OCR A-level Chemistry pH calculations
This page covers the following topics:
1. Determination of pH
2. The ionic product of water
3. The dissociation constant
4. Buffer calculations
pH and the hydrogen ion concentration are related by the equation pH = -log₁₀[H⁺].
Water molecules dissociate into hydrogen and hydroxide ions in solution. The ionic product of water describes the relationship between the two at equilibirum.
In solution, strong acids/bases completely dissociate into ions, but weak acids/bases only partially dissociate. The acid dissociation constant is Ka and is related to pKa by the expression pKa = -log₁₀Ka. The pH of a weak acid can be calculated by pH = 1/2 pKa - 1/2 log₁₀c.
A buffer solution maintains a constant pH when small volumes of acid or base are added. The pH of a buffer solution can be calculated by the equation pH = pKa - log₁₀[acid]/[salt].
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