A pipette measures a fixed volume of alkali into a conical flask. The burette delivers acid while a suitable indicator shows the end point. The volume delivered is the final burette reading minus the initial reading.
A rough titration helps locate the end point. Accurate repeats should agree closely. Calculate the mean from concordant titres; check any result that differs substantially before deciding which measurements to use.
Adding distilled water to the flask after pipetting the alkali dilutes it but leaves its number of moles unchanged. Rinse the burette with the acid it will contain and the pipette with the alkali it will measure.
Higher tier: multiply the known concentration by the pipetted volume in dm³ to find moles of alkali. Apply the balanced neutralisation equation to find moles of acid. Divide by the mean acid titre in dm³ to find its concentration.
Hydrochloric acid reacts 1:1 with sodium or potassium hydroxide. Sulfuric acid reacts 1:2. To convert an acid concentration in mol/dm³ to g/dm³, multiply by its relative formula mass.
Worked example
For an initial reading of 0.30 cm³ and a final reading of 24.60 cm³, the delivered titre is 24.30 cm³. Two concordant titres of 24.20 and 24.30 cm³ give a mean of 24.25 cm³.
Higher: 25.00 cm³ of 0.100 mol/dm³ NaOH contains 0.00250 mol. If 25.00 cm³ of H₂SO₄ neutralises it, 0.00125 mol of acid is required. Its concentration is 0.0500 mol/dm³.
Specification references
AQA 8462 section 4.4.2.5, required practical 2; Edexcel 1CH0 Issue 4 sections 3.18 and 5.9C. Both tiers study the method; AQA 4.4.2.5 and Edexcel 5.8C, 5.10C require concentration calculations at Higher only.
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Why should a student exclude an anomalous titre when calculating the mean, and how does the neutralisation equation affect the final concentration?
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Check your explanation for
Identify the titres that agree and explain the selection.
Subtract each initial reading from its final reading.
For Higher tier, explain how acid and alkali moles are related.
Example answer
The mean should use concordant accurate titres. Each titre is final minus initial reading. For Higher tier, the balanced equation gives the acid:alkali mole ratio before the unknown concentration is calculated.
An answer to improve
Include the rough run and every accurate run in the average, then assume all acids react 1:1 with alkalis.
Improve your answer
Use the burette readings and reacting mole ratio to check the claim.
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