MCAT GENCHEM

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1. Which of the following best characterizes electron flow in an electrolytic cell?
a.Electrons flow from anode to cathode due to a potential difference between the electrodes.
b.Electrons flow from cathode to anode due to a potential difference between the electrodes.
c.Electrons flow from anode to cathode due to the driving force provided by an external source.
d.Electrons flow from cathode to anode due to the electrostatic attraction of the respective poles.
• c
• Electrons always flow from anode to cathode (eliminate choices B and D)
• in galvanic or electrolytic cells. In galvanic cells, this flow of
• electrons occurs spontaneously as electrons flow down their potential
• gradient. For an electrolytic cell (which we are being asked about
• here), the potential difference between the anode and cathode is
• negative, indicating the given reaction is nonspontaneous and the flow
• of electrons from anode to cathode requires assistance from an external
• source, such as a battery (choice A is wrong and choice C is correct).
2. What is the final pH after 30 milliliters of 3 x 10–3 M HBr is mixed with 20 milliliters of 4 x 10–3 M NaOH?
• 3.7
• In the reaction of a strong acid and a strong base, protons generated by the acid react with hydroxide ions generated by the base and any remaining protons or hydroxide ions are then responsible for the final pH of the solution. Here we have (0.03 L)(3 x 10–3 M) = 9 x 10–5 mol H+ and (0.02 L)(4 x 10–3 M) = 8 x 10–5 mol OH resulting in 1 x 10–5 mol of H+ remaining. This means the solution should be acidic, so we can eliminate choices C and D. The final proton concentration is 1 x 10–5 mol/0.05 L = 0.2 x 10–3 = 2 x 10–4 M H+ resulting in a pH of 3.7 (choice B is correct).
3. When 1 mole of K3PO4 is dissolved in 1 L of water, how many moles of ions are produced?
• 4
• Because potassium phosphate is an ionic compound, it is a strong electrolyte and will dissociate in solution (eliminate choice A). The covalent bonds between the P and O will not break: only the ionic bonds will, so choice D can be eliminated. The question asks for the total number of ions formed, not just the number of different types of ions. Since three K+ ions and one PO43– ion will form for every unit of K3PO4, choice C is the better answer.
4. Which of the following pairs will generate an electrolytic cell with the least stable products?

 Al3+ + 3 e− → Al(s) E° = –1.68 V Cd2+ + 2 e− → Cd(s) E° = –0.40 V Ag+ + e− → Ag(s) E° = 0.80 V Br2(l) + 2 e− → 2Br− E° = 1.08 V
Al3+ and Ag(s)

The electrolytic cell with the most negative electrical potential will have the least stable products (choice A is correct). To determine the overall potential of any given cell, the reduction and oxidation half-reactions are added. To find the oxidation half-reactions, the reduction half-reaction in question can be reversed and its potential reversed in sign. In this instance to find the oxidation half-reaction, the reduction of silver above can be reversed to Ag(s) → Ag+ + e E° = –0.80 V which can then be add to the reduction half-reaction Al3+ + 3 e → Al(s) E° = –1.68 V. Note that while the oxidation half-reaction is multiplied 3 to appropriately balance the reaction, this does not impact its electrical potential. The overall electrical potential can be found by adding the two half-reaction potentials: –1.68 V + (–0.80 V) = –2.49 V.
5. A medical student inadvertently dumps a sample of K2C2O4
on his leg while in lab and frantically searches for a solution to
neutralize the burn. Which of the following would best help neutralize
the student’s burn?

(A) H2O
(B) NaHCO3
(C) C6H12O6
(D)HCH3CO2
• HCH3CO2
• The question asks for a solution to neutralize the
• burn. To determine whether this unknown salt is a acid or base, we
• inspect each ion independently. Potassium is a group I metal and is
• neutral (does not react with water) while the anion (oxalate) is the
• conjugate base of a weak acid and is therefore a base. While selecting
• water may seem like a good idea, in order to have a neutralization
• reaction, we would require our base to react with an acid; the only
• answer choice which is a stronger acid than water is acetic acid, so
• choice D is correct and A is not. It should be noted that a
• neutralization reaction does not necessitate that the final pH will be
• neutral (although it will be closer to neutral than the starting
• solutions). Addition of NaHCO3 would only exacerbate the issue as it is a base, and addition of glucose would have no effect (choices B and C are incorrect).
6. A medical student inadvertently dumps a sample of K2C2O4
on his leg while in lab and frantically searches for a solution to
neutralize the burn. Which of the following would best help neutralize
the student’s burn?

