MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
Acids and Bases
Answers to Concept Checks
· 10.1
1.
Definition |
Acid |
Base |
Arrhenius |
Dissociates to form excess H+ in solution |
Dissociates to form excess OH− in solution |
Brønsted—Lowry |
H+ donor |
H+ acceptor |
Lewis |
Electron pair acceptor |
Electron pair donor |
2.
Anion |
Acid Formula |
Acid Name |
MnO4− |
HMnO4 |
Permanganic acid |
Titanate (TiO32−) |
H2TiO3 |
Titanic acid |
I− |
HI |
Hydroiodic acid |
IO4− |
HIO4 |
Periodic acid |
3.
Reaction |
Amphoteric Reactant |
Amphiprotic? |
HCO3− + HBr → H2CO3 + Br− |
HCO3− |
Yes |
3 HCl + Al(OH)3 → AlCl3 + 3 H2O |
Al(OH)3 |
No |
2 HBr + ZnO → ZnBr2 + H2O |
ZnO |
No |
· 10.2
1. An amphoteric species can act as an acid or a base.
2. High Ka indicates a strong acid, which will dissociate completely in solution. Having a Ka slightly greater than water means the acid is a weak acid with minimal dissociation.
3. High Kb indicates a strong base, which will dissociate completely in solution. Having a Kb slightly greater than water means the base is a weak base with minimal dissociation.
4.
pH |
[H3O+] |
pOH |
[OH−] |
Acid or Base? |
4 |
10−4 M |
10 |
10−10 M |
Acid |
3.05 |
8.89 × 10−4 M |
10.95 |
1.12 × 10−11 M |
Acid |
8.81 |
1.55 × 10−9 M |
5.19 |
6.46 × 10−6 M |
Base |
8.27 |
5.32 × 10−9 M |
5.73 |
1.88 × 10−6 M |
Base |
5. (Note: Exact answers are provided; your rounded answers should be relatively close to those listed here.)
6. Ka × Kb = Kw
7.
Reaction |
Acid |
Base |
Conjugate Acid |
Conjugate Base |
H2CO3 + H2O ⇌ HCO3− + H3O+ |
H2CO3 |
H2O |
H3O+ |
HCO3− |
H2PO4− + H2O ⇌ H3PO4 + OH− |
H2O |
H2PO4− |
H3PO4 |
OH− |
8. Therefore, x2 = 3.6 × 10−6 → x ≈ 2 × 10−3 M (actual = 1.9 × 10−3 M). Then, pH = −log H3O+ ≈ 3 − 0.2 = 2.8 (actual = 2.72)
· 10.3
1. Acids use moles of H+ (H3O+) as an equivalent. Bases use moles of OH− as an equivalent.
2. 6 N Al(OH)3; 32 N H2SO4
· 10.4
1. The buffering region occurs when [HA] ≈ [A−] and is the flattest portion of the titration curve (resistant to changes in pH). The half-equivalence point is the center of the buffering region, where [HA] = [A−]. The equivalence point is the steepest point of the titration curve, and occurs when the equivalents of acid present equal the equivalents of base added (or vice-versa). The endpoint is the pH at which an indicator turns its final color.
2. Phenolphthalein would be the preferred indicator for this titration.
3. A strong acid and weak base have an equivalence point in the acidic range. A strong base and weak acid have an equivalence point in the basic range. A strong acid and strong base have an equivalence point at pH = 7 (neutral). A weak acid and weak base can have an equivalence point in the acidic, neutral, or basic range, depending on the relative strengths of the acid and base.
4. A buffer solution is designed to resist changes in pH and has optimal buffering capacity within 1 pH point from its pKa.
5. Recall from the example on page 356 that the concentrations of the conjugate acid and conjugate base in the final solution must first be calculated due to the dilution from mixing the two solutions together. The calculation below follows that step, but leaves the values unsolved for so that some of their components can be cancelled and simplified: