Mandatory Laboratory Experiments and Practical Skills
What the exam tests
Mandatory Laboratory Experiments and Practical Skills examines the prescribed experiments and the skills behind them: measuring, controlling variables, plotting graphs, interpreting slopes and evaluating accuracy.
Core rules
- Titration: rinse the burette with the solution it will contain, use a white tile, read the bottom of the meniscus at eye level, and take concordant results within 0.1 cm³.
- Calorimetry: use a polystyrene cup to reduce heat loss and record the maximum temperature reached.
- Rates: measure the time for a cross to disappear behind sulfur, or the volume of gas collected; change one variable at a time.
- Test for anions: carbonate gives effervescence with acid; sulfate gives a white precipitate with barium chloride; chloride gives a white precipitate with silver nitrate that darkens in light.
- Flame tests: lithium crimson, sodium yellow, potassium lilac, copper blue-green.
- Uncertainty for a scale is ± half the smallest division; percentage error = (uncertainty/measurement) × 100.
- On a graph, the slope usually gives the physical constant being measured.
Worked example
A 25.0 cm³ sample of NaOH is neutralised by 20.0 cm³ of 0.100 M HCl. Find the concentration of the NaOH. n(HCl) = cV = 0.100 × 0.0200 = 2.00 × 10⁻³ mol. NaOH + HCl → NaCl + H₂O, so the ratio is 1 : 1 and n(NaOH) = 2.00 × 10⁻³ mol. c = n/V = 2.00 × 10⁻³ / 0.0250 = 0.0800 M.
Traps
- Using the average of all titres instead of the concordant ones.
- Forgetting to rinse the pipette with the solution being measured.
- Reading a thermometer to more decimal places than the scale allows.
- Forcing a straight line through the origin when the data do not support it.
- Describing a colour change without stating the initial and final colours.