(A) H2O
(B) NaHCO3
(C) C6H12O6
(D)HCH3CO2
HCH3CO2

The question asks for a solution to neutralize the burn. To determine whether this unknown salt is a acid or base, we inspect each ion independently. Potassium is a group I metal and is neutral (does not react with water) while the anion (oxalate) is the conjugate base of a weak acid and is therefore a base. While selecting water may seem like a good idea, in order to have a neutralization reaction, we would require our base to react with an acid; the only answer choice which is a stronger acid than water is acetic acid, so choice D is correct and A is not. It should be noted that a neutralization reaction does not necessitate that the final pH will be neutral (although it will be closer to neutral than the starting solutions). Addition of NaHCO3 would only exacerbate the issue as it is a base, and addition of glucose would have no effect (choices B and C are incorrect).
7. Which of the following is a possible set of of quantum numbers?

 n = 1, l = 0, ml = 0, ms = +1/3 n = 1, l = 1, ml = 0, ms = –1/2 n = 2, l = 1, ml = 0, ms = +1/2 n = 2, l = 0, ml = –1, ms = +1/2
n = 2, l = 1, ml = 0, ms = +1/2

The ms number can only be +1/2 or –1/2 (eliminate choice A). The l quantum number has allowed integer values from 0 to (n – 1), eliminating choice B. The ml quantum number is an integer from –l to l (eliminate choice D). Thus choice C must be the correct answer.
8. Non-volatile solutes depress the vapor pressure of a solution compared to the pure solvent because the solute particles:
a. shift the liquid ⇄ gas equilibrium to the products by increasing the number of particles in the liquid phase.
b. have stronger intermolecular attractions for solvent molecules than
solvent molecules have for each other, slowing solvent evaporation.
c. decrease the disorder of a solution compared to the pure solvent, lowering the entropic driving force for evaporation.
d. act as a physical barrier for solvent molecules and prevent them from evaporating from the liquid surface.
• d.
• Choice A is a false statement. The solute is not a part of the
• equilibrium indicated since the non-volatile solute particles are not
• able to evaporate themselves. While if choice B were true it would
• justify the lowering of vapor pressure, without knowledge of the
• identity of the solute and solvent there is no way to know which
• particles would have stronger attractions for each other. Therefore,
• this is not a valid justification for vapor pressure depression if it is
• not always a true statement. Entropy increases when a solute and
• solvent mix, so choice C is also false. Since vapor pressure is defined
• as the partial pressure of a gas that has evaporated from the surface
• of a liquid, if the solute particles prevent the solvent from accessing
• the surface, less solvent will evaporate, lowering the vapor pressure.
• This makes choice D the best answer.
9. A radioactive nucleus decays by emitting a positron particle. What is true about the resulting daughter nucleus?
a Its mass number increases
b Its mass number decreases.
c Its atomic number increases
d Its atomic number decreases.
• d
• Beta particles, both positive and negative, cause no change in mass
• number (eliminate choices A and B). However, positive beta particles
• (positrons) essentially convert one of the protons into a neutron, thus
• decreasing the atomic number during the process and making choice D
• correct.
10. Which of the following would have the biggest decrease in entropy for a gas in a piston cylinder?
(A)Compression
(B)Doing work on the gas
(C)Cooling
(D)Deposition
• D. An increase in entropy means an
• increase in randomness, or a decrease in order. Therefore, this
• question is asking for the change that would make the system most
• ordered. Compression and doing work on the gas are synonymous, so
• choices A and B can be eliminated. Cooling the piston cylinder would
• slow the movements of the gas, causing the volume to decrease and
• bringing molecules closer together. This definitely decreases entropy
• and could perhaps even increase intermolecular forces to the point that
• the gas might condense, decreasing entropy even more. Deposition is the
• process of going from a vapor to a solid, so since this phase change
• from the least ordered to most ordered phase is made explicit, choice D
• is a better answer than choice C.
11. A chemist seals a rigid reaction vessel containing a gaseous reaction at
equilibrium and begins pumping in helium gas. She fails to see a
change in the relative quantity of reactants and products in the system
of study. Which of the following best explains these observations?
