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Cambridge IAL Chemistry study materials
Cambridge IAL Chemistry study resources

Cambridge IAL Chemistry: Advanced Practical Skills

Cambridge IAL Chemistry · Work through your exam, one topic at a time.

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Advanced Practical Skills

Advanced practical skills in Cambridge IAL Chemistry tests how reliably you can plan, carry out and evaluate an experiment. Paper 3 questions commonly involve titration, qualitative analysis, enthalpy measurement, gas collection, uncertainty and graph work, with marks for method as well as final value.

Core rules

  • Titration: rinse the burette with the titrant, fill the tip, remove air bubbles and read the bottom of the meniscus. Record burette readings to 0.05 cm3; use concordant titres within 0.10 cm3 and average only those.
  • Uncertainty: a burette reading has uncertainty ±0.05 cm3, so a titre from two readings has ±0.10 cm3. Percentage uncertainty = (uncertainty ÷ measurement) × 100. Use the total volume delivered, not the final reading alone.
  • Qualitative tests: flame tests for cations; NaOH gives coloured precipitates (Cu(II) blue, Fe(II) green, Fe(III) red-brown); carbonate gives CO2 with acid; sulfate gives white BaSO4 insoluble in acid; halides give AgX with AgNO3 acidified with HNO3.
  • Enthalpy experiments: q = mcΔT, with m as the mass of solution and c = 4.18 J g−1 K−1. Plot temperature against time and extrapolate to the mixing point to correct for heat loss.
  • Planning: identify independent, dependent and control variables; choose apparatus with suitable precision; state safety for hazardous reagents.

Worked example

25.0 cm3 of 0.100 mol dm−3 NaOH is titrated with HCl; the mean titre is 24.30 cm3. Moles NaOH = 0.100 × 25.0/1000 = 2.50 × 10−3 mol. The equation is 1:1, so moles HCl = 2.50 × 10−3 mol. [HCl] = 2.50 × 10−3 ÷ (24.30/1000) = 0.103 mol dm−3. The burette uncertainty for the titre is ±0.10 cm3, so percentage uncertainty = 0.10 ÷ 24.30 × 100 = 0.41%. Quote the concentration to three significant figures and combine this with the uncertainty in the NaOH concentration if required.

Traps

Including the rough titre in the mean; reading the top of the meniscus; forgetting to subtract the initial burette reading; using c = 4.18 J g−1 K−1 with moles instead of mass; not extrapolating the cooling curve; confusing a white precipitate with a colourless solution; and quoting impossible precision, such as 0.10345 mol dm−3 from a burette read to 0.05 cm3.