Kaplan MCAT General Chemistry Review

7. Which of the following situations would most favor the change of water from a liquid to a solid?

I. Decreased solute concentration of a substance

II. Decreased temperature of a substance

III. Decreased pressure on a substance

A. I only

B. II only

C. I and II only

D. II and III only

8. The heats of vaporization of four substances are given in the table below. Which of these substances has the lowest boiling point?

Comparative Heats of Vaporization

Liquid Heat Required (cal/g) Chlorine67.4Ether9.4Carbon dioxide72.2Ammonia295

A. Chlorine

B. Ether

C. Carbon dioxide

D. Ammonia

9. What factor(s) determine whether or not two liquids are miscible?

A. Molecular size

B. Molecular polarity

C. Density

D. Both B and C

10. Alloys are mixtures of new metals in either the liquid or solid phase. Which of the following is usually true of alloys?

A. The melting/freezing point of an alloy will be lower than that of either of the component metals because the new bonds are stronger.

B. The melting/freezing point of an alloy will be lower than that of either of the component metals because the new bonds are weaker.

C. The melting/freezing point of an alloy will be greater than that of either of the component metals because the new bonds are weaker.

D. The melting/freezing point of an alloy will be greater than that of either of the component metals because the new bonds are stronger.

11. The osmotic pressure at STP of a solution made from 1 L of NaCl (aq) containing 117 g of NaCl is

A. 44.77 atm.

B. 48.87 atm.

C. 89.54 atm.

D. 117 atm.

Small Group Questions

1. On water’s pressure-temperature phase diagram, the boundary between liquid and solid has a negative slope. Explain this phenomenon, focusing on how density affects pressure and temperature.

2. Which would have a greater effect on the boiling point of one liter of water: 2 moles of sodium nitrate or 1 mole of carbonic acid? What pieces of information do we need to calculate this answer?

Explanations to Practice Questions

1. D

For the scope of the MCAT, water is the only substance that has a solid/liquid equilibrium line with a negative slope. We are not expected to know specific phase diagrams for other compounds.

2. A

Intermolecular forces hold molecules closer to one another, which relates to the compound’s phase. As there are more intermolecular forces, the amount of heat needed to change phase (such as in vaporization) increases as well, thus showing a proportional, positive relationship.

3. B

This molecule is likely to have unequal sharing of electrons, which means it will be polar. Polarity often leads to increased bonding because of the positive-negative attractive forces between molecules. Greater bonding or stronger intermolecular forces both lead to higher melting points, because more heat is needed to break these interactions and cause liquefaction. (A) is nonpolar, making its intermolecular forces weaker than (B) or (C). (D) is only weakly polar, because the geometry of this molecule will make some of the polarized bonds cancel each other out. (C) will have some polarity but less than (B), because only one bond is polarized.

4. D

We can imagine a high-pressure situation as one in which molecules are in close proximity (low-volume container) and forced to interact with one another. Intermolecular attractions are necessary in the solid and liquid phases but assumed to be negligible in the gas phase. So we can assume that high pressure is not conducive to the gaseous state. What about temperature? Because temperature is a measure of average kinetic energy of the molecules, an increase in the average kinetic energy increases molecules’ ability to move apart from one other, ultimately entering a gaseous state. Decreasing the temperature (or kinetic energy) or increasing pressure both favor the more organized liquid and solid phases.

5. C

Melting point depresses upon solute addition, making (A) and (B) incorrect. Solute particles interfere with lattice formation, the highly organized state in which solid molecules align themselves. Colder than normal conditions are necessary to create the solid structure.

6. C

This answer choice shows movement from the solid to gas phase, which is, by definition, sublimation.

7. C

Both I and II are correct. Dissolved solutes interfere with the crystalline lattice bonds of a solid and, therefore, favor the liquid side of the equilibrium. Conversely, a decreased amount of solute would favor a solid state (I). Additionally, decreased temperature (II) is a decrease in the average kinetic energy of the molecules, making lower-energy phases (i.e. solid) more likely to exist. Finally, decreasing pressure (III) would actually favor higher-energy phases (i.e., liquid or gas) over solids. The only answer choice that includes both I and II is (C).

8. B