a.Total pressure has increased with no change in the reaction quotient.
b.Total pressure has increased with compensatory changes in Q and Keq.
c.Total pressure has decreased with no change in the reaction quotient.
d.Total pressure has decreased with compensatory changes in Q and Keq.
• A. Sealing a rigid container and
• adding a gas to the vessel will increase total pressure (choices C and D
• are incorrect). Addition of an inert gas also results in a
• corresponding decrease in the mole fraction for each gas in the mixture,
• resulting in no change in partial pressures and therefore no change in Q (choice A is correct and choice B is incorrect). Note that changing reactant or product concentrations cannot change Keq, only a change in temperature changes the value of K.
12. Which of the following is true about the amount of shielding the highest energy electrons of calcium and arsenic experience?
a.The electrons of calcium have a greater amount of shielding than the electrons of arsenic.
b.The electrons of calcium have a lesser amount of shielding than the electrons of arsenic.
c.The electrons of calcium have the same amount of shielding as the electrons of arsenic.
d.The amount of shielding between the electrons of calcium and of arsenic can only be determined experimentally.
• B. The amount of shielding that the
• highest energy electrons of an atom feels is determined by the number of
• filled shells in that atom (inner core electrons), as well as subshells
• within an energy level (eliminate choice D). Because calcium and
• arsenic are in the same period (row), they have the same number of inner
• core electrons. However, the highest energy electrons in arsenic are
• in the p subshell and are shielded by the electrons in the s subshell. Since the highest energy electrons in calcium are in the s subshell, there is more shielding for arsenic, eliminating choices A and C.
13. An intern in a laboratory is asked to clean up after a new graduate
student in the lab who was making a HEPES buffered solution. The intern
finds a white solid powder left on the counter from the buffer
preparation and is uncertain how to dispose of it. Which of the
following is the most likely identity of this white powder?
a.Na
b.H2SO4
c.C3H6O
d.NaC3H5O3
• NaC3H5O3
• The intern was disposing of a white powder (solid), thus we are looking for the compound that will have the strongest intermolecular forces (and therefore is most likely to be solid). H2SO4 (sulfuric acid) is capable of hydrogen bonding, C3H6O (acetone) is capable of dipole-dipole interaction, and NaC3H5O3 (sodium lactate) is an ionic compound. Ionic interactions are stronger than any other form of IMF, making NaC3H5O3 the answer (choices B and C are incorrect, D is the correct answer). While sodium is a metal, pure sodium would not appear as a white powder, nor would it be added to any traditional solution given its volatile reactivity when added to water (choice A is incorrect).
14. In the standard formation reaction of carbon dioxide, graphite acts as a(n):
a.oxidizing agent.
b.reducing agent.
c.catalyst.
d.intermediate.
• B. Carbon (graphite) begins as a solid
• with an oxidation state of 0 and ends as carbon dioxide with an
• oxidation state of +4. This reaction is therefore an oxidation of
• carbon, which indicates that it was functioning as a reducing agent
• (choice B is correct, choice A is not). Graphite is a starting reagent
• (so choice D can be eliminated) and is consumed in the process which
• means it cannot be a catalyst (eliminate choice C).
15. Which of the following is true when heat is added to a system?
a.Temperature always increases
b.Temperature always decreases
c.Temperature may increase, or it may remain unchanged
d.Temperature may increase, or it may decrease
C. If heat is added to a system it makes the molecules of that system move faster (increases the average kinetic energy), then the temperature will increase (eliminate choice B). However, there are instances when adding heat to a system will not increase the temperature of the system. One such situation is during a phase change, when the added energy is used to break the intermolecular forces that hold molecules together. In this case, the temperature remains unchanged. Similarly, in an isothermal system, adding heat to a system (heat > 0) is counterbalanced by the system doing work against its environment (w < 0), so temperature remains the same because the internal energy of the system does not change. This eliminates choices A and D (adding heat can never make temperature decrease), and makes choice C the correct answer.
16. Which of the following is the most likely identity of a gas that effuses four times slower than hydrogen?

