AAMC Flashcards - Chemistry

Approximately how many moles of Al3+ are reduced when 0.1 faraday of charge passes through a cell during the production of Al? (Note: Assume there is excess Al3+ available and that Al3+ is reduced to Al metal only.)

A. 0.033 mol A faraday is equal to one mole of electric charge. Because each aluminum ion gains 3 electrons, 0.1 faraday of charge will reduce 0.1/3 moles of aluminum, or 0.033 moles of aluminum.

Which of the following chemical species is NOT isoelectronic with a neon atom? A.He B. F- C. Mg2+ D. Na+

A.He If two atoms are isoelectronic, they have the same number of electrons. Neon has ten electrons. F- has 9 + 1 = 10 electrons and so is isoelectronic with Ne. Mg2+ and Na+ have 12 - 2 = 10 and 11 - 1 = 10 electrons respectively. Only He, with two electrons, is not isoelectronic with neon. Answer choice A is the best answer.

Which of the following valence electron configurations corresponds to an atom in an excited state? 1s22s2 2s22p1 3s23d1 4s23d2

B. 2s^2 2p^1 The ground state electron configuration of an atom is generally written as follows: 1s22s22p63s23p64s23d104p65s2... with the orbital energetics following the (n + l) rule. When an electron is excited, it "jumps" to a higher energy subshell. Since the 3d subshell (n + l = 5) differs by more than one unit of (n + l) from the 3s subshell (n + l = 3) it is clear that the electron in the 3d subshell is in an "excited" state. Thus, C is the best answer. So in each one of these you want something that could have jumped from a lower energy level to higher energy. For A: 2s electron couldn't have jumped from the lower electron level 1s because that is full. For B: the energy level below 2p is 2s but that's already full so an electron couldn't have jumped. For D: 3D then the energy level below 3D is 4S which again is already full. That leaves only C where 3d1 could have jumped an energy level from the lower energy level 3p.

The following reaction occurs spontaneously. Cd(s) + 2 H+(aq) → Cd2+(aq) + H2(g) Which of the following has the highest electron affinity? Cd(s) H+(aq) Cd2+(aq) H2(g)

B. H+(aq) The reaction equation shows the reduction of H+ by Cd. (aqueous --> gas is reduction). Because the H+accepts the electron readily from Cd, it can be determined that H+ has the highest electron affinity. Electron affinity = energy associated with the addition of an electron to a gaseous atom. aka a neutral atom's likelihood of gaining an electron Ex Cl(g) + e- --> Cl- (g) EA = = 349 kj/mol: the sign is negative because energy is usually related in this process, as opposed to ionization energy, which requires energy. A more negative electron affinity correspond to a greater attraction for an electron. Trend: electron affinity becomes less negative down a group. Electron affinity decreases or increases (usually increases) across a period depending on electron configuration

Which of the following substances is most likely to be more soluble in 1.0 M HCl than in 1.0 M NaOH? A. AgCl B. Pb(OH)2 C. CaF2 D. HI

B. Pb(OH)2 When an ionic substance dissolves, it dissociates into its constituent ions. The solubility of any substance whose anion is basic will be increased in solutions of low pH, as the basic anion reacts with the H+ in solution and is drawn out of the equilibrium. As the basic anion is consumed, the dissolution reaction is driven to the right by Le Chatelier's principle. Of the substances given as choices, Pb(OH)2 has the most basic anion, OH-. Thus it is most likely to show increased solubility in an HCl solution.

An electrochemical cell is designed to produce pure copper from CuSO4. An increase in which of the following cell conditions will most effectively increase the rate at which pure copper is produced? A. The concentration of SO42+(aq) B. The current of electricity C. The size of the cathode D. The size of the anode

B. The current of electricity An electrolytic cell drives a non-spontaneous reaction using electrical energy. In the electrolytic cell, electrons are driven in through/flows out of the cathode (-) and drawn out through/electricity moves into the anode (+). The rate at which this occurs is the current. By increasing the current, electrons are forced in faster, increasing the rate of reduction, and are withdrawn faster thus increasing the rate of oxidation. Answer choice B is the best answer

why is the boiling point of ammonia higher than phosphine?

Because the degree and magnitude of intermolecular bonding between ammonia molecules is much greater than that between phosphine, and arsine molecules. Ammonia forms stronger intermolecular bonds than phosphine Both phosphine (PH3) and ammonia (NH3) have London dispersion forces. Phosphine with a molecular mass of 34g/mole is a larger and heavier molecule than ammonia with its molecular mass of 17g/mole. Hence, phosphine has stronger London dispersion forces. However, ammonia, unlike phosphine, has hydrogen bonds due to the presence of a covalent bond between hydrogen and highly electronegative nitrogen with a lone pair. Hydrogen bonds are stronger than London dispersion forces, therefore ammonia has stronger intermolecular forces and higher boiling point.

