Question

A litre of buffer solution contains $01$ mole of each of $NH _3$ and $NH _4 CL$ On the addition of $002$ mole of $HCl$ by dissolving gaseous $HCl$, the $pH$ of the solution is found to be ____ $\times 10^{-3}$ (Nearest integer) [Given : $pK _5\left( NH _3\right)=4745$ $\log 2=0301$ $\log 3=0477$ $T =298 K ]$

Answer 1

The correct answer is 9079.
In resultant solution
nNH3​​=0.1−0.02=0.08
nNH4​Cl​=nNH4+​​=0.1+0.02=0.12
pOH=pKb​+log[NH3​][NH4+​]​
=4.745+log0.080.12​
=4.745+log23​
=4.745+0.477−0.301
pOH=4.921
pH=14−pH
=9.079