N2 O2 Cl2 Ar
• O2
• Graham’s law can be used when the relative rates of effusion of two gases are known to find the molar mass of an unknown gas: • • The unknown gas has a molar mass of 32 g/mol, which matches the molar mass of O2, making choice B the best answer.
17. Which of the following metal ions will have the strongest ionic force with a chloride ion?
a.Magnesium b.Potassium c.Calcium d.Sodium
A. Coulomb’s Law states that the force between two charged particles can be explained by the equationF = (k*q1*q2)/r2, where k is a constant, q1 and q2are the magnitude of the charges, and r is the distance between the charges. Since Ca and Mg have larger charges (+2) compared to Na and K (+1), they will have a greater attractive force with chloride, so eliminate choices B and D. As the distance r between the charges increases (with increasing ionic radius), the attractive force between the charges decreases. Magnesium is a smaller ion than calcium since it is higher on the periodic table, so choice C can be eliminated.
18. What percentage of HF (pKa = 3.17) dissociates in one liter of solution containing two moles of the acid?
• 1.8%
• Here we are asked to find the percentage of a weak acid (HF) which has dissociated. To do this, we can use an ICE table.

•  HF+ H2O H3O++ F– Initial 2 M Change – x + x + x Equilibrium 2 – x x x
•   19. Which of the following changes to an equilibrated system is most likely to change the proportion of reactants and products at equilibrium? (A)Increasing activation energy (B)Increasing product stability(C)Increasing reactant concentration (D)Increasing reaction rate
B. Increasing product stability will increase the relative proportion of products at equilibrium (choice B is correct). Changes in reaction rate (and other kinetic changes) are not likely to impact equilibrium (or other thermodynamic properties – choices A and D are wrong). Increasing reactant concentration will temporarily decrease the reaction quotient, Q, but once equilibrium has been re-established, the same proportions of reactants and products will exist (choice C is wrong).
20. A cook picked up a pan on the stove and, realizing it was too hot, quickly threw it into a nearby pot on the counter filled with water (at 99.9°C). After treating his hand for the burn, he found approximately one liter of water had vanished. Assuming no heat exchange with the surroundings, approximately how much energy did the pan lose in the vaporization of the water? [Note: ΔHvap (water) = 40.6 kJ/mol]
• 2200 kJ
• This question involves the calculation of the energy required for the vaporization of water. One liter of water, weighing one kilogram, contains 55.5 moles (1000 g / 18 g/mole = 55.5 moles). Thus here we would expect 40.6 kJ/mol * 55.5 moles to result in approximately 2200 kJ of energy required to vaporize one liter of water (choice C is correct). Note that if the water had been at a temperature below boiling, more energy would have been required to heat the solution before vaporization could have occurred.
21. Which of the following can be deduced from the following reaction?
a.The reaction is nonspontaneous.
b.At equilibrium, the products are favored.
c.The products are less stable than the reactants
d.The oxidation half-reaction is more spontaneous than the reduction half-reaction.

