Question

Which of the following mixtures will have the lowest pH at 298 K ?

• A. $10\mathrm{ml}0.05\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+5\mathrm{ml}0.1\mathrm{N}{\mathrm{NH}}_4\mathrm{OH}$
• B. $5\mathrm{ml}0.1\mathrm{N}{\mathrm{NH}}_4\mathrm{Cl}+5\mathrm{ml}0.2\mathrm{N}{\mathrm{NH}}_4\mathrm{OH}$
• C. $5\mathrm{ml}0.01\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+10\mathrm{ml}0.05\mathrm{N}{\mathrm{CH}}_3\mathrm{COONa}$
• D. $5\mathrm{ml}0.1\mathrm{N}\mathrm{CH}3\mathrm{COOH}+5\mathrm{ml}0.2\mathrm{N}\mathrm{NH}4\mathrm{OH}$

Answer 1

The correct option is C

$5\mathrm{ml}0.01\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+10\mathrm{ml}0.05\mathrm{N}{\mathrm{CH}}_3\mathrm{COONa}$

The explanation for the correct option:

(c ) $5\mathrm{ml}0.01\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+10\mathrm{ml}0.05\mathrm{N}{\mathrm{CH}}_3\mathrm{COONa}$

• This is an example of an acidic buffer
• So pH is less than $7$
• Hence this option is correct

Explanation for the incorrect options:

(a) $10\mathrm{ml}0.05\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+5\mathrm{ml}0.1\mathrm{N}{\mathrm{NH}}_4\mathrm{OH}$

• ${\mathrm{pK}}_{\mathrm{a}}$ value of ${\mathrm{CH}}_3\mathrm{COOH}$ and ${\mathrm{pK}}_{\mathrm{b}}$ value of ${\mathrm{NH}}_4\mathrm{OH}$ are same
• so it will have pH of $7$ since they neutralize completely.
• Thus, this option s incorrect.

(b)$5\mathrm{ml}0.1\mathrm{N}{\mathrm{NH}}_4\mathrm{Cl}+5\mathrm{ml}0.2\mathrm{N}{\mathrm{NH}}_4\mathrm{OH}$

• This is an example of a basic buffer
• so $\mathrm{pH}$ is more than $7$.
• Thus,this option s incorrect.

(d) $5\mathrm{ml}0.1\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+5\mathrm{ml}0.2\mathrm{N}{\mathrm{NH}}_4\mathrm{OH}$

• This is an aqueous solution ${\mathrm{CH}}_3\mathrm{COONa}$ that hydrolyses to form a basic solution.
• so $\mathrm{pH}$ is more than$7$.
• Thus, this option s incorrect.

Hence the correct option is (c ) $5\mathrm{ml}0.01\mathrm{N}{\mathrm{CH}}_3\mathrm{COOH}+10\mathrm{ml}0.05\mathrm{N}{\mathrm{CH}}_3\mathrm{COONa}$