14.3 Relative Strengths of Acids and Bases

Explain why the neutralization reaction of a weak acid and a strong base gives a weakly basic solution.

The odor of vinegar is due to the presence of acetic acid, CH₃CO₂H, a weak acid. List, in order of descending concentration, all of the ionic and molecular species present in a 1-M aqueous solution of this acid.

Explain why the ionization constant, K_a, for H₂SO₄ is larger than the ionization constant for H₂SO₃.

Gastric juice, the digestive fluid produced in the stomach, contains hydrochloric acid, HCl. Milk of Magnesia, a suspension of solid Mg(OH)₂ in an aqueous medium, is sometimes used to neutralize excess stomach acid. Write a complete balanced equation for the neutralization reaction, and identify the conjugate acid-base pairs.

What is the ionization constant at 25 °C for the weak acid $\text{CH}_{3} \text{NH}_{3} ^{+} ,$ the conjugate acid of the weak base CH₃NH₂, K_b = 4.4 $\times$ 10⁻⁴.

Which base, CH₃NH₂ or (CH₃)₂NH, is the stronger base? Which conjugate acid, $\left(\text{CH}_{3}\right)_{2} \text{NH}_{2} ^{+}$ or $\text{CH}_{3} \text{NH}_{3} ^{+}$, is the stronger acid?

Which is the stronger base, (CH₃)₃N or $\text{H}_{2} \text{BO}_{3} ^{−} ?$

Predict which compound in each of the following pairs of compounds is more acidic and explain your reasoning for each.

(a) $\text{HSO}_{4} ^{−}$ or $\text{HSeO}_{4} ^{−}$

(b) NH₃ or H₂O

(c) PH₃ or HI

(d) NH₃ or PH₃

(e) H₂S or HBr

Rank the compounds in each of the following groups in order of increasing acidity or basicity, as indicated, and explain the order you assign.

(a) acidity: NaHSO₃, NaHSeO₃, NaHSO₄

(b) basicity: $\text{BrO}_{2} ^{−} ,$ $\text{ClO}_{2} ^{−} ,$ $\text{IO}_{2} ^{−}$

(c) acidity: HOCl, HOBr, HOI

(d) acidity: HOCl, HOClO, HOClO₂, HOClO₃

(e) basicity: $\text{NH}_{2} ^{−} ,$ HS⁻, HTe⁻, $\text{PH}_{2} ^{−}$

(f) basicity: BrO⁻, $\text{BrO}_{2} ^{−} ,$ $\text{BrO}_{3} ^{−} ,$ $\text{BrO}_{4} ^{−}$

The active ingredient formed by aspirin in the body is salicylic acid, C₆H₄OH(CO₂H). The carboxyl group (−CO₂H) acts as a weak acid. The phenol group (an OH group bonded to an aromatic ring) also acts as an acid but a much weaker acid. List, in order of descending concentration, all of the ionic and molecular species present in a 0.001-M aqueous solution of C₆H₄OH(CO₂H).

What two common assumptions can simplify calculation of equilibrium concentrations in a solution of a weak acid or base?

Which of the following will increase the percent of NH₃ that is converted to the ammonium ion in water?

(a) addition of NaOH

(b) addition of HCl

(c) addition of NH₄Cl

What is the effect on the concentrations of $\text{NO}_{2} ^{−} ,$ HNO₂, and OH⁻ when the following are added to a solution of KNO₂ in water:

(a) HCl

(b) HNO₂

(c) NaOH

(d) NaCl

(e) KNO₂

Why is the hydronium ion concentration in a solution that is 0.10 M in HCl and 0.10 M in HCOOH determined by the concentration of HCl?

From the equilibrium concentrations given, calculate K_a for each of the weak acids and K_b for each of the weak bases.

(a) NH₃: [OH⁻] = 3.1 $\times$ 10⁻³ M;
$\left[\text{NH}_{4} ^{+}\right]$ = 3.1 $\times$ 10⁻³ M;

[NH₃] = 0.533 M;

(b) HNO₂: $\left[\text{H}_{3} \text{O}^{+}\right]$ = 0.011 M;
$\left[\text{NO}_{2} ^{−}\right]$ = 0.0438 M;

[HNO₂] = 1.07 M;

(c) (CH₃)₃N: [(CH₃)₃N] = 0.25 M;
[(CH₃)₃NH⁺] = 4.3 $\times$ 10⁻³ M;

[OH⁻] = 3.7 $\times$ 10⁻³ M;

(d) $\text{NH}_{4} ^{+} :$ $\left[\text{NH}_{4} ^{+}\right]$ = 0.100 M;

[NH₃] = 7.5 $\times$ 10⁻⁶ M;
[H₃O⁺] = 7.5 $\times$ 10⁻⁶ M

Determine K_a for hydrogen sulfate ion, $\text{HSO}_{4} ^{−} .$ In a 0.10-M solution the acid is 29% ionized.

Calculate the ionization constant for each of the following acids or bases from the ionization constant of its conjugate base or conjugate acid:

(a) HTe⁻ (as a base)

(b) $\left(\text{CH}_{3}\right)_{3} \text{NH}^{+}$

(c) $\text{HAsO}_{4} ^{2–}$ (as a base)

(d) $\text{HO}_{2} ^{−}$ (as a base)

(e) $\text{C}_{6} \text{H}_{5} \text{NH}_{3} ^{+}$

(f) $\text{HSO}_{3} ^{−}$ (as a base)

Calculate the concentration of all solute species in each of the following solutions of acids or bases. Assume that the ionization of water can be neglected, and show that the change in the initial concentrations can be neglected.

(a) 0.0092 M HClO, a weak acid

(b) 0.0784 M C₆H₅NH₂, a weak base

(c) 0.0810 M HCN, a weak acid

(d) 0.11 M (CH₃)₃N, a weak base

(e) 0.120 M $\text{Fe} \left(\text{H}_{2} \text{O}\right)_{6} ^{2+}$ a weak acid, K_a = 1.6 $\times$ 10⁻⁷

White vinegar is a 5.0% by mass solution of acetic acid in water. If the density of white vinegar is 1.007 g/cm³, what is the pH?

Nicotine, C₁₀H₁₄N₂, is a base that will accept two protons (K_b1 = 7 $\times$ 10⁻⁷, K_b2 = 1.4 $\times$ 10⁻¹¹). What is the concentration of each species present in a 0.050-M solution of nicotine?

The pH of a 0.15-M solution of $\text{HSO}_{4} ^{−}$ is 1.43. Determine K_a for $\text{HSO}_{4} ^{−}$ from these data.

The pH of a solution of household ammonia, a 0.950 M solution of NH_3, is 11.612. Determine K_b for NH₃ from these data.