Class XII students were asked to prepare one litre of buffer solution of pH   8 . 26 by their chemistry…

A mixture of ${\mathrm{NH}}_3$ and ${\mathrm{NH}}_4\mathrm{Cl}$ will form a basic buffer.

$\mathrm{pH}$ of final solution $=8.26$

$\mathrm{pOH}$ of final solution$=14-8.26$ $=5.74$

Let the number of moles of ${\mathrm{NH}}_4\mathrm{Cl}$ added to $1 \mathrm{L}$ solution of be $\mathrm{x}$.

Using Henderson’s equation.

$\mathrm{pOH}={\mathrm{pK}}_{\mathrm{b}}+\log \frac{\left[{\mathrm{NH}}_4\mathrm{Cl}\right]}{\left[{\mathrm{NH}}_3\right]}$

$5.74=4.74+\log \frac{\mathrm{x}}{0.2}$

$\therefore \mathrm{x}=2$

The amount of ammonium chloride to be dissolved by the student in $0.2\mathrm{M}$ ammonia solution to make one litre of buffer$=$$2\times 53.5=107\mathrm{g}$