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Buffering capacity; NH₄Cl for pH 9 buffer

Short answer

Buffering capacity is moles of strong acid or base needed to change pH of 1 L buffer by one unit. For pH 9, pOH=5, [NH4Cl]=0.20×10^(0.25)=0.356 M. Mass of NH4Cl = 0.356 × 53.49 = 19.0 g.

What is meant by buffering capacity? A research student desires to prepare one litre of a solution buffered at pH 9.00. How many grams of ammonium chloride have to be added to one litre of 0.20 M NH₃ to make such a buffer? [pKb value of ammonia is 4.75]

Buffering capacity

The number of moles of strong acid or base required to change the pH of one litre of the buffer by one unit. It is greatest when [salt] = [base] (or acid).

Calculation

Basic buffer: NH₃ + NH₄Cl, pKb = 4.75.

pOH = 14 − 9.00 = 5.00
pOH = pKb + log([NH₄Cl]/[NH₃])
5.00 = 4.75 + log([NH₄Cl]/0.20)
log([NH₄Cl]/0.20) = 0.25  →  ratio = 1.778
[NH₄Cl] = 0.20 × 1.778 = 0.3557 M

Moles in 1 L = 0.3557 mol
Molar mass NH₄Cl = 53.49 g/mol
Mass = 0.3557 × 53.49 = 19.0 g

Answer: about 19.0 g of NH₄Cl.

✓ Answer checked by Claude Opus 5.5 (high effort)

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