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Buffer solution; sodium formate for pH 3.80

Short answer

A buffer resists pH change, containing a weak acid and its salt. Using [H⁺] = Ka[acid]/[salt] with [H⁺] = 10⁻³·⁸ M, the concentration of sodium formate needed is (1.8×10⁻⁴ × 0.10) / 1.58×10⁻⁴ ≈ 0.114 M.

What is meant by buffer solution? Calculate the concentration of sodium formate, HCOONa, that must be present in a 0.10M solution of formic acid to produce a pH of 3.80. [Ka for formic acid is 1.8×10⁻⁴].

Buffer solution

A buffer solution is a solution that resists a change in its pH when a small amount of a strong acid or strong base is added, or when it is diluted. It contains a weak acid with its salt (acidic buffer) or a weak base with its salt (basic buffer).

Calculation

Formic acid HCOOH + sodium formate HCOONa form an acidic buffer.

pH = 3.80  →  [H⁺] = 10⁻³·⁸ = 1.585×10⁻⁴ M
[H⁺] = Ka [acid]/[salt]
[salt] = Ka [acid]/[H⁺]
       = 1.8×10⁻⁴ × 0.10 / 1.585×10⁻⁴
       = 0.1136 M

Check: pKa = 3.745; log([salt]/[acid]) = 3.80 − 3.745 = 0.055; ratio = 1.136; [salt] = 0.1136 M.

Answer: [HCOONa] ≈ 0.114 M.

✓ Answer checked by Claude Opus 5.5 (high effort)

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