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Q.
$50\, mL\, 0.2\, M$ ammonia solution is treated with $25\, mL$ of $0.2\, M\, HCl$. If $pK _{ b }$ of ammonia solution is $4.75$, the $pH$ of the mixture will be______
Equilibrium
Solution:
In the given reaction, all the $HCl$ will be neutralized and $NH _{4} Cl$ will be formed and equivalent amount of $NH _{4} OH$ will remain in the solution. Thus, the final solution will contain $NH _{4} OH$ and $NH _{4} Cl$, which can act as basic buffer solution. The concentration of $NH _{4} Cl$ and $NH _{4} OH$ will be same.
$pOH = pK _{ b }+\log _{10} \frac{[\text { Salt }]}{[\text { Base }]}$
$pOH =4.75+\log _{10} \frac{1}{1}$
$pOH =4.75$
$pH =14- pOH $
$=14-4.75=9.25$