Class XII students were asked to prepare one litre of buffer...

A mixture of ${NH}_{3}$ and ${NH}_{4}Cl$ will form a basic buffer.

$pH$ of final solution $= 8 . 26$

$pOH$ of final solution$= 14 - 8 . 26$ $= 5 . 74$

Let the number of moles of ${NH}_{4}Cl$ added to $1L$ solution of be $x$.

Using Henderson’s equation.

$pOH = {pK}_{b} + log\dfrac{\left[{NH}_{4}Cl\right]}{\left[{NH}_{3}\right]}$

$5 . 74 = 4 . 74 + log\dfrac{x}{0 . 2}$

$∴ x = 2$

The amount of ammonium chloride to be dissolved by the student in $0 . 2M$ ammonia solution to make one litre of buffer=$2 \times 53 . 5 = 107g$