Kaplan MCAT General Chemistry Review

There are three definitions of acids and bases: Arrhenius—acids produce hydrogen ions, and bases produce hydroxide ions in aqueous solutions; Brønsted-Lowry—acids donate hydrogen ions, and bases accept hydrogen ions; and Lewis—acids accept electron pairs, and bases donate electron pairs.

Water is an amphoteric species (both acid and base) and auto-ionizes to produce equilibrium concentrations of hydrogen ions and hydroxide ions equal to 10^-7 M at 298 K. The K_w for water at 298 K is 10^-14. All aqueous acid or base solutions are defined by the equilibrium constant for water.

pH and pOH scales are logarithmic and express the negative log of the molar concentration of the hydrogen ions or hydroxide ions, respectively. For aqueous solutions at 298 K, a pH less than 7 is acidic, and a pH greater than 7 is basic; a pH 7 is neutral.

Strong acids and bases dissociate completely in aqueous solutions. Examples of strong acids include HCl, H₂SO₄, and HNO₃. Examples of strong bases include NaOH and KOH.

Weak acids and bases dissociate incompletely in aqueous solutions. They have K_a’s or K_b’s less than 1. Examples of weak acids include CH₃COOH, H₂CO₃, and H₂O. Examples of weak bases include NH₃ and H₂O.

Brønsted-Lowry acid-base reactions always involve chemical pairs called conjugates: Strong acid produces weak conjugate base; strong base produces weak conjugate acid; a weaker acid produces a stronger conjugate base; a weaker base produces a stronger conjugate acid.

An equivalent is equal to one mole of charge (hydrogen ions or hydroxide ions). Equivalent weight is the mass in grams of an acid or base compound that yields one acid or base equivalent (i.e., one mole of charge). Normality is the number of acid or base equivalents per liter of solution.

Acid-base titration is used to determine the molar concentration of a known acid or base solution. The equivalence point is the point in the titration at which the equivalents of acid equal the equivalents of base.

Strong acid/strong base titration has an equivalence point at pH 7. Strong acid/weak base titration has an equivalence point at pH less than 7. Strong base/weak acid titration has an equivalence point at pH greater than 7. Indicators approximate the equivalence point by a steady color change at end point.

Buffers are weak acid/conjugate base or weak base/conjugate acid systems that act to absorb strong acids or bases from solutions, thereby minimizing pH changes within the buffered region. Buffers are most effective within ±1 of pK_a or pK_b.

EQUATIONS TO REMEMBER

Practice Questions

1. Which of the following is not a Brønsted-Lowry base?

2. What is the pH of a solution containing 5 mM H₂SO₄?

A. 1

B. 1.5

C. 2

D. 4

3. Which of the following is chloric acid?

A. HClO₃

B. ClO₃^-

C. HClO₂

D. HClO

4. Which of the following bases is the weakest?

A. KOH

B. NH₃

C. CH₃NH₂

D. Ca(OH)₂

5. The function of a buffer is to

A. speed up reactions between acids and bases.

B. resist changes in pH when small amounts of acid or base are added.

C. slow down reactions between acids and bases.

D. maintain a neutral pH.

6. What is the pH of the following solution shown below?

pK_b = 3.45[NH₄⁺] = 70 mM[NH₃] = 712 mMA. 4.45B. 7.55C. 11.56D. 10.65

Answer questions 7-9 based on the titration curve of acid X shown below:

7. What is the approximate value of the first pK_a?

A. 1.9

B. 2.9

C. 3.8

D. 4.1

8. Where is the second equivalence point?

A. pH = 3.0

B. pH = 4.1

C. pH = 5.9

D. pH = 7.2

9. What is the approximate value of the second pK_a?

A. 3.6

B. 4.1

C. 5.5

D. 7.2

10. What is the approximate gram equivalent weight of phosphoric acid?

A. 25 g

B. 33 g

C. 49 g

D. 98 g

11. What is the [H⁺] of a 2 M aqueous solution of a weak acid “HXO₂” with K_a = 3.2 × 10^-5?

A. 8 .0 × 10^-3 M

B. 6.4 × 10^-5 M

C. 1.3 × 10^-4 M

D. 4 .0 × 10^-3 M

12. A solution is prepared with an unknown concentration of a theoretical compound whose K_a is exactly 1.0. What is the pH of this solution?

A. Higher than 7

B. Exactly 7

C. Lower than 7

D. Impossible to determine

13. Which of the following is NOT a characteristic of an amphoteric species?

A. Can act as a base or an acid, depending on its environment.

B. Can act as an oxidizing or reducing agent, depending on its environment.

C. Is always protic.

D. Is always a nonpolar species.

14. What is the approximate pH of a 1.2 × 10^-5 M aqueous solution of NaOH?

A. 4.85

B. 7.50

C. 9.15

D. 12.45

Small Group Questions

1. The Henderson-Hasselbach equation cannot be applied to all acids and bases. Explain why not.

2. Phosphoric acid is a polyprotic acid with K_a’s of 7.1 × 10^-3, 6.3 × 10^-8, and 4.5 × 10^-13. Estimate its pK_b values.

Explanations to Practice Questions

1. D

A Brønsted-Lowry base is defined as a proton acceptor. (A), (B), and (C) can each accept a proton. (D), HNO₂, cannot.

2. C