Why some salt solutions are acidic or basic, buffer solutions and how they resist a change in pH, the solubility product and the common ion effect.
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Created by StemPy LabA salt dissolved in water can make the solution acidic or basic. The ion that came from a weak acid or a weak base reacts with water: this is salt hydrolysis. Salts of a weak acid and a strong base give basic solutions. Salts of a strong acid and a weak base give acidic ones. The pH follows from pKa or pKb and the concentration.
Adding a salt of a weak acid pushes back the acid's ionisation: this is the common ion effect. A mixture of a weak acid and its salt, or a weak base and its salt, is a buffer: it keeps its pH almost steady when a little acid or base is added. Its pH is given by the Henderson-Hasselbalch equation, .
A sparingly soluble salt in contact with its saturated solution is in equilibrium with its ions. The product of the ion concentrations, each raised to its number in the formula, is the solubility product, . From you can find the molar solubility. A common ion lowers the solubility, and a precipitate forms when the ionic product exceeds .
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