Practical Chemistry and Project Work
This section covers Practical Chemistry and Project Work: qualitative salt analysis, volumetric analysis, pH and kinetics experiments, chromatography, and the planning and reporting of a chemistry project. The exam tests accurate observations, calculations and identification of sources of error.
Core rules
- Cation analysis uses group reagents: dilute HCl for group I, H₂S in acidic medium for group II, NH₄OH/NH₄Cl for group III, H₂S in basic medium for group IV, and ammonium carbonate for group V.
- In titration, use a primary standard such as oxalic acid or potassium hydrogen phthalate; rinse the burette with the titrant and the pipette with the solution being measured.
- Record burette readings to 0.05 mL and use the concordant readings.
- In project work, state a testable aim, control variables, repeat trials, and analyse errors quantitatively.
Worked example
25.0 mL of NaOH solution is neutralised by 20.0 mL of 0.100 M HCl. Moles of HCl = 0.0200 × 0.100 = 2.00 × 10⁻³ mol. Since NaOH + HCl → NaCl + H₂O, moles of NaOH = 2.00 × 10⁻³ mol. Molarity of NaOH = 2.00 × 10⁻³/0.0250 = 0.0800 M. The calculation uses the 1:1 stoichiometry and the exact volume of NaOH delivered by pipette.
Traps
Do not read the burette from the top; do not use the endpoint as the equivalence point without choosing the indicator correctly; do not forget to subtract the initial burette reading; do not report a single titre when concordant readings are required; in salt analysis, do not add group reagents out of order.