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Buffer criteria; NH₄Cl weight calculation

Short answer

A buffer requires a weak base-conjugate acid pair, stable pOH, and capacity. For NH₄OH buffer pH 9, pOH = 5, [OH⁻] = 10⁻⁵. Using [OH⁻] = Kb × [base]/[salt] gives [NH₄Cl] = (1.8×10⁻⁵ × 0.2) / 10⁻⁵ = 0.36 M. In 2 L, mass = 0.72 mol × 53.5 g/mol = 38.5 g.

What are the criteria for buffer system? Calculate the weight in gram of NH₄Cl required to prepare buffer solution with 2 litre of 0.2N NH₄OH solution with pH = 9. (Kb = 1.8×10⁻⁵)

Criteria for a buffer system

  • It must resist a change in pH when a small amount of acid or alkali is added.
  • It must contain a weak acid and its conjugate base (salt), or a weak base and its conjugate acid, in appreciable amounts.
  • The ratio of the two components should be close to 1 (between 0.1 and 10), so that pH = pKa ± 1.
  • The pH should not change much on dilution or on keeping for a long time.
  • The components must not react with each other and must be stable; the buffer should have a reasonable buffer capacity.

Calculation (basic buffer NH₄OH + NH₄Cl)

Given: NH₄OH = 0.2 N (0.2 M, as NH₄OH has one OH⁻), volume = 2 L, pH = 9, Kb = 1.8×10⁻⁵.

pOH = 14 − 9 = 5   →  [OH⁻] = 10⁻⁵ M
For a basic buffer: [OH⁻] = Kb × [base]/[salt]
10⁻⁵ = 1.8×10⁻⁵ × 0.2/[NH₄Cl]
[NH₄Cl] = 1.8×10⁻⁵ × 0.2 / 10⁻⁵ = 0.36 M

Moles of NH₄Cl in 2 L = 0.36 × 2 = 0.72 mol
Molar mass NH₄Cl = 53.5 g/mol
Mass = 0.72 × 53.5 = 38.52 g

Answer: about 38.5 g of NH₄Cl.

✓ Answer checked by Claude Opus 5.5 (high effort)

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