Question

For the preparation of a buffer of $pH=8.26$ , the amount of $\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4$ required to be mixed with one litre of $0.1(\mathrm{M}) \mathrm{NH}_3$ (aq), $\mathrm{pK}_{\mathrm{b}}=4.74$ is ?

• A. $1.0 \text{ moles}$
• B. $10.0 \, mole$ s
• C. $0.50 \, mole$ s
• D. $5 \text{ moles}$

Answer 1

Answer: (C)

$\text{pOH} = \text{pK}_{\text{b}} + log \frac{\left[\text{NH}_{4}^{+}\right]}{\left[\text{NH}_{3}\right]}$

$5.74 = 4.74 \, + log \frac{\left[\text{NH}_{4}^{+}\right]}{0.1}$

or $log \frac{\left[N H_{4}^{+}\right]}{0.1}= \, 1$

or $\frac{\left[N H_{4}^{+}\right]}{0.1}= \, 10$

or $\left[N H_{4}^{+}\right]= \, 1 \, \left(M\right)$

$\therefore \left(\text{NH}_{4}^{+} \, \right)_{2} \left(\text{SO}\right)_{4}$ required $= 0.5 \text{ mole}$