Titrations and concentration calculations
Chemistry · GCSE · The School
A titration finds exactly how much of one solution reacts with another
A known volume of acid is measured into a flask with an indicator. Alkali of known concentration is added slowly from a burette until the indicator just changes colour — the END POINT, where the reaction is exactly complete. The volume of alkali used is read from the burette. Repeating the titration checks the result is reliable rather than a one-off reading.
Why the method is so careful about volumes
A pipette measures a fixed, precise volume of acid, more accurately than a measuring cylinder. The burette is read from the bottom of the meniscus, and titrations are usually repeated until two results agree within 0.10 cm³ — called concordant results. Precision matters here because the whole calculation afterwards depends on the volume being right.
Concentration in mol/dm³
Concentration = moles ÷ volume in dm³. A solution containing 0.5 mol of a solute dissolved in 2 dm³ of water has a concentration of 0.5 ÷ 2 = 0.25 mol/dm³. Rearranged, moles = concentration × volume — the version used most often once a titre volume is known.
A worked titration calculation
25 cm³ of NaOH solution exactly neutralises 30 cm³ of 0.1 mol/dm³ HCl. Moles of HCl = 0.1 × (30 ÷ 1000) = 0.003 mol. Since HCl and NaOH react 1:1, moles of NaOH = 0.003 mol too. Concentration of NaOH = 0.003 ÷ (25 ÷ 1000) = 0.12 mol/dm³. Every step converts volume to dm³ first — the single most common place marks are lost.
Concentration in g/dm³
Some questions ask for concentration in grams per dm³ instead of moles per dm³. Convert by multiplying: mass in g/dm³ = concentration in mol/dm³ × molar mass. For the NaOH above (molar mass 40 g/mol): 0.12 × 40 = 4.8 g/dm³. Always check which unit a question asks for before converting — losing a mark for the right number in the wrong unit is a common and avoidable mistake.
Why an indicator is chosen, not just added
The indicator must change colour sharply and clearly AT the end point, and not react with either the acid or the alkali itself. Universal indicator is unsuitable for a titration because it changes gradually through many colours — phenolphthalein or methyl orange, which snap between two colours at one point, are used instead. Choosing the wrong indicator can make the end point impossible to see precisely.