Question

What is the degree of dissociation of weak acid $HA$ having concentration $=0.1 M$ in the presence of strong acid HB having concentration $=0.1 \,M$. Given $K_{a}$ for weak acid $=10^{-6}$

• A. $10^{-5}$
• B. $10^{-6}$
• C. $10^{-4}$
• D. $10^{-3}$

Answer 1

Answer: (A)

For weak acid,



$K _{ a }=\frac{\left( H ^{+}\right)\left( A ^{-}\right)}{( HA )}$



$\left( H ^{+}\right)$ in the solution $=0.1 M (\because$ of strong acid $)$

$K$, will remain constant

$\therefore 10^{-6}=\frac{(0.1)( C \alpha)}{( C - C \alpha)}$

$10^{-6}=\frac{0.1 C \alpha}{ C (1-\alpha)}$

$10^{-6}=\frac{0.1 \times \alpha}{1-\alpha}$

$1-\alpha \approx 1$

$10^{-6}=0.1 \times \alpha$

$10^{-6}=10^{-1} \times \alpha$

$\alpha=10^{-5}$