Mistake Master
Acid-base titrations
A titration curve packs a lot into one S-shaped line. The two landmarks that matter are the equivalence point — where the moles match — and the half-equivalence point, where pH equals pKa. Neither is set by hitting pH 7.
§1
Landmarks on the curve.
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A titration curve plots pH against the volume of titrant added. Its key landmark is the equivalence point: where the moles of added titrant match the analyte by the stoichiometry.
The equivalence point is not always at pH 7. Only a strong-acid/strong-base titration reaches equivalence at neutral; a strong titrant against a weak analyte reaches equivalence at an acidic or basic pH, because the conjugate species left behind is acidic or basic.
A second landmark, the half-equivalence point (halfway to equivalence in a weak-acid titration), is where pH = pKa. All these landmarks are set by matched moles, not by equal volumes.
§2
Reading the curve.
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Find equivalence by moles; read pKa at half-equivalence.
- Locate the equivalence point. Where the moles of titrant match the analyte (the steep rise), not a fixed pH.
- Judge the equivalence pH. Strong-strong: pH 7; strong-weak: acidic or basic, from the leftover conjugate.
- Find the half-equivalence point. Halfway to equivalence in a weak-acid titration: pH = pKa here.
- Use moles, not volumes. Equivalence is set by matched moles, so unequal concentrations give unequal volumes.
§3
The pieces you'll meet.
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Two landmarks, both mole-based.
§4
Worked example: weak acid with strong base.
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Half-equivalence. Halfway to equivalence, half the weak acid is converted to its conjugate base, so [HA] = [A⁻] and pH = pKa — a direct read of pKa off the curve.
Equivalence. At equivalence, all the weak acid has become its conjugate base A⁻, which is basic, so the equivalence pH is above 7 (basic), not neutral.
Moles, not volume. The equivalence volume depends on the concentrations; equal volumes do not signal equivalence.
Contrast. A strong-acid/strong-base titration would instead reach equivalence right at pH 7, with no leftover conjugate to shift it.
§5
Mistakes that cost real points.
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"The equivalence point is always at pH 7."
Only strong-acid/strong-base titrations reach equivalence at pH 7. A strong base titrating a weak acid reaches equivalence at a basic pH (the conjugate base remains); a strong acid titrating a weak base reaches it at an acidic pH. The equivalence pH depends on the leftover conjugate.
Fix. Judge the equivalence pH from what remains: strong-strong → 7; strong-weak → basic or acidic.
"The half-equivalence point and the equivalence point are the same landmark."
They are different. The half-equivalence point is halfway to equivalence, where pH = pKa; the equivalence point is where the moles fully match. Confusing them mislocates pKa and the endpoint.
Fix. Keep them separate: half-equivalence gives pKa; equivalence is the full mole match (the steep rise).
"Equivalence is reached when equal volumes are mixed."
Equivalence is set by matched moles, not equal volumes. Solutions of different concentrations reach equivalence at different volumes. Reading equivalence off equal volumes ignores the concentrations.
Fix. Determine equivalence by matching moles (concentration × volume), not by equal volumes.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.