Find the pH of a buffer from its pKa and the concentrations of the conjugate base [A⁻] and the weak acid [HA], using the Henderson–Hasselbalch equation.
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From the article Henderson–Hasselbalch Equation: Buffer pH in Three Steps.
A buffer contains 0.100 M CH₃COOH and 0.100 M CH₃COONa. Ka = 1.8 × 10⁻⁵. Find the pH.
pKa = −log(1.8 × 10⁻⁵) = 5 − log 1.8 = 5 − 0.255 = 4.74
Ratio [A⁻]/[HA] = 0.100 / 0.100 = 1, and log 1 = 0
pH = 4.74 + 0 = 4.74
Key result to remember: when acid and salt are equal, pH = pKa. That is also the half-equivalence point of a weak-acid titration, and it is where the buffer works best.
Worked in full in Henderson–Hasselbalch Equation: Buffer pH in Three Steps.
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