Question

What is $\left[ H ^{+}\right]$of a solution having $0.1 \,M \,HCN$ and $0.2\, M \,NaCN$ ?
$\left(K_{a}\right.$ for $\left.HCN =6.2 \times 10^{-10}\right)$

• A. $ 3.1\times {{10}^{10}} $
• B. $ 6.2\times {{10}^{5}} $
• C. $ 6.2\times {{10}^{-10}} $
• D. $ 3.2\times {{10}^{-9}} $

Answer 1

Answer: (D)

$p H=-\log K_{a}+\log \frac{[N a C N]}{H C N}$
$=-\log 6.2 \times 10^{-10}+\log \frac{0.2}{0.1}$
$=-\log 6.2+10 \log 10+\log 2-\log 1$
$p H=-0.7924+10+0.3010-0$
$=9.5086$
$ \because p H=-\log \left[H^{+}\right]$
$0.5086=-\log \left[H^{+}\right] $
$\left[H^{+}\right]=3.225 \times 10^{-9} \,M$