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Acid dissociation constant formula

The acid dissociation constant formula as it is used in A Level Chemistry, taken from the A Level Chemistry formula sheet.

The formula

Ka=[H+][A−][HA],pKa=−log⁡10KaK_a = \dfrac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}, \quad \text{p}K_a = -\log_{10} K_a

How to use it in the exam

  • Write the formula down before substituting — most mark schemes award a method mark for a correct, clearly quoted formula.
  • Check every quantity is in consistent units before you substitute, then substitute in one step so the working stays easy to follow.
  • Show the rearrangement rather than jumping to the answer, and give the final value to the accuracy the question asks for.

Related acids & bases formulas

pH
pH=−log⁡10[H+]\text{pH} = -\log_{10}[\text{H}^+]
Hydrogen ion concentration
[H+]=10−pH[\text{H}^+] = 10^{-\text{pH}}
Ionic product of water
Kw=[H+][OH−]=1.0×10−14 (298 K)K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}\ \text{(298 K)}

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