$\left[\right.$ આપેલ : $pK _{ b }\left( NH _3\right)=4.745$
$\log 2=0.301$
$\log 3=0.477$
$T =298\,K ]$
$n _{ NH _3}=0.1-0.02=0.08$
$n _{ NH _4 Cl }= n _{ NH _4^{+}}=0.1+0.02=0.12$
$pOH = pK _{ b }+\log \frac{\left[ NH _4^{+}\right]}{\left[ NH _3\right]}$
$=4.745+\log \frac{0.12}{0.08}$
$=4.745+\log \frac{3}{2}$
$=4.745+0.477-0.301$
$pOH =4.921$
$pH =14- pH$
$=9.079$
$A = NH_4Cl$; $ B = CH_3COONa$; $ C = NH_4OH$; $D = CH_3COOH$