Proton transfer reactions: notes and practice questions

The acid dissociation constants, $K_a$, for four weak acids at 298 K are given below.

Ethanoic acid, $CH_3COOH$: $K_a = 1.7 \times 10^{-5}$

Phenol, $C_6H_5OH$: $K_a = 1.0 \times 10^{-10}$

Carbonic acid, $H_2CO_3$: $K_{a1} = 4.5 \times 10^{-7}$

Ammonium ion, $NH_4^+$: $K_a = 5.6 \times 10^{-10}$

Which acid has the strongest conjugate base?

A. Ethanoic acid

B. Phenol

C. Carbonic acid

D. Ammonium ion

The indicator phenolphthalein has a pH range of $8.2$ to $10.0$. Which titrations would this indicator be suitable for?

I. A weak acid with a strong base

II. A strong acid with a weak base

III. A strong acid with a strong base

A. I only

B. III only

C. I and III only

D. I, II and III

Propanoic acid ($CH_3CH_2COOH$) is a weak acid used in various industrial applications. What is the pH for a $0.200 \text{ mol dm}^{-3}$ solution of propanoic acid if its acid dissociation constant, $K_a$, is $1.34 \times 10^{-5}$?

A $1.00$

B $2.50$

C $2.79$

D $3.09$

Which of the following aqueous solutions will have a pH below 7 at 298 K?

A. Sodium cyanide

B. Lithium sulfate

C. Aluminium chloride

D. Potassium carbonate

A chemist prepares a buffer solution by mixing equal molar amounts of a weak acid, propanoic acid ($CH_3CH_2COOH$), and its conjugate base, sodium propanoate ($CH_3CH_2COONa$). The resulting solution has a pH of $5.15$ at $298 \text{ K}$. What is the $K_a$ value for propanoic acid at this temperature?

A $7.08 \times 10^{-6}$

B $7.08 \times 10^{-5}$

C $5.15 \times 10^{-6}$

D $1.00 \times 10^{-5}$

Which graph correctly represents the change in pH when $0.100 \text{ mol dm}^{-3}$ hydrochloric acid is added from a burette to $25.0 \text{ cm}^3$ of $0.100 \text{ mol dm}^{-3}$ aqueous ammonia in a flask?

A.

B.

C.

D.

Under physiological conditions, the ion product constant of water ($K_w$) is approximately $2.4 \times 10^{-14}$ at $37 \text{ } ^\circ C$. What is the pH of pure water at this temperature?

A $6.5$

B $6.8$

C $7.0$

D $7.2$

A chemist mixes $150 \text{ cm}^3$ of $0.50 \text{ mol dm}^{-3}$ sodium hydroxide solution with $350 \text{ cm}^3$ of $0.30 \text{ mol dm}^{-3}$ hydrochloric acid solution.

What is the pH of the resulting solution?

A $0.80$

B $1.10$

C $1.30$

D $7.00$

A student is preparing buffer solutions in the laboratory. Which of the following mixtures will result in the formation of a buffer solution?

I. $50 \text{ cm}^3$ of $0.20 \text{ mol dm}^{-3}$ carbonic acid ($H_2CO_3$) and $50 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3}$ potassium hydroxide ($KOH$)

II. $100 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3}$ methylamine ($CH_3NH_2$) and $50 \text{ cm}^3$ of $0.20 \text{ mol dm}^{-3}$ hydrobromic acid ($HBr$)

III. $50 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3}$ sodium carbonate ($Na_2CO_3$) and $50 \text{ cm}^3$ of $0.20 \text{ mol dm}^{-3}$ hydrochloric acid ($HCl$)

A. I only

B. II only

C. I and II only

D. II and III only

Which aqueous solution has a pH greater than 7?

A. $CH_3COONa$

B. $NH_4NO_3$

C. $KCl$

D. $CH_3COONH_4$

What is the best indicator to use in the titration of aqueous ethanoic acid with aqueous sodium hydroxide?

