Cleaning products that remove limescale contain sulfamic acid (HA). Q is the product diluted ×10 (100 cm³ made up to 1.00 dm³). Titrating 25.0 cm³ of Q with P = 0.200 mol/dm³ NaOH and thymolphthalein (colourless in acid, blue in alkali) gives an end-point when one drop of P leaves a blue that stays.
HA + NaOH → NaA + H₂O
Average titre 18.70 cm³ → 0.200 × 0.01870 = 0.00374 mol NaOH = 0.00374 mol HA; ÷ 0.0250 dm³ = 0.150 mol/dm³ in Q; ×10 for the dilution → 1.50 mol/dm³ in the product. Forgetting the ×10 is THE classic error in this paper.
2HA + Na₂CO₃ → CO₂ + H₂O + 2NaA. Weigh solid HA, react with excess sodium carbonate, collect the CO₂ over water in an inverted measuring cylinder. Moles CO₂ = V/24000, moles HA = 2× that, Mr = mass ÷ moles. Because CO₂ dissolves in water, V reads low → Mr reads high; a gas syringe fixes it.
H₂ escapes, so the flask loses mass. Hotter runs start faster but level off at the same final loss. Average rate over 7 min (Expt 1) = 39 mg ÷ 7 min = 5.57 mg/min — and the units mark is a whole mark of the three!