25.1.4—[H+(aq)] and pH values

For a strong monoprotic acid, [H⁺]≈c; for a strong alkali, [OH⁻]≈c and use Kw to obtain [H⁺]. For a weak acid, use Ka and the equilibrium concentration rather than assuming complete dissociation.

Include stoichiometric H⁺/OH⁻ numbers for polyprotic or multi-hydroxide species when appropriate. Check that the weak-acid approximation is small relative to the initial concentration.

0.010 mol dm⁻³ HCl has pH 2.00; 0.010 mol dm⁻³ NaOH has pOH 2.00 and pH≈12.00 at 25 °C.

Do not use strong-acid shortcuts for weak acids or forget that pH + pOH depends on temperature.