Chemistry · Chemical equilibrium. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Water ionises slightly. The product [H+][OH−] is constant at a given temperature and is called the ionic product of water, Kw. pH and pOH are a short way to write small concentrations of H+ and OH−.
A weak acid ionises only partly. Its Ka shows how far: the larger Ka (the smaller pKa), the stronger the acid. Adding an ion that is already present pushes an ionic equilibrium back. This is the common ion effect.
A buffer resists a change in pH when a little acid or base is added. It is a weak acid with its salt, or a weak base with its salt. Ksp applies to saturated solutions of sparingly soluble salts.
In the Henderson equation students put the acid on top. The salt goes on top: pH = pKa + log([salt] ÷ [acid]). More salt than acid gives a pH above pKa, less salt gives a pH below it.
Answer: pH = 3.74 (below pKa, because there is less salt than acid).
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