Practical Chemistry and Laboratory Skills
This domain tests qualitative salt analysis, functional-group detection, titration technique, safe handling of reagents, and correct recording of observations and data.
Salt analysis
Cations are separated by group reagents: Group I (Pb²⁺, Ag⁺, Hg₂²⁺) with dilute HCl; Group II (Cu²⁺, Bi³⁺, Cd²⁺, Sn²⁺, As³⁺) with H₂S in dilute HCl; Group III (Al³⁺, Cr³⁺, Fe³⁺) with NH₄OH/NH₄Cl; Group IV (Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺) with H₂S in ammoniacal medium; Group V (Ba²⁺, Sr²⁺, Ca²⁺) with (NH₄)₂CO₃; Group VI (Mg²⁺, NH₄⁺, K⁺) by elimination. Anions: dilute-acid gases identify CO₃²⁻, SO₃²⁻, S²⁻ and NO₂⁻; SO₄²⁻ gives a white BaSO₄ precipitate insoluble in conc. HCl; halides give precipitates with AgNO₃ after acidifying with dilute HNO₃.
Functional-group tests
Lassaigne's sodium fusion detects N (Prussian blue), S (black PbS or violet colour with sodium nitroprusside) and halogens (AgNO₃ precipitate). 2,4-DNP signals C=O; Tollens and Fehling detect aldehydes; NaHCO₃ effervescence confirms a carboxylic acid but not a phenol.
Titration and safety
Use M₁V₁/n₁ = M₂V₂/n₂. Rinse the burette with the solution it will contain, read the meniscus at eye level against a white tile, and record to 0.05 cm³. Always add acid to water, never water to acid; handle bromine and permanganate in a fume hood with gloves.
Worked example
25.0 cm³ of NaOH needs 20.0 cm³ of 0.100 M HCl. Find the NaOH molarity. Moles of HCl = 0.100 × 0.0200 = 2.00 × 10⁻³ mol; the reaction is 1:1, so NaOH moles are the same. M(NaOH) = 2.00 × 10⁻³/0.0250 = 0.0800 M.
Traps that cost marks
- Adding AgNO₃ without acidifying first, so carbonate gives a false halide result.
- Using NH₄Cl alone as the Group IV reagent instead of H₂S in ammoniacal medium.
- Choosing phenolphthalein for a strong acid–weak base titration (methyl orange is correct).
- Reporting more significant figures than the burette can support.