 2 Fe2+ + H2O2 + 2 H+ → 2 Fe3+ + 2 H2O E°= 1.0 V
B. Reaction potentials provide much of the same information as ΔG andKeq. As this reaction has a positive potential, it is spontaneous and the products must therefore be more stable than the reactants and favored at equilibrium (choice B is correct while choices A and C are eliminated). The overall reaction cannot directly tell us which of the two half-reactions is more spontaneous (choice D is incorrect).
22. An expanding gas in an adiabatic piston cylinder system:
a cools because internal energy is decreasing.
b cools because heat is decreasing.
c warms because internal energy is increasing.
d warms because heat is increasing.
A. Adiabatic means that the boundary is insulated, and no heat transfer can occur. Thus, the heat term in the first law goes to zero, and choices B and D can be eliminated. As the gas expands, it does work on the surroundings, which results in a negative work term. This means that internal energy has to decrease, causing the gas to cool (choice A is correct).
23. A sample of nitrogen gas is cooled from 80°C to 20°C in a rigid-walled container. What impact would this have on pressure? • Decrease to 0.8 times the original pressure
•  Pressure and temperature increase and decrease proportionally, thus a decrease in temperature results in a decrease in pressure (choices A and B are incorrect). As volume, the number of moles of gas, and the ideal gas constant remain unchanged in this process, we can rearrange the ideal gas law to P1/T1 = P2/T2, orP2/P1 = T2/T1. Remembering to convert temperature to Kelvin when using the gas law equations, we can solve for P2/P1 thus: 293 K / 353K ~ 0.8. The pressure therefore also decreases to roughly 0.8 times the original pressure, making choice C correct.
24. What order is the following reaction?
CO(g) + NO3(g) → NO2(g) + CO2(g)
• 2nd order
• The rate law for this elementary step is rate = k[CO][NO3] which, due to each reactant being first order, is a second order process overall (choice C is correct).
25. Which of the following statements about solids is true?
a.The intramolecular bonds of a molecular solid are the same as its intermolecular forces.
b.Ionic solids have a lower melting point than molecular solids.
c.Network solids have greater electrical conductivity than metallic solids.
d.Unlike network solids, metallic solids are ductile.
D. Metallic solids are unique in that their covalently-bound lattice of nuclei and inner shell electrons are surrounded by a “sea” of valence electrons. Thus, they are excellent conductors of electricity and heat (eliminate choice C). They are also malleable and ductile. The intramolecular bonds of an ionic solid and of a network solid are the same as their respective intermolecular forces, but molecular solids have covalent intramolecular forces and van der Waals intermolecular forces (hydrogen bonds, dipole-dipole forces, or London dispersion forces), making choice A incorrect. Ionic solids have stronger intermolecular forces than molecular solids, thus they have a higher melting point (eliminate choice B).
26. Removal of a solid bromine sample from a –80°C freezer results in a phase transition as it warms to room temperature. Which of the following terms best explains this process?
Condensation
Fusion
Vaporization
Deposition
• Fusion
•  Bromine is removed from the freezer as a solid and its temperature will increase until it reaches its melting point. Once there, it melts, a process also known as fusion (choice B is correct), and once completely melted it will continue to increase in temperature until room temperature is reached. Note that bromine is a liquid at room temperature, but this was not necessary information to answer this question as the starting material is a solid, and the only phase transition listed here that begins with a solid is fusion.
27. A chemist attempts to determine the rate law of a poorly characterized reaction. Propose the best rate law consistent with the following data. • rate = k [H2O2] [I
•  In the isolation method of determining reaction rate, two trials should be identified where the initial concentration of only one reactant changes at a time so the order of that reactant can then be determined based upon the change observed in the initial reaction rate. Trials 1 and 4 can be compared to determine the order of hydrogen peroxide: given rate = [x]y, 2 = 2y and y = 1. In other words, when the concentration of peroxide increases by a factor of two, the reaction rate increases by the same factor. Therefore the reaction is first order with respect to peroxide. (Incidently, trials 2 and 3 can be used to find the order of peroxide as well.) Similarly, trials 1 and 2 can be compared to determine the order of iodide: given rate = [x]y, 2 = 2y and y = 1. Thus the rate law is rate = k [H2O2] [I–] (choice C is correct).
28. The rule of thumb for solubility is “like dissolves like,” meaning polar solutes are soluble in polar solvents, and likewise for nonpolar things. Since water is a polar molecule that has hydrogen bonds as intermolecular forces, other hydrogen-bonding compounds like NH3 and CH3OH should be very soluble in water (choices A and C are wrong). Since LiF is an ionic compound that dissociates into charged particles in solution, it will need a polar solvent to dissolve (choice D is wrong). CS2, however, is a nonpolar compound and is the least soluble in water, making it the best choice (choice B is correct).
29. Which of the following terms describes a vertical line on a P-V diagram?
Isobaric
Isochoric
Isothermal
• Isochoric
• A P-V diagram plots variations in pressure and volume of a system while keeping all other variables constant. A vertical line would indicate one of these variables changes while the other remains constant. Isothermal means constant temperature (eliminate choice C), while adiabatic means no heat exchange between system and surroundings (eliminate choice D). You should expect volume to be on the horizontal axis of the graph, indicating a constant volume, or isochoric system (choice B). Isobaric means constant pressure (eliminate choice A).
30. Given the reaction below, which of the following is the anode?
3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
a. Cu(s)
b. Graphite
c. NO3(aq)
d. H+(aq)
A. The anode is the site of oxidation (an ox) and in the above reaction, copper is oxidized (loses an electron) from Cu(s) to Cu+(aq) making Cu(s) the anode (choice A is correct). While nitrate is the species being reduced to nitrogen monoxide, this would be taking place at the cathode (eliminate choice C). Hydrogen is neither oxidized nor reduced in the reaction (eliminate choice D) and graphite is not mentioned in the stem as being part of the cell, although it could be functioning as a cathode (eliminate choice B).
31. Pure acetic acid freezes at a temperature of 16.6°C. If the kf for acetic acid is 3.90 °C/m, what is the freezing point of a solution made from 8.0 g CH3OH dissolved in 500 g acetic acid?
14.6°C

