Class XII students were asked to prepare one litre of buffer solution of pH 8.26 by their Chemistry teacher: The amount of ammonium chloride to be dissolved by the student in 0.2 M ammonia solution to make one litre of the buffer is :
(Given: pK_b(NH₃)=4.74
Molar mass of NH₃=17 g mol⁻¹
Molar mass of NH₄ Cl=53.5 g mol⁻¹ )

Solution

Chemistry ma’am · 4 steps

Ammonia is a base, so convert pH to pOH first. Then Henderson says the salt must be ten times the ammonia.

1. Go to pOH. NH₃ with NH₄Cl is a basic buffer, and you are given pK_b. So work with pOH: pOH = 14 - 8.26 = 5.74.

   pOH = pK_b + log[NH₄⁺]/[NH₃]

2. Find the ratio. 5.74 = 4.74 + log[NH₄⁺]/0.2, so log[NH₄⁺]/0.2 = 1. The salt concentration is 10 times the base: [NH₄⁺] = 10 × 0.2 = 2 M.
3. Convert to grams. In one litre, 2 M means 2 mol of NH₄Cl. Its mass is 2 × 53.5 = 107 g.
4. So. The student must dissolve 107.0 g of NH₄Cl, option (c).

The idea to keep. For a basic buffer, turn pH into pOH and use pOH = pK_b + log[salt]/[base]. Each unit of the log means a factor of 10 in the ratio.

Answer from the NTA final answer key. Solution by PrepWiser.