Question

A litre of buffer solution contains $0.1$ mole of each of $NH _3$ and $NH _4 CL$ On the addition of $0.02$ mole of $HCl$ by dissolving gaseous $HCl$, the $pH$ of the solution is found to be ____ $\times 10^{-3}$ (Nearest integer)
[Given : $pK _5\left( NH _3\right)=4.745$
$ \log 2=0.301$
$\log 3=0.477$
$T =298 K ]$

Answer 1

In resultant solution
$n _{ NH _3}= 0.1-0.02=0.08 $
$ n _{ NH _4 Cl }= n _{ NH _4^{+}}=0.1+0.02=0.12$
$pOH = pK _{ b }+\log \frac{\left[ NH _4^{+}\right]}{\left[ NH _3\right]}$
$=4.745+\log \frac{0.12}{0.08}$
$=4.745+\log \frac{3}{2}$
$=4.745+0.477-0.301 $
$ pOH =4.921 $
$ pH =14- pH$
$=9.079$