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Corrosion; pH after adding NaOH to acetic acid

Short answer

Corrosion is the slow destruction of metals by chemical/electrochemical attack of the environment, e.g., rusting of iron. Adding 0.1 g NaOH to 200 mL 0.1 M CH3COOH yields a buffer with 0.0175 mol acid and 0.0025 mol salt; pH = 4.74 + log(0.0025/0.0175) = 3.90.

What is corrosion? Calculate the pH of a resulted solution when 0.1 gm of NaOH is added to 200ml of 0.1M acetic acid solution. (pKa = 4.74).

Corrosion

The slow destruction of a metal by chemical or electrochemical attack of its surroundings (air, moisture, gases) is called corrosion. Example: rusting of iron, 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (hydrated ferric oxide); formation of green patina on copper.

Calculation

NaOH converts part of the acetic acid into sodium acetate, giving an acidic buffer.

Moles NaOH = 0.1/40 = 0.0025 mol
Moles CH₃COOH = 0.1 × 200/1000 = 0.020 mol
Acid left = 0.020 − 0.0025 = 0.0175 mol
Salt formed = 0.0025 mol

pH = pKa + log([salt]/[acid])
   = 4.74 + log(0.0025/0.0175)
   = 4.74 − 0.845 = 3.895

Answer: pH ≈ 3.90

✓ Answer checked by Claude Opus 5.5 (high effort)

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