Practical Chemistry and Laboratory Skills
Domain 15: Practical Chemistry and Laboratory Skills
This domain tests qualitative salt analysis, volumetric calculations, purification, and safe, accurate bench technique.
Core rules
- Cation analysis uses group reagents: group I HCl (Pb2+, Ag+, Hg2+), group II H2S in acidic medium, group III NH4OH/NH4Cl, group IV H2S in basic medium, group V (NH4)2CO3. Confirmatory tests follow separation.
- Anion analysis: carbonate/bicarbonate give CO2 with dilute acid; sulphite gives SO2; sulphide gives H2S; chloride gives white AgCl soluble in ammonia; sulphate gives white BaSO4 insoluble in acid.
- Flame tests: Na golden yellow, K lilac, Ca brick red, Sr crimson, Ba apple green, Cu blue-green. Clean the wire with conc. HCl between tests.
- Titration: use a primary standard (e.g. oxalic acid, Na2CO3), rinse the burette with the titrant, read the bottom of the meniscus, and choose an indicator matched to the titration type.
- Purification: crystallisation, sublimation, distillation, fractional distillation, chromatography. Purity can be checked by melting/boiling point.
Worked example
25.0 mL of NaOH is neutralised by 20.0 mL of 0.100 M HCl: NaOH + HCl -> NaCl + H2O. Moles HCl=0.0200 x 0.100=2.00 x 10^−3 mol, so moles NaOH is the same. [NaOH]=2.00 x 10^−3/0.0250=0.0800 M. Use the burette volume that corresponds to the endpoint, not the initial reading.
Traps
- Adding group reagents in the wrong order and precipitating several groups together.
- Failing to clean the flame-test wire.
- Reading the top of the meniscus or parallax.
- Using phenolphthalein for a strong acid–weak base titration without considering endpoint pH.
- Reporting a titre without subtracting the initial burette reading.