Question

Match the List-I (solution of salts) with List-II (pH of the solution) and select the correct answer using the codes given below the lists:

List-I	List-II
A. Weak acid and strong base	1. $1/2p{K}_w$
B. Strong acid and weak acid	2. $1/2[p{K}_w-p{K}_b+logc]$
C. Weak acid and weak base	3. $1/2[p{K}_w-p{K}_b-logc]$
D. Strong acid and strong base	4. $1/2[p{K}_w+p{K}_a+logc]$

• A. A- 4 B-3 C-2 D-1
• B. A- 1 B-2 C-3 D-4
• C. A-2 B-3 C-4 D-1
• D. A- 3 B-2 C-1 D- 4

Answer 1

Answer: (A)

A- 4 B-3 C-2 D-1

(A) Weak acid and strong base:-

We know that

$H^{+}=\sqrt{\frac{K_w.K_a}{C}}$

taking log both side

$pH=\frac{1}{2}[p{K}_w+p{K}_a+\log C]$

(B) For strong acid and weak acid, same as (A), just concentration would be subtract as (in term of $p{K}_b$)

$pH=\frac{1}{2}[p{K}_w-p{K}_b-\log C]$

(C) For weak acid and weak base

$H^{+}=\sqrt{\frac{K_a.K_w}{K_b}}$

$pH=\frac{1}{2}[p{K}_w+p{K}_a-p{K}_b]$ or $[p{K}_w-p{K}_b+\log C]$

(D) For strong acid and strong base

$H^{+}={10}^{-7}$

$pH=\frac{p{K}_w}{2}$ $[\because p{K}_w=14]$