The freezing point of a solution is depressed compared to the freezing point of the pure solvent. Therefore, choices C and D can be eliminated. The amount the freezing point is depressed by can be calculated by the following expression: ΔT = kf i m. Since methanol is a non-electrolyte, its van’t Hoff factor is 1. To find the molality of the solution: • Therefore, • Since the freezing point should be depressed by about 2°C, the best answer is choice A.
32. What is the hybridation of the central atom in XeOF4?
Xenon is the central atom in this compound since it is the only atom allowed to have an expanded octet (it's also listed first in the formula). The Lewis dot structure of XeOF4 should look like this:Since Xe has six electron groups around it (five bonds and one lone electron pair), it needs six hybrid orbitals. Therefore, one s, three p, and two datomic orbitals combine to give six sp3d2 hybrid orbitals.
33. What is the final phase of a 36 g water vapor sample at 400°C after removal of 15 kJ of energy? [Note: ΔHfus and ΔHvap = 6.0 kJ/mol and 40.6 kJ/mol, respectively; c of solid, liquid, and gaseous water = 38.0 J/molˑK, 74.5 J/molˑK, and 36.0 J/molˑK, respectively]
Aq.
Solid
Liquid
Gas
• Gas
• As we are uncertain of the end phase of the water, let us first calculate the amount of energy required to cool the sample to its boiling point. Givenq = mcΔT, we have (2 mol)(36.0 J/molˑK)(300 K) = 21,600 J, or 21.6 kJ, which is far more energy than the amount we removed. Thus, no phase change takes place (choice D is correct). Choice A could have been eliminated before doing any calculations because aqueous is not a phase, but describes a solution in which a solute is surrounded by water.
34. What is the mole fraction of NaOH in a solution made from 80 g NaOH in 180 g H2O?

• 0.17
• Mole fraction is defined as the number of moles of a given component of a mixture divided by the total number of moles of all components in the mixture. Since the molar mass of NaOH is 40 g/mol, 80 g NaOH is two moles of solute. Similarly, since the molar mass of water is 18 g/mol, 180 g of water is 10 mol of solvent. Therefore, the mole fraction of NaOH can be calculated by 2 mol NaOH / (2 mol NaOH + 10 mol H2O), or 0.17.
35. Which of the following best describes the reduction half-reaction in the following reaction?
Cu(s)  +  2 NO3(aq)  +  4 H+(aq) →  Cu2+(aq) +  2 NO2(g)  +   2 H2O(l)

