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Propionic acid buffer pH; effect of HCl

Short answer

A buffer resists pH changes. For propionic acid buffer, pH = pKa + log([salt]/[acid]) = 4.873 + log(0.25/0.5) = 4.57. After adding 0.1 M HCl, salt decreases to 0.15 M, acid increases to 0.60 M, giving pH = 4.27.

What is a buffer solution? The Ka of propionic acid is 1.34×10⁻⁵. What is the pH of a solution containing 0.5M propionic acid and 0.25M sodium propionate? What will be the pH of this solution after 0.1M HCl has been added to the buffer?

Buffer solution

A solution that resists a change of pH on the addition of small amounts of acid or alkali. Propionic acid + sodium propionate is an acidic buffer.

pH of the buffer

pKa = −log(1.34×10⁻⁵) = 4.873
pH = pKa + log([salt]/[acid])
   = 4.873 + log(0.25/0.5)
   = 4.873 − 0.301 = 4.572

pH after adding 0.1 M HCl

HCl (strong acid) reacts with propionate: C₂H₅COO⁻ + H⁺ → C₂H₅COOH. Taking 0.1 mol per litre and no change of volume:

Salt   = 0.25 − 0.10 = 0.15 M
Acid   = 0.50 + 0.10 = 0.60 M
pH = 4.873 + log(0.15/0.60) = 4.873 − 0.602 = 4.271

Answer: pH of buffer = 4.57; after adding HCl, pH = 4.27 (a fall of only 0.30 unit, whereas 0.1 M HCl in pure water would give pH 1).

✓ Answer checked by Claude Opus 5.5 (high effort)

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