Question

The hydrogen ion concentration of $0.1\, M$ hydrocyanic acid is $6.3 \times 10^{-6} M ,$ the $K_{a}$ is

• A. $3.96 \times 10^{-10}$
• B. $3.78 \times 10^{-9}$
• C. $3.86 \times 10^{-8}$
• D. $3.70 \times 10^{-11}$

Answer 1

Answer: (A)

$\left[ H ^{+}\right]=6.3 \times 10^{-6} M$

$\alpha$ ' degree of dissociation $=\frac{\left[ H ^{+}\right]}{C}=\frac{6.3 \times 10^{-6}}{0.1}=6.3 \times 10^{-5}$

According to Ostwald's formula for weak electrolyte,

$K_{a}=\alpha^{2} C=\left(6.3 \times 10^{-5}\right)^{2} \times 0.1=3.96 \times 10^{-10}$