Titration calculations

A titration result lets you find an unknown concentration or an unknown volume. First work out the moles of the known solution using moles = concentration (mol/dm³) × volume (dm³), converting cm³ to dm³ by dividing by 1000. Next use the balanced equation to see the ratio in which the acid and alkali react. Finally divide the moles of the unknown by its volume in dm³.

Example: 25.0 cm³ of NaOH of unknown concentration is neutralised by 20.0 cm³ of 0.100 mol/dm³ HCl. The equation HCl + NaOH → NaCl + H₂O shows a 1:1 ratio, so the moles are equal. Moles of HCl = 0.100 × 0.0200 = 0.00200 mol, so the NaOH is also 0.00200 mol. Its volume is 0.0250 dm³, so its concentration is 0.00200 ÷ 0.0250 = 0.0800 mol/dm³.

Not every reaction is 1:1. In H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O the ratio is 1:2, so 0.0010 mol of H₂SO₄ reacts with 0.0020 mol of NaOH, and the alkali moles are twice the acid moles, so 0.0060 mol of NaOH would need 0.0030 mol of H₂SO₄. To find an unknown volume instead, rearrange to volume = moles ÷ concentration.