Ethanedioic (oxalic) acid crystallises with water: C₂H₂O₄·xH₂O. Solution A holds 8.90 g of the crystals per dm³. Titrating 25.0 cm³ of A with B = 0.150 mol/dm³ NaOH and thymolphthalein C (colourless in acid, pale blue in alkali) gives an end-point when one drop of B leaves a pale blue that does not change on swirling.
C₂H₂O₄ + 2NaOH → C₂O₄Na₂ + 2H₂O
Average titre 22.93 cm³ → 0.150 × 0.02293 = 0.00344 mol NaOH; ÷2 = 0.00172 mol acid; ×40 = 0.0688 mol/dm³; × Mr 90 = 6.19 g of anhydrous acid. The rest of the 8.90 g is water: 2.71 g = 0.151 mol → ratio 0.0688 : 0.151 ≈ 1 : 2 → x = 2.
Mg + 2H⁺ → Mg²⁺ + H₂. The gas is collected by downward displacement of water in an inverted 100 cm³ measuring cylinder. A dropper of air sets the start level to the 10 cm³ mark, so the volume COLLECTED = final − initial = 56.0 − 10.0 = 46.0 cm³. Magnesium is limiting — no metal is left at the end.
The graph of Table 2.2 plateaus at 92 cm³ (acid limiting). Rate over the first 25 s = volume at 25 s ÷ 25 ≈ 57.5/25 = 2.30 cm³/s (3 s.f.). Halving the limiting acid's concentration halves the moles → line X levels off at ≈46 cm³.
Pencil start line, capillary spots of the sample AND the three pure amino acids, solvent kept below the line, mark the solvent front, dry, then a locating agent — amino acids are colourless. Matching spot heights (same Rf) identify the three acids.