Units

Ka and Kb from pH Calculator

Finds the strength of a weak acid or base from a pH reading.

Ka or Kb

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pKa or pKb —

Percent ionised — %

Ion concentration at equilibrium — mol/L

Reading this —

The formula

x = 10^(−pH) for an acid, or 10^(pH − 14) for a base K = x² ÷ (C − x)

x is the concentration of each ion at equilibrium, and C − x is what stays un-ionised.

A pH meter reading usually carries two decimals, which limits K to about two significant figures.

What is Ka for a 0.1 mol/L acid with a pH of 2.88?

1.76 × 10⁻⁵, which is close to ethanoic acid.

How is this done for a weak base?

Choose weak base: the calculation uses pOH, 14 − pH, in place of pH, and returns Kb.