Question

A solution contains $0.2$M $N{H}_{4}OH$ and $0.2$M $N{H}_{4}Cl$. If $1.0$ mL of $0.001$ M HCl is added to it. What will be the $\left[O{H}^{-}\right]$ of the resulting solution $[K_b=2\times {10}^{-5}]$?

• A. $2\times {10}^{-5}$
• B. $5\times {10}^{-10}$
• C. $2\times {10}^{-3}$
• D. None of these

Answer 1

The correct option is A $2\times {10}^{-5}$

Concentration oF $N{H}_{4}OH=0.2M$

Concentration of $N{H}_{4}Cl=0.2M$

Concentration OF $HCl=0.001M$

$K_b=2\times {10}^{-5}$

Applying Henderson Equation

$POH=P^{K_a}+\log \left[\frac{salt}{base}\right]$

$POH=5-\log 2+\log \frac{0.2}{0.2}$

$=5-\log 2$

Hence,

$\left[O{H}^{-}\right]=2\times {10}^{-5}$

so, the option $\left(A\right)$ is the correct answer.