Which of the following describes the orbital geometry of an sp3 hybridized atom?

C. Tetrahedral Orbital geometry is also known as electron domain geometry. Hybridization is based on the number of electron domains surrounding an atom. An sp3-hybridized atom, such as C in CH4 or N in :NH3, mixes the s and p subshells (one s and three porbitals) to form four equivalent orbitals. Four equivalent orbitals point at the vertices of a tetrahedron. Thus, C is the correct answer.

With which of the above metals can copper form a galvanic cell in which copper is reduced? A.With silver only B. With lead only C. With lead and zinc D. With silver and zinc

C. With lead and zinc In order for an oxidation-reduction reaction to proceed spontaneously, the cell potential Ecell° for the net reaction must be positive. The cell potential Ecell° can be found by subtracting the standard reduction potential Ered° for the substance being oxidized from that of the substance being reduced. The question asks which metals can reduce copper ions. This means that the metals in question are oxidized. The difference in standard reduction potentials Ecell° from the table is positive for lead and zinc but negative for silver. Thus, the correct answer is C.

Compound X is classified as a: A. dipeptide that contains one peptide bond. B. dipeptide that contains two peptide bonds. C. tripeptide that contains two peptide bonds. D. tripeptide that contains three peptide bonds.

C. tripeptide that contains two peptide bonds. Peptides are formed when two amino acid residues are joined through a common amide linkage ("peptide bond"); this bond is formed between the N-terminus (amino end) of one amino acid residue and the C-terminus (carboxyl end) of another amino acid residue. Compound X is composed of three amino acid residues linked by two peptide bonds.

Calculation of Empirical Formula

Calculation of an Empirical Formula Step 1: Obtain the mass of each element present in grams. Element % = mass in g = m. Step 2: Determine the number of moles of each type of atom present. m/atomic mass = Molar amount (M) Step 3: Divide the number of moles of each element by the smallest number of moles. M / least M value = Atomic Ratio (R) Step 4: Convert numbers to whole numbers. R * whole number = Empirical Formula

Which pair of formulas represents the empirical and molecular formulas, respectively, of caffeine? A.CHNO and C8H10N2O2 B. C4H5N2O and C4H5N2O C. C8H10N4O2 and C8H10N4O2 D. C4H5N2O and C8H10N4O2

D. C4H5N2O and C8H10N4O2 Counting the numbers of atoms of different elements in the structure of caffeine gives the molecular formula, C8H10N4O2. The empirical formula of a compound is the simplest whole-number ratio of the moles of the elements in the compound. Dividing the molecular formula of caffeine by 2 gives the empirical formula, C4H5N2O. Thus, D is the best answer.

Which of the following functional groups is found in benzoin, C6H5CH(OH)C(O)C6H5? Carboxylic acid Ether Aldehyde Ketone

D. Ketone Benzoin contains a ketone carbonyl group and an alcohol hydroxyl group. It has no carboxylic acid, ether, or aldehyde. Thus, answer choice D is the best answer.

When a weak acid (HA) is titrated with sodium hydroxide in the presence of an indicator (HIn), the pH at which a color change is observed depends on the:

D. pKa of HIn The indicator will change color over a specific pH range. The range at which the color change takes place depends on the point at which HIn is converted to In-, and this depends on the pKa of the indicator which is answer D.

Oxidation and Reduction

oxidation: gaining oxygen reduction: losing oxygen (anything that leads back to the free mental state) Magnesium gets oxidized by losing 2 electrons to oxygen, which gets reduced by accepting 2 electrons from magnesium

catalyst

substance that speeds up the rate of a chemical reaction by lowering the activation energy of a reaction. It can do NOTHING to affect the energies of products and reactants and thus cannot affect the position of the equilibrium = SO IT WILL REMAIN THE SAME. It only allows the reaction to reach equilibrium more quickly.

A gas that occupies 10 L at 1 atm and 25oC will occupy what volume at 500 atm and 25oC?

Somewhat more than 0.02 L because of the space occupied by the individual gas molecules Ex: The ideal gas law makes the assumption that molecules have no volume. This assumption is adequate when the gas is at 1 atm, but when the pressure is increased to 500 atm the volume of the gas molecules is no longer negligible. Ideal gas - consists of a large number of particles (atoms or molecules) in constant motion - collisions between particles and the walls of a container are perfectly elastic (kinetic energy is conserved) - attractive forces between particles do not exist - intermolecular forces are 0 expect during instantaneous collisions - the total volume of particles is very small compared with the volume of the container

18 O-labeled methanol (CH3O*H)

The methoxy product (OCH3) in the product comes from the 18^O labeled methanol