1. 1
   An antacid tablet contains 0.500 g of calcium carbonate, CaCO₃, which neutralises excess stomach acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Calcium carbonate is an ionic compound made of Ca²⁺ and CO₃²⁻ ions.

   (a)

   How many oxygen atoms are present in the calcium carbonate in one tablet?

   [1 mark]

   • A3.01 × 10²¹
   • B9.02 × 10²¹
   • C9.03 × 10²³
   • D9.02 × 10²⁴

   (b)

   What is the total number of ions in the calcium carbonate in one tablet?

   [1 mark]

   • A3.01 × 10²¹
   • B6.01 × 10²⁴
   • C1.50 × 10²²
   • D6.01 × 10²¹

   (c)

   A rival tablet contains 0.500 g of magnesium carbonate, MgCO₃, which also reacts with hydrochloric acid in a 1 : 2 mole ratio. Deduce, with a calculation, which tablet neutralises more acid.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethyl hexanoate gives pineapple-flavoured sweets their smell. Combustion analysis shows that it contains only carbon, hydrogen and oxygen, with percentage composition by mass C 66.61%, H 11.20% and O 22.19%. Mass spectrometry gives its molar mass as 144 g mol⁻¹.

   (a)

   What is the empirical formula of ethyl hexanoate?

   [1 mark]

   • AC₄H₈O
   • BC₈H₁₆O₂
   • CC₂H₄O
   • DC₆HO₂

   (b)

   What is the molecular formula of ethyl hexanoate?

   [1 mark]

   • AC₄H₈O
   • BC₁₂H₂₄O₃
   • CC₈H₁₆O₂
   • DC₉H₂₀O

   (c)

   Calculate the number of carbon atoms in 5.00 g of ethyl hexanoate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   20.0 cm³ of a gaseous hydrocarbon, CₓHᵧ, is mixed with 150.0 cm³ of oxygen (an excess) and ignited. After cooling to the original room temperature and pressure, at which the water produced is a liquid, the volume of gas remaining is 110.0 cm³. This gas is then passed through aqueous sodium hydroxide, which absorbs carbon dioxide only, and the volume falls to 50.0 cm³. All gas volumes are measured at the same temperature and pressure.

   (a)

   Deduce the volume of carbon dioxide produced and the volume of oxygen used in the reaction, and state the law that allows these gas volumes to be used as a mole ratio.

   [3 marks]

   (b)

   Determine the molecular formula of the hydrocarbon. Show your reasoning.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Washing soda is hydrated sodium carbonate, Na₂CO₃·xH₂O. A student dissolves 7.15 g of fresh washing-soda crystals in distilled water and makes the solution up to exactly 250.0 cm³ in a volumetric flask. 25.00 cm³ portions of this solution are titrated with 0.200 mol dm⁻³ hydrochloric acid: Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g). The concordant titres are 25.05 cm³, 24.95 cm³ and 25.00 cm³.

   (a)

   Determine the value of x in Na₂CO₃·xH₂O and the concentration of sodium carbonate in the volumetric flask.

   [6 marks]

   (b)

   Washing soda from a jar left open in a warm room for several months is analysed by heating instead. 5.00 g of these crystals are heated to constant mass, leaving 2.52 g of anhydrous Na₂CO₃. Calculate x for this sample, suggest why it differs from your answer in (a), and evaluate how the result would be affected if the student stopped heating before constant mass was reached.

   [6 marks]

   Total for question 4: 12 marks

End of questions