📖 What's the chemistry?
Q1 — thermometric titration (continuous variation)
Neutralisation is exothermic: OH⁻(aq) + H⁺(aq) → H₂O(l). Seven mixes of acid P and
alkali Q keep the total volume at 50 cm³, so the temperature rise is largest at the mix
where neither reagent is in excess.
- Rising line: Q is limiting — more Q, more heat. Falling line: P is limiting — extra Q just soaks up the heat.
- The two straight best-fit lines intersect at the equivalence mix, 28.0 cm³ of Q (with 22.0 cm³ of P).
- moles OH⁻ = 1.25 × 0.0280 = 0.0350 mol = moles H⁺ → [H⁺] = 0.0350 ÷ 0.0220 = 1.59 mol/dm³.
- [H⁺] ≈ 2 × the acid's 0.75 mol/dm³ → each molecule gives TWO protons → sulfuric acid.
Q2 — gravimetric purity + QA
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Both products of decomposition escape, so every x mol of
H₂O lost carries x mol of CO₂ with it: mass lost = 18x + 44x = 62x g.
- 0.44 g lost → x = 0.0071 mol → 2x × 84.0 = 1.19 g NaHCO₃ in 1.50 g of R → 79.5 %.
- Weigh only at room temperature — a hot tube sets up convection currents and the balance drifts.
- Residue S: acid → CO₂ (carbonate), AgNO₃ → white ppt (chloride), Ba(NO₃)₂ → nothing
(no sulfate) → impurity = NaCl. Nitric acid is used because all nitrates are soluble.
Writing answers the examiner accepts
- Say "colourless", not "clear" — a coloured solution can still be clear.
- Name the gas AND its test result: "the limewater turns milky, so the gas is carbon dioxide".
- Q3 planning: titrate with an indicator → repeat with half the NaOH volume without
indicator → evaporate to saturation, cool, filter, wash, dry.