The p Ka of a weak acid, benzoic acid and p Kb of a weak base, ammonium hydroxide are 4.25 and 4.75 respectively. Then, the pH of 0.1 M solution of ammonium benzoate will be

Question

The p Ka of a weak acid, benzoic acid and p Kb of a weak base, ammonium hydroxide are 4.25 and 4.75 respectively. Then, the pH of 0.1 M solution of ammonium benzoate will be (A) 7.10 (B) 7.50 (C) 6.75 (D) 6.50

Tardigrade Admin · Accepted Answer

Ammonium benzoate ( C 6 H 5 COONH 4) is the salt weak benzoic acid ( C 6 H 5 COOH , p K a=4.25) and weak ammonium hydroxide ( NH 4 OH , p Kb=4.75) p H=7+(1/2)(p Ka-p Kb) =7+(1/2)(4.25-4.75)=7-0.25=6.75

Q. The $p K_{a}$ of a weak acid, benzoic acid and $p K_{b}$ of a weak base, ammonium hydroxide are $4.25$ and $4.75$ respectively. Then, the $p H$ of $0.1 M$ solution of ammonium benzoate will be__________

7.10

7.50

6.75

6.50

Ammonium benzoate $(C_{6} H_{5} COON H_{4})$ is the salt weak benzoic acid $(C_{6} H_{5} COO H, p K_{a} = 4.25)$ and weak ammonium hydroxide $(N H_{4} O H, p K_{b} = 4.75)$
$p H = 7 + \frac{1}{2} (p K_{a} - p K_{b})$
$= 7 + \frac{1}{2} (4.25 - 4.75) = 7 - 0.25 = 6.75$

Q. The pKa​ of a weak acid, benzoic acid and pKb​ of a weak base, ammonium hydroxide are 4.25 and 4.75 respectively. Then, the pH of 0.1M solution of ammonium benzoate will be__________