Practical Chemistry
Practical Chemistry tests volumetric calculations, salt analysis, organic functional-group tests, chromatography and lab-record/viva reasoning.
Volumetric analysis
Use M1V1/n1 = M2V2/n2 for titration stoichiometry. A primary standard is pure, stable and non-hygroscopic. Choose an indicator whose pH range matches the equivalence point: phenolphthalein for weak acid–strong base, methyl orange for strong acid–weak base. Read the bottom of the meniscus at eye level and use concordant titres.
Salt analysis
Cations are identified by group reagents and confirmatory tests; anions by dilute acid, BaCl2, AgNO3 and other specific tests. Interfering ions must be removed before confirmatory tests. For example, carbonate gives effervescence with dilute acid, sulphate gives a white BaSO4 precipitate insoluble in HCl.
Organic tests and chromatography
Tollens/Fehling for aldehydes, iodoform for methyl ketones, FeCl3 for phenols, and sodium bicarbonate for carboxylic acids. In paper chromatography, Rf = distance travelled by solute / distance travelled by solvent front; Rf is characteristic under fixed conditions.
Worked example
25.0 mL of HCl is neutralised by 20.0 mL of 0.100 M NaOH. Since n1 = n2 = 1, M(HCl) = (0.100 × 20.0)/25.0 = 0.0800 M. Moles NaOH = 0.00200 mol, so moles HCl = 0.00200 mol in 0.0250 L. Check units and significant figures before writing the record.
Mark-losing traps
Using the wrong stoichiometric ratio, choosing an unsuitable indicator, reading the top of the meniscus, reporting an unconverged titration, and inverting the Rf ratio cost marks. In viva, state observations and inferences separately.