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Understanding Ka and Acid Strength in Water

In the Bronsted‑Lowry framework, an acid donates a proton (H⁺) to the solvent, generating a conjugate base (A⁻). In water, this proton becomes part of the hydronium ion (H₃O⁺). The equilibrium constant, denoted Ka, quantifies the acid’s tendency to release protons.

Calculating Ka

Ka is determined from the equilibrium concentrations of the species involved:

Ka = [H₃O⁺][A⁻] / [HA]

From Ka to pKa

Because Ka values span many orders of magnitude, chemists use the pKa, the negative base‑10 logarithm of Ka:

pKa = –log₁₀ Ka

For example, hydrochloric acid has Ka ≈ 10⁷, giving pKa = –7. In contrast, ascorbic acid (vitamin C) has Ka = 1.6 × 10⁻¹², yielding pKa = 11.80. A lower pKa indicates a stronger acid.

Computing Logarithms

While logarithms can be tabulated or computed with a calculator, the relationship log₁₀ x = y can be inverted by raising 10 to the power y, yielding x = 10ʸ. The pKa formula is the negative of this, so a large pKa corresponds to a very small Ka.

On a scientific calculator, simply press the “log” button after entering the Ka value to obtain the pKa.