Question

How much water must be added to $300$ mL of $0.2$ M solution of ${CH}_{3}COOH{\left(K\right)}_a={1.8\times 10}^{-5}$ for the degree of dissociation of the acid to double?

• A. 600 mL
• B. 900 mL
• C. 1200 mL
• D. 1500 mL

Answer 1

Answer: (B)

900 mL

As we know, $C{\alpha }^2=K_a;\alpha =\sqrt{K_a/C}$.

According to the given condition,

${\alpha }_2/{\alpha }_1=2=\sqrt{C_1/C_2};C_2=0.2/4$

Again,

$M_1{V}_1=M_2{V}_2,$

$V_2=300\times 0.2\times 4/0.2=1200$ ml.

So, 900 (=1200-300) mL of water should be added.

Hence, the correct option is $\text{B}$