Titrations and Calculations from Titration Data — AQA GCSE Chemistry (Higher / Separate)

Titration is an accurate technique to find the volume of acid and alkali that exactly react, and hence calculate an unknown concentration.

The titration method

1. Use a pipette to measure a known volume of one solution (e.g. alkali) into a conical flask, and add an indicator (e.g. phenolphthalein or methyl orange).
2. Fill a burette with the other solution (e.g. acid) and record the starting volume.
3. Add the acid slowly, swirling, until the indicator just changes colour (the end point).
4. Record the final burette reading; the difference is the titre.
5. Repeat until you get concordant results (within 0.10 cm³) and take a mean.

Calculating concentration (Higher Tier)

1. Calculate moles of the substance you know (moles = concentration × volume in dm³).
2. Use the balanced equation ratio to find moles of the unknown substance.
3. Calculate the unknown concentration (concentration = moles ÷ volume in dm³).

Worked outline

25.0 cm³ of NaOH neutralised by 20.0 cm³ of 0.10 mol/dm³ HCl (1:1 ratio):

• moles HCl = 0.10 × (20.0 ÷ 1000) = 0.002 mol → moles NaOH = 0.002 mol.
• concentration NaOH = 0.002 ÷ (25.0 ÷ 1000) = 0.08 mol/dm³.

Exam tips

• Use a pipette (fixed volume) and a burette (variable, read to 0.05 cm³).
• Take readings until results are concordant and use a mean titre.
• Convert all volumes to dm³ in calculations.
• Indicators give a sharp colour change at the end point (universal indicator is not used).