2 NO3(aq) + 2 e + 4 H+(aq)→ 2 NO2(g) +2 H2O(l)

This question is asking for a reduction half-reaction which therefore must involve only accepting electrons (choices A and B are wrong). The reaction described here involves the reduction of nitrogen in nitrate and the oxidation of copper metal to copper(II) cation. While answer choice C describes the oxidation of oxygen, this reaction is not a part of the overall reaction as oxygen gas is not involved (choice C is wrong). The reduction of nitrate to nitrogen dioxide gas describes the reduction half-reaction in the overall reaction (choice D is correct).
36. Twenty milliliters of a 8.4 x 10–3 M solution of HBr is added to sixty milliliters of water. What is the final pH of the solution?
• 2.7
• This problem requires us to perform a dilution before calculating the pH. Using C1VC2V2, we find our final concentration to be 2.1 x 10–3 M. Since we are dealing with a strong acid, the concentration of hydronium ion is the same as the molarity. To find the pH, we need only find –log [H+] and find our answer to be between a pH of 2 and 3 after appropriate rounding to 1 x 10–2 M and 1 x 10–3 M(choice C is correct).
37. A 0.20 M NaCl solution is separated from a 0.30 M glucose solution by a semi-permeable membrane. What change to the system should be observed?
• Water will flow into the salt solution because the osmotic pressure of the salt solution is greater.
• Osmotic pressure increases with increasing concentration, or total number of dissolved solute particles in the solution. While the glucose solution has a larger molarity, since it is a non-electrolyte, the NaCl solution (a strong electrolyte) with the smaller molarity actually has more dissolved particles in it. Therefore, the salt solution has the higher osmotic pressure (eliminate choices A and B). Water flows across a semi-permeable membrane from the less concentrated solution into the more concentrated solution. Therefore, water should flow into the salt solution, making D the best answer.
38. Which of the following represents the ground state electron configuration for silver?
a. [Kr] 5s2 5d9
b. [Kr] 5s2 4d9
c. [Kr] 5s1 4d10
d. [Kr] 5s1 3f14 4d10
[Kr] 5s1 4d10

The f subshell appears for the first time in the fourth energy level, so 3f orbitals do not exist (eliminate choice D). The d subshell in any given energy level is one principle quantum number behind the s subshell. Therefore, 5s is followed by 4d, not 5d (eliminate choice A). Since completely filled d subshells are more stable for transition metals, we observe that one s electron is promoted into the d subshell in the copper family, of which silver is a member. This means choice C is better than choice B.
39. If weights are added to a well-insulated piston cylinder, what are the signs of heat and work for this process?
• Heat is zero, work is positive
• The keywords “well-insulated” indicate that the system is adiabatic, and does not transfer heat (eliminate choices A and B). Since weights are being added, the surroundings are doing work on the system, so the system is increasing in energy meaning that work is positive. Thus choice C is correct.
40. Which of the following is the most stable resonance structure for NO2–?     The most stable resonance structures occur when charge separation is minimized (eliminate choices C and D). The most stable resonance structures of ions occur when the negative charge is found on the more electronegative atom(s) and the positive charge is found on the more electropositive atom(s). Oxygen is more electronegative than nitrogen, thus oxygen will preferentially have the negative charge (eliminate choice A). Nitrogen is also not able to have an expanded octet, so choice A can be eliminated for this reason as well.
41. When aqueous solutions of AgNO3 and Ba(OH)2 are combined, one should expect to observe:
1.
no visible change since all ion combinations create soluble compounds.
2.
the formation of a precipitate since AgOH is insoluble.
3.
the formation of a precipitate since Ba(NO3)2 is insoluble.
• 2 only.
• The reaction suggested is the double displacement:

AgNO3 (aq) + Ba(OH)2 (aq) → Ba(NO3)2 (aq) + AgOH(s)

