Practical Chemistry
What this domain tests
Volumetric analysis and calculations, titration procedure and indicator choice, qualitative salt analysis, simple organic preparations and the reporting of project work.
Core rules
- Molarity and normality: use n-factors correctly. KMnO₄ in acid medium n = 5, K₂Cr₂O₇ n = 6, oxalic acid n = 2, Mohr's salt n = 1.
- Indicators: KMnO₄ is self-indicating; K₂Cr₂O₇ titrations use diphenylamine or starch–iodide; iodine titrations use starch; acid–base titrations use phenolphthalein or methyl orange according to the pH at the endpoint.
- Cation groups: Group I with dilute HCl (Pb²⁺, Ag⁺, Hg₂²⁺); Group II with H₂S in acidic medium (Cu²⁺, Cd²⁺, Bi³⁺); Group III with NH₄OH (Al³⁺, Fe³⁺); Group IV with H₂S in ammoniacal medium (Zn²⁺, Mn²⁺, Ni²⁺); Group V with (NH₄)₂CO₃ (Ba²⁺, Sr²⁺, Ca²⁺); Group VI (Mg²⁺, NH₄⁺).
- Anion tests: carbonate gives CO₂ with acid; sulphite gives SO₂, which decolourises KMnO₄; sulphide turns lead acetate paper black; chloride gives a white precipitate with AgNO₃ that dissolves in NH₄OH; sulphate gives a white precipitate with BaCl₂ that is insoluble in concentrated HCl; nitrate gives the brown ring test.
- Organic preparations: acetanilide from aniline and acetic anhydride; dibenzalacetone from benzaldehyde and acetone. Record yield, melting point and purity.
Worked example
25.0 mL of 0.1 M oxalic acid is titrated against KMnO₄ and 20.0 mL is required. The reaction is 2MnO₄⁻ + 5H₂C₂O₄ + 6H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O, so moles of MnO₄⁻ = (2/5) × 0.0025 = 0.0010 mol. Molarity = 0.0010/0.020 = 0.05 M. The 2:5 ratio, not a 1:1 ratio, is the key step.
Traps that cost marks
KMnO₄ n-factor changes with the medium. Group II and Group IV both use H₂S but at different pH, so the conditions matter. Read the burette to two decimal places and use concordant readings. In salt analysis, confirm a cation with a specific test rather than relying only on the group precipitate.