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Buffer criteria; sodium benzoate calculation

Short answer

A buffer needs a weak acid-conjugate base pair, stable pH near pKa, and adequate capacity. Using pH = pKa + log([salt]/[acid]), 4.3 = 4.2 + log([salt]/0.13) yields [salt]=0.1637 M. For 160 mL, mass = 0.02619 mol × 144.1 g/mol = 3.77 g.

What are the criteria for buffer system? How many grams of sodium benzoate should be added to 160 mL of 0.13 M benzoic acid solution to obtain a buffer with a pH of 4.3? [pKa value of benzoic acid is 4.2]

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

Benzoic acid (C₆H₅COOH) + sodium benzoate (C₆H₅COONa) form an acidic buffer.

pH = pKa + log([salt]/[acid])
4.3 = 4.2 + log([salt]/[acid])
log([salt]/[acid]) = 0.1  →  [salt]/[acid] = 1.259
[salt] = 1.259 × 0.13 = 0.1637 M

Moles in 160 mL = 0.1637 × 160/1000 = 0.02619 mol
Molar mass of C₆H₅COONa = 144.1 g/mol
Mass = 0.02619 × 144.1 = 3.77 g

Answer: about 3.77 g of sodium benzoate. (Volume change on adding the solid is neglected.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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