A. Methyl orange, $pK_a = 3.7$

B. Bromocresol green, $pK_a = 4.7$

C. Bromothymol blue, $pK_a = 7.0$

D. Phenolphthalein, $pK_a = 9.6$

A chemist is preparing various aqueous solutions for an experiment. Which of the following solutions would have a pH of approximately $11.0$ at $298 \text{ K}$?

(Assume $K_w = 1.0 \times 10^{-14}$ at $298 \text{ K}$ and $K_b$ for $NH_3 = 1.8 \times 10^{-5}$)

A. $1.0 \times 10^{-3} \text{ mol dm}^{-3} \text{ NH}_3 \text{ (aq)}$

B. $1.0 \times 10^{-3} \text{ mol dm}^{-3} \text{ NaOH(aq)}$

C. $1.0 \times 10^{-5} \text{ mol dm}^{-3} \text{ NaOH(aq)}$

D. $1.0 \times 10^{-5} \text{ mol dm}^{-3} \text{ NH}_3\text{(aq)}$

What is the pH of a buffer solution prepared by mixing $0.300 \text{ mol dm}^{-3}$ propanoic acid ($CH_3CH_2COOH$) and $0.450 \text{ mol dm}^{-3}$ sodium propanoate ($CH_3CH_2COONa$)?

The $pK_a$ for propanoic acid is $4.87$.

A. $4.69$

B. $4.87$

C. $5.05$

D. $5.23$

Which of these bases has the strongest conjugate acid?

Given the following $K_b$ values at $298 \text{ K}$:

A. Aniline ($C_6H_5NH_2$): $K_b = 4.3 \times 10^{-10}$

B. Hydrazine ($N_2H_4$): $K_b = 8.5 \times 10^{-7}$

C. Pyridine ($C_5H_5N$): $K_b = 1.7 \times 10^{-9}$

D. Hydroxylamine ($NH_2OH$): $K_b = 1.1 \times 10^{-8}$

Which aqueous solution has the highest pH?

A. $0.10\text{ mol dm}^{-3} CH_3COONa$

B. $0.10\text{ mol dm}^{-3} NH_4Cl$

C. $0.10\text{ mol dm}^{-3} KBr$

D. $0.10\text{ mol dm}^{-3} Na_2CO_3$

A household cleaner is found to have a $pH$ of $11.80$ at $298.15 \text{ K}$. What is the concentration of $OH^-(aq)$, in $mol \text{ dm}^{-3}$, in this solution?

$pH = -\log_{10} [H^+]$ $[H^+] = 10^{-pH}$

$pOH = -\log_{10} [OH^-]$ $[OH^-] = 10^{-pOH}$

$K_w = [H^+][OH^-]$

A. $1.58 \times 10^{-12}$

B. $6.31 \times 10^{-3}$

C. $6.31 \times 10^{-11}$

D. $1.58 \times 10^{-2}$

Which aqueous solution has the highest pH?

A. $1.0 \text{ mol dm}^{-3}$ $NH_3$

B. $0.5 \text{ mol dm}^{-3}$ $KOH$

C. $1.0 \text{ mol dm}^{-3}$ $H_2SO_4$

D. $0.5 \text{ mol dm}^{-3}$ $Ba(OH)_2$

The indicator bromocresol green is a weak acid, HIn, with a $pK_a$ of 4.7. Its equilibrium in aqueous solution is shown below.

$HIn(aq) \rightleftharpoons H^+(aq) + In^-(aq)$

yellow blue

Which statements are correct?

I. In a solution with $pH = 2$, the indicator will appear yellow.

II. The indicator is most effective for a titration with an equivalence point at $pH \approx 4.7$.

III. In a solution of $pH = 6$, the concentration of the blue form is greater than the concentration of the yellow form.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

Which term cannot be used to describe the chemical behaviour of water, $H_2O$?

A. Lewis acid

B. Ligand

C. Brønsted-Lowry base

D. Nucleophile