Henderson–Hasselbalch equation (pH = pKa + log([A-]/[HA]))

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pH = pKa + log([A-]/[HA])

What each symbol means

SymbolWhat it stands for
pK_a-log10 Ka
[A^-]/[HA]Ratio of conjugate base to acid (same volume ⇒ mole ratio)

When to use this

Buffer with weak acid HA and A-; best when both significant and not extremely dilute; assumes [HA],[A-] ≫ [H+] from water.