What a learner can do afterwards
- Writes the Ka expression for a named weak acid
- Calculates the pH of a weak acid solution and names the two approximations the calculation makes
- Converts between Ka and pKa and ranks acids by strength from either
- Explains why a weak acid and a strong acid at the same concentration differ in pH yet need the same volume of alkali to neutralise
1 · Read
Vinegar holds ethanoic acid and lemon holds citric acid. Both are weak acids that only partly split into ions. For a weak acid HA splitting into H+ and A-, the equilibrium constant is Ka = [H+][A-] over [HA]. At the same concentration and temperature, a bigger Ka means a bigger share has split, so bigger Ka means a stronger weak acid.
Take ethanoic acid, the acid in vinegar, as a 0.10 mol per cubic decimetre solution with Ka 1.7 x 10-5. The working gives [H+] near 1.3 x 10-3, so the pH is about 2.9. It leans on two quiet guesses: the water gives almost no H+ next to the acid, and 0.10 stays near 0.10 since so little splits.
pKa = -log Ka puts strength on a pH-like scale: lower pKa wins, and each single unit is a factor of ten in Ka. Ranking by pKa and ranking by Ka always agree.
Take equal volumes of 0.10 mol per cubic decimetre ethanoic acid and 0.10 mol per cubic decimetre hydrochloric acid. The strong acid shows a lower pH because it is fully split. Yet both need the same volume of the same alkali to neutralise. A monoprotic acid can give one proton per molecule, so both flasks hold the same total neutralisable amount, and the titre cannot tell them apart.
Ka measures the split, pKa rescales it, approximations unlock the pH, and titre counts total neutralisable amount, not strength.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.