + 8 A + 2 A = 14 A. The current leaving node B equals the current across Re because there are no other nodes in between.

Which of the following statements accurately orders the speed of sound through the following mediums: air (ρair = 1.275 kg/m3), water (ρwater = 1000 kg/m3), and copper (ρcopper = 8960 kg/m3)? A. Air > water > copperB. Air > copper > waterC. Copper > water > airD. Copper > air > water

This question asks the examinee to identify the correct order for the velocity of sound waves through various media. The velocity of sound is given by the equation, v = (B/ρ)1/2, where B is the bulk modulus and ρ is the density of the material. The densities of solids are the greatest because of the strong intermolecular forces, which create lattice structures with closely bonded molecules. While solids tend to be densest, they also tend to be the least compressible, which corresponds to a high bulk's modulus. As a result, the B/ρ quotient is the highest for solids, followed by liquids, and then gases. Therefore, the velocity of sound through copper is greater than water, which is greater than air. Thus, C is the correct answer.

The combustion of octane is a common reaction in automobile engines. 1) 2 C8H18 (l) + 25 O2 (g) -> 16 CO2 (g) + 18 H2O (g) ΔH1 = -250 kJ/mole 2) 18 H2O (l) -> 18 H2O (g) ΔH2 = 88 kJ/mole Net Reaction: 3) 2 C8H18 (l) + 25 O2 (g) -> 16 CO2 (g) + 18 H2O (l) ΔH3 = ?

This question asks you to determine the change in enthalpy for a net reaction given the enthalpy change for two other reactions. Hess's Law tells us that the sum of two reactions gives us the enthalpy change for the net reaction: ΔH1 + ΔH2 = ΔH3. Since the net reaction has H2O (l) as a product, reaction 2 must be reversed and the sign of the enthalpy change negated (-ΔH2). The sum of the reaction enthalpies for the net reaction is as follows: ΔH3 = ΔH1 + (-ΔH2) ΔH3 = -250 kJ/mole + (-88 kJ/mole) ΔH3 = -338 kJ/mole

Which of the following best describes the elbow joint in a human? A. Gliding joint B. Ball and socket joint C.Hinge joint D. Saddle joint

This question is testing your understanding of the basic types of joints in the human body. Each type of joint allows a different type of movement. The elbow contains a hinge joint allowing for movement in only one plane, like the opening and closing of a door. Thus, C is the correct answer. Gliding joints are common at the junction between flat bones and allow the bones to slide past one another. An example would be the synovial joints between vertebrae in the spinal column. A ball and socket joint permits the greatest range of motion and allows for 360o of rotation in most cases. The shoulder and hip are examples of ball and socket joints. A saddle joint is formed between reciprocal concave and convex bones. An example is the carpometacarpal joint of the thumb.

What proportion of offspring with the genotype AABBCC will result from a trihybrid cross for the unlinked genes A, B, and C? 1/8 1/16 1/32 1/64

This question tests your ability to calculate the probability of a possible genotype resulting from a trihybrid cross between two heterozygotes. One method to determine the probability is to make a Punnett square. An organism that is heterozygous for three genes can produce eight different gamete possibilities. Thus, resultant Punnett square would contain an 8×8 table with a total of 64 genotype combinations. A more efficient method is to calculate the probability of each event occurring separately and then use the rule of multiplication to determine the likelihood of all three events occurring at the same time. Probability of obtaining an AA genotype from a cross between Aa and Aa = ¼ Probability of obtaining a BB genotype from a cross between Bb and Bb = ¼ Probability of obtaining a CC genotype from a cross between Cc and Cc = ¼ Total probability of all three possibilities = ¼ x ¼ x ¼ = 1/64 Therefore, D is the correct answer.

The pH inside the stomach is very closely regulated. Which of the following is a major function for the pH in the stomach?

This question tests your knowledge of the physiological functions that result from the low pH within the stomach cavity. While the acid environment serves several functions, one important one is denaturing the tertiary structure of proteins in the food. This lets proteases break up the protein into smaller peptides. Denaturing proteins to allow for digestive processes

An infectious agent is probably a virus instead of a bacterium if it: A. lacks membrane-bound organelles B. cannot reproduce in acellular culture C. has a length of 1μm, about the size of a mitochondrion D. contains DNA

Viruses possess DNA or RNA but not both, require a host cell's reproductive machinery in order to reproduce, and have a size 10-100nm. For the MCAT: eukaryotic cell > bacterium (roughly size of mitochondrion) > virion (roughly size of protein, roughly 100x smaller than bacterium, 1000x smaller than eukaryotic cell, cannot be seen with optical microscope). B is the correct answer

The circuit shown below consists of a capacitor, resistor, and battery with a switch that completes the circuit. When the switch connects A to B, which of the following describes the conversion of energy that occurs as current flows through the circuit?

When the switch connects node A to B, the circuit consists of a battery in series with a resistor. Batteries store chemical energy, which is converted to electrical energy as current flows through the circuit. At the resistor, the electrical energy is converted to thermal energy making B the correct answer. A. Chemical to potential to thermal B. Chemical to electrical to thermal C. Electrical to chemical to thermal D. Potential to kinetic to electrical

The fundamental wavelength of a standing wave in a 1 meter long pipe, if the pipe has one open end and one closed end is:

ans: 4m Standing waves consist of wave frequencies at which wave nodes occur at closed pipe ends and antinodes occur at open pipe ends. For a pipe closed at one end, the equation relating the length of the pipe, L, to the λ of a standing wave in the pipe is λ = 4L / n, where n is the harmonic number, and can be any odd integer. When n = 1, the standing wave is the "fundamental" wavelength. This is also called the "first harmonic". A. 4m, correct, the fundamental wavelength for a pipe with one closed end is λ1= 4L. B. 1m, incorrect , the pipe length does not correspond to the fundamental wavelength. C. 2m, incorrect, 2m corresponds to the fundamental wavelength of a pipe with both ends open λ1= 2L. D. 4/3m, incorrect, 4/3m corresponds to the second resonant wavelength of a pipe with one end open, λ2= 4L/3.

An infectious agent is probably a virus instead of a bacterium if it:

cannot reproduce in acellular culture, correct. Viruses require host cells in order to reproduce.

Alpha-carbohydrate

α- carbohydrates have the OH (anomeric) and CH2OH group on opposite sides of the ring

Beta-carbohydrate

β-Carbohydrates have the OH (anomeric) and CH2OH group on the same side of the ring