Calculating Concentration

Once a mean titre has been found, the unknown concentration is calculated in three steps.1. Moles of the known solution: amount = concentration × volume (dm³). This is usually the solution in the conical flask (measured with a pipette) or the standard solution in the burette.2. Mole ratio: write the balanced equation and use it to find the amount of the other reactant.3. Unknown concentration: concentration = amount ÷ volume (dm³) of that solution.Worked example: 25.0 cm³ of 0.100 mol dm^-3 sodium hydroxide was titrated with hydrochloric acid. The mean titre was 22.50 cm³. Find the concentration of the acid.NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)Step 1: amount of NaOH = 0.100 × (25.0 ÷ 1000) = 2.50 × 10^-3 molStep 2: ratio NaOH : HCl = 1 : 1, so amount of HCl = 2.50 × 10^-3 molStep 3: concentration of HCl = 2.50 × 10^-3 ÷ (22.50 ÷ 1000) = 0.111 mol dm^-3Use the mean titre of the concordant results, not a single reading and never the rough titre. Keep unrounded values in your calculator between steps and give the final answer to 3 significant figures, matching the data.Setting the working out in these three labelled steps also earns method marks if an arithmetic slip is made.