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 : $\mathrm{pK_b(NH_3)=4.745}$
$\mathrm{\log2=0.301}$
$\mathrm{\log3=0.477}$
$\mathrm{T=298~K]}$
Answer (integer)
9079
Solution
In resultant solution<br/><br/>
$$
\begin{aligned}
& \mathrm{n}_{\mathrm{NH}_3}= 0.1-0.02=0.08 \\\\
& \mathrm{n}_{\mathrm{NH}_4 \mathrm{Cl}}= \mathrm{n}_{\mathrm{NH}_4^{+}}=0.1+0.02=0.12 \\\\
& \mathrm{pOH}= \mathrm{pK}_{\mathrm{b}}+\log \frac{\left[\mathrm{NH}_4^{+}\right]}{\left[\mathrm{NH}_3\right]} \\\\
&=4.745+\log \frac{0.12}{0.08} \\\\
&=4.745+\log \frac{3}{2} \\\\
&=4.745+0.477-0.301 \\\\
& \mathrm{pOH}=4.921 \\\\
& \mathrm{pH}=14-\mathrm{pH} \\\\
&=9.079 = 9079\times 10^{-3}
\end{aligned}
$$
About this question
Subject: Chemistry · Chapter: Equilibrium · Topic: Chemical Equilibrium and Kc, Kp
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