Two common ionic solid solubility rules are that nitrate compounds are soluble, while most Pb, Hg, and Ag solids are insoluble. Because the AgOH forms a precipitate, Roman numeral I is false while Roman numeral II is true. Since all nitrates are soluble, Roman numeral III is also false, making B the best answer.
42. describe the orbital geometry and molecular shape of AsCl3, respectively
• Tetrahedral, trigonal pyramidal
• The orbital geometry of a molecule is determined by the total number of electron groups around the central atom (bonds and lone electron pairs), while the shape is determined by the number of atoms bonded to the central atom. AsCl3 has four electron groups surrounding the central arsenic, three single bonds to chlorine and one lone electron pair. Therefore its orbital geometry is tetrahedral (eliminate choices A and B). Since it only has three atoms attached, however, its shape is trigonal pyramidal. A molecule can only have a trigonal planar shape if its orbital geometry is also planar (eliminate choice C).
43. Acetylsalicylic acid, also known as aspirin, possesses a pKa of 3.5. What is the pH of a 1M solution of acetylsalicylic acid?
• 1.8
• Here we are asked to find the pH of a solution containing a weak acid (HA). To do this, we can use an ICE table to find the solution.

•  HA+ H2O H3O++ A– Initial 1 M Change – x + x +x Equilibrium 1 –x x x  •    • Thus choice A correct.
44. A researcher investigates an endothermic reaction found to be non-spontaneous in a temperature-independent manner. Which of the following is the reaction being studied?
2 H2O(g)  →  2 H2(g) + O2(g)
CO2(s) → CO2(l)
N2(g) + 2 O2(g) → 2 NO2(g)
N2O4(g) → 2 NO2(g)
• N2(g) + 2 O2(g) → 2 NO2(g)
• The reaction described is non-spontaneous in a temperature-independent manner. As it is endothermic, this indicates that the reaction must have a negative ΔS given ΔG = ΔH – TΔS. The only reaction displaying a negative change in entropy is N2(g) + 2 O2(g) → 2 NO2(g) because the total number of gaseous moles decreases (choice C is correct, and choices A and D are not). In addition, choice B can be eliminated since as a substance changes phase from solid to liquid to gas, the particles are more disordered, so entropy increases.
45. Rank the following species by increasing boiling point.

• CH3CHO

• F2

• CH3OH

• KBr

• II < I < III < IV
• Coulomb’s Law states that the force between two charged particles can be explained by the equationF = (kq1q2)/r2, where k is a constant, q1 and q2 are the magnitude of the charges, and r is the distance between the charges. When comparing molecules, the weaker intermolecular forces occur between molecules with a lesser magnitude of charge. The charge (or partial charge) is due to polarity, which occurs due to differences in electronegativity. KBr will have ionic forces, the strongest option, because of the full charges on the K+ and Br– ions (choice B is correct). F2 is a nonpolar molecule and will form instantaneous-induced dipoles (also known as London dispersion forces), the weakest of all intermolecular forces (choices A and C are wrong). CH3OH and CH3CHO are both polar molecules, but methanol is able to hydrogen-bond due to the –OH group, while weaker dipole-dipole forces hold together the CH3CHO.
46. What is the relationship between the mass of an atom and the sum of the masses of its constituent particles?
The mass of the atom is greater.
The sum of the masses of the constituent particles is greater.
Both masses will be equal.
It depends on the number of particles comprising the atom.
• The sum of the masses of the constituent particles is greater.
• From a stability perspective, the bound atom has to have a lower energy compared to the constituent particles (or else nature would prefer free protons and neutrons moving in space). From Einstein's equation, E = mc2, this difference in energy translates into a mass defect in the bound atom (albeit a very small difference). In other words, the mass of the atom is smaller than the sum of the masses of its constituent atoms because the energy of the bound atom is smaller than the individual particles. Thus, choice B is correct.
47. • KNO3, because it has a lower solubility at low temperatures.
• A solid needs to be soluble at high temperatures and most importantly insoluble at low temperatures in order to recover it as a precipitate via recrystallization. Choice A should be eliminated since a high solubility at low temperatures means the solid will stay in solution. Choices B and C have no bearing on recovering the solute.
 Author: ccraigr ID: 201931 Card Set: MCAT GENCHEM Updated: 2013-02-21 03:07:04 Tags: 105 106 Problem Based Review Folders: Description: MCAT CHEM problem based review Show Answers: