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?

(Y or N)

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. Image 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:

Image