Match each salt in aqueous solution to the correct chemical description of its hydrolysis … — JAMB UTME

Match each salt in aqueous solution to the correct chemical description of its hydrolysis behavior and resulting pH at $25^\circ\text{C}$ .

Ammonium sulfate, $(NH_4)_2SO_4$ Acidic ( $pH < 7$ ) because only the cation undergoes hydrolysis, yielding hydronium ions.
Sodium propanoate, $CH_3CH_2COONa$ Alkaline ( $pH > 7$ ) because only the propanoate anion undergoes hydrolysis, yielding hydroxide ions.
Potassium nitrate, $KNO_3$ Neutral ( $pH = 7$ ) because neither the alkali metal cation nor the conjugate base of the strong acid undergoes hydrolysis.
Ammonium cyanide, $NH_4CN$ Alkaline ( $pH > 7$ ) because both ions hydrolyze, but $K_b$ of the weak base component exceeds $K_a$ of the weak acid component.

Answer

Ammonium sulfate pairs with the acidic cation-hydrolyzing description; Sodium propanoate pairs with the alkaline anion-hydrolyzing description; Potassium nitrate pairs with the neutral non-hydrolyzing description; Ammonium cyanide pairs with the alkaline dual-hydrolyzing description where

K_b > K_a

The solution pH resulting from salt hydrolysis depends directly on the relative strengths of the parent acid and base. Salts derived from weak bases and strong acids yield acidic solutions via cation hydrolysis. Salts derived from strong bases and weak acids yield alkaline solutions via anion hydrolysis. Salts of strong acids and strong bases do not undergo net hydrolysis, remaining neutral. For salts derived from both a weak acid and a weak base, both ions undergo hydrolysis, and the solution acidity or alkalinity is determined by comparing the

K_b

of the weak base to the

K_a

of the weak acid.

Step-by-Step Solution

Identify the parent acid and parent base for each salt.

(NH_4)_2SO_4

comes from

NH_3

(weak base) and

H_2SO_4

(strong acid).

CH_3CH_2COONa

comes from

NaOH

(strong base) and

CH_3CH_2COOH

(weak acid).

KNO_3

comes from

KOH

(strong base) and

HNO_3

(strong acid).

NH_4CN

comes from

NH_3

(weak base) and

HCN

(weak acid).

The strength of parent acids and bases determines which ions undergo hydrolysis in water.

Determine which ion hydrolyzes for single-weak component salts.

(NH_4)_2SO_4

NH_4^+

hydrolyzes to produce

H_3O^+

(acidic,

pH < 7

). In

CH_3CH_2COONa

CH_3CH_2COO^-

hydrolyzes to produce

OH^-

(alkaline,

pH > 7

). In

KNO_3

, neither ion hydrolyzes (neutral,

pH = 7

Conjugate ions of weak species react with water, whereas conjugate ions of strong species do not hydrolyze.

Compare ionization constants for the weak acid-weak base salt.

For

NH_4CN

, compare

K_b(NH_3) = 1.8 \times 10^{-5}

with

K_a(HCN) = 6.2 \times 10^{-10}

. Since

K_b > K_a

CN^-

anion hydrolysis produces more

OH^-

than

NH_4^+

cation hydrolysis produces

H_3O^+

, resulting in an alkaline solution (

pH > 7

When both ions hydrolyze, the relative magnitudes of

K_a

and

K_b

govern whether

OH^-

H_3O^+

is in excess.

Key Concept

Salt Hydrolysis and Solution Acidity/Alkalinity

Estimated Time:1m 30s

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