Kaplan MCAT General Chemistry Review

Salt bridges contain inert electrolytes. Ionic compounds, such as (A), (C), and (D), are known to be strong electrolytes because they completely dissociate in solution. (B) cannot be considered an electrolyte because its atoms are covalently bound and will not dissociate in aqueous solution. (B) and (C) may appear similar, but there is an important distinction to be made. (C) implies that Mg²⁺ and SO₃^2- are the final, dissociated ionic constituents, while (B) implies that SO₃^2- might want to dissociate into smaller elements.

11. C

In NaClO (sodium hypochlorite), sodium carries its typical +1 charge, and oxygen carries its typical -2 charge. This means that the chlorine atom must carry a +1 charge in order to balance the overall -1 charge. Although this may seem atypical, it is not uncommon (NaClO, for instance, is the active ingredient in household bleach).

12. C

Current is defined in amperes, a unit that breaks down to coulombs per second. An increased current will mean that more electrons (coulombs) will be transported per second. (A) is incorrect because an increase in resistance will decrease the current, thereby producing the opposite of the desired effect. (B) is incorrect because the amount of electrolyte will only affect the amount of final product produced; it does not limit the rate. (D) is not always relevant because the pH only affects electrolytic reactions that involve acids and bases.

13. D

A strong oxidizing agent will be easily reduced, meaning that it will have a tendency to gain electrons. Atoms usually gain electrons if they are one or two electrons away from filling up their valence shell. (A) has a full 4s-orbital, meaning that it can only gain an electron if it gains an entire p subshell. (B) has a stable, half-full d-orbital, so it is unlikely to pick up electrons unless it can gain five. (C) has only a single electron in the outer shell, which can easily be lost upon ionization. (D) would fill up its p-orbital by gaining one electron, but the five electrons currently present would not be easily lost through ionization. (D) is the correct answer.

Chapter 12: High-Yield Problem Solving Guide for General Chemistry

High-Yield MCAT Review

This is a High-Yield Questions section. These questions tackle the most frequently tested topics found on the MCAT. For each type of problem, you will be provided with a stepwise technique for solving the question and key directional points on how to solve for the MCAT specifically.

For each topic, you will find a “Takeaways” box, which gives a concise summary of the problem-solving approach, and a “Things to Watch Out For” box, which points out any caveats to the approach discussed above that usually lead to wrong answer choices. Finally, there is a “Similar Questions” box at the end so you can test your ability to apply the stepwise technique to analogous questions.

We’re confident that this guide, and our award-wining Kaplan teachers, can help you achieve your goals of MCAT success and admission into medical school!

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Melting Points

Key Concepts

Chapter 3

Polarity

Molecular symmetry

Melting points

Arrange the following compounds in order of increasing melting point:

1) Separate the compounds by general polarity.

In this series, we can separate the compounds into three groups of two. The alkanes (3 and 4) will be the least polar and, therefore, will melt at the lowest temperature; the alkenes (1 and 5) will be in the middle, and the aromatic compounds (2 and 6) will melt at the highest temperature.

Takeaways

Forces that stabilize a molecule more in the solid state than in the liquid state will cause a molecule to have a higher melting point.

Things to Watch Out For

Be careful not to confuse melting points with boiling points. Remember that in general, symmetry raises melting points, whereas branching lowers them.

2) Examine each grouping for trends in polarity and/or molecular symmetry.

For the lowest-melting-point compounds, notice that cyclohexane has a higher degree of molecular symmetry than does n–hexane; this will cause it to melt at a significantly higher temperature.

With the alkenes, the trans alkene has more symmetry than the cis alkene, because the cis alkene has a rather large “kink” in the middle of the chain that prevents it from packing together as well in the crystal, thus lowering its melting point.

Finally, acetanilide (6) is significantly more polar than aniline (2), because the amide carbonyl bond is highly polar, causing these molecules to stick together better and consequently raising their melting point.

Therefore, the ordering of the compounds’ melting points is as follows:

3 < 4 < 5 < 1 < 2 < 6