Kaplan MCAT General Chemistry Review

First, we’ll convert the concentration to 5 × 10^-3 M. Next, since sulfuric acid is a strong acid, we can assume that both protons will dissociate. So the concentration of hydrogen ions is really 2 × 5 × 10^-3, which simplifies to 1 × 10^-2. The equation for pH is pH = -log[H⁺]. If [H⁺] = 1 × 10^-2 M, then pH = 2.

3. A

Answering this question is simply a matter of knowing nomenclature. Acids ending in –ic are derivatives of anions ending in –ate, while acids ending in –ous are derivatives of anions ending in –ite. ClO₃^- is named chlorate because it has more oxygen than the other occuring ion, ClO₂^-, which is named chlorite. Therefore, HClO₃ is chloric acid.

4. B

Members of the IA and IIA columns on the periodic table combined with OH^- are always strong bases. This means (A) and (D) can be eliminated. (B) and (C) are both weak bases, but (B) is weaker than (C) because a positive charge would sit solely on ammonia’s nitrogen. Methylamine, on the other hand, has a neighboring carbon (remember, alkyl groups are electron donating), which will help reduce the positive charge’s stress on nitrogen.

5. B

The purpose of a buffer is to resist changes in the pH of a reaction. Buffers will not affect the kinetics of a reaction, so (A) and (C) are wrong. (D) is correct only in specific circumstances that require a pH of 7. Many natural buffer systems maintain pHs in the acidic or basic ranges.

6. C

The question is asking for pH, but because of the information given, we must first find the pOH and then subtract it from 14 to get the pH. The equation for pOH is:

pOH = pK_b + log. When the given values are substituted into this equation ([conjugate acid] = 70mM; [weak base] = 712mM), we find that pOH = 2.44, so the pH = 14 - 2.44 = 11.56

7. A

The first pK_a in this curve can be estimated by eye. It is located between the starting point (when no base had been added yet) and the first equivalence point. This point is approximately at 7-8 mL added, which corresponds to a pH of approximately 1.9. Notice that this region experiences very little change in pH. This is the defining characteristic of a buffer zone!

8. C

The second equivalence point is the midpoint of the second quick increase in slope. This corresponds approximately to pH = 5.9.

9. B

The value of the second pK_a is found at the midpoint between the first and second equivalence points. In this curve, that corresponds to pH = 4.1. Just like the first pKa, it is in the center of a flat buffering region.

10. B

Gram equivalent weight is the weight (in grams) that would release 1 acid equivalent. Because H₃PO₄ contains 3 acid equivalents, we find the gram equivalent weight by dividing the mass of one mole of the species by 3. Therefore, (B) is the correct answer.

11. A

This question requires the application of the acid dissociation formula:

Weak acids do not dissociate completely; therefore, all three species that appear in the formula will be present in solution. Hydrogen ions and conjugate base anions dissociate in equal amounts, so [H⁺] = [X^-]. We don’t know exactly how much they’ll dissociate though, so we’ll just stick in an x for both of those species. How much HX do we have? The original concentration minus x.

Each of these x terms is quite small. The numerator becomes more significant because we’ll end up with x², but we can simply ignore the x in the denominator. Let’s also plug in our K_a value:

Now all we have to do is isolate x, which leads us to find that the [H⁺] is 8 .0 × 10^-3 M.

12. C

A higher K_a implies a stronger acid. Consider the following theoretical reaction, which defines the K_a of acid HA, HA H⁺ + A^-. In such a reaction, K_a = [H⁺][A^-] / [HA]. A K_a near 1 therefore implies that there are enough hydrogen ions present to affect the pH significantly. Weak acids usually have a K_a that is several orders of magnitude below 1. Yet a detailed understanding of K_a is not necessary to answer this question. According to the pH scale, which sets the K_a of water at 10^-7, a compound with a K_a above 10^-7 is acidic; even if the acid is very weak, it will still cause the pH to drop below 7.

13. D

An amphoteric species is one that can act either as an acid or a base, depending on its environment. Proton transfers are classic redox reactions, so (A) and (B) are true. (C) must be true because, by definition, an amphoteric molecule needs to have a proton to give up in order to act like an acid. (D) is false, and thus the correct answer, because amphoteric species can be either polar or nonpolar in nature. Some examples: HSO₄^-, NH₃, H₂O.

14. C

NaOH Na⁺ + OH^-

The balanced equation shows the same coefficients in front of each of the three species. So if the initial concentration of NaOH is 1.2 × 10^-5, then the concentration of Na⁺ and OH^- must be also, because NaOH will completely dissociate.