Quantitative analysis: moles, concentration and titration
The quantitative analysis standard tests your ability to perform and interpret a volumetric analysis: preparing a standard solution, using a pipette and burette accurately, calculating amounts with n = cV and n = m/M, and commenting on the precision and accuracy of the result. Excellence answers carry units and significant figures consistently and explain an error rather than merely naming it.
Core rules
- n = m/M and n = cV. Concentration is in mol L⁻¹ (identical to mol dm⁻³); V must be in litres (20.0 mL = 0.0200 L).
- Dilution: c₁V₁ = c₂V₂, because the moles of solute are unchanged.
- The balanced equation supplies the mole ratio; convert moles of the known substance to moles of the unknown before dividing by volume.
- Titres are concordant when within 0.10 mL. Discard the rough titre and any outlier, then average the concordant values and record to two decimal places.
- Burette readings are estimated to ±0.05 mL; titre = final reading − initial reading.
Worked example
A 20.0 mL pipette sample of 0.100 mol L⁻¹ NaOH is titrated with H₂SO₄; concordant titres average 18.40 mL. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O n(NaOH) = 0.100 × 0.0200 = 2.00 × 10⁻³ mol n(H₂SO₄) = 2.00 × 10⁻³ ÷ 2 = 1.00 × 10⁻³ mol c(H₂SO₄) = 1.00 × 10⁻³ ÷ 0.01840 = 0.0543 mol L⁻¹ The 2:1 ratio is the step that decides the mark; ignoring it doubles the answer.
Traps that cost marks
- Using the titre volume in mL without converting, or adding the pipette volume to the titre.
- Averaging non-concordant titres instead of repeating the titration.
- Rounding intermediate moles too early, then quoting four significant figures.
- Not subtracting the initial burette reading, or failing to rinse the burette with the titrant.
- Calling an error 'human error' instead of naming a specific effect (parallax reading the meniscus, an air bubble in the jet) and its direction.