A-LevelChemistryAtoms, molecules and stoichiometry9701/21 · May/June 2021 · Q110 Marks

Ethanedioic acid, HO₂CCO₂H, has a relative molecular mass of 90.0.

(a)(i)Explain what is meant by the term relative molecular mass.[2]
(a)(ii)State the empirical formula of ethanedioic acid.[1]
(a)(iii)Calculate how many atoms of carbon are present in 0.18 g of ethanedioic acid, HO₂CCO₂H. Show your working. atoms of carbon present =[3]
(b)(i)Solid ethanedioic acid reacts with aqueous calcium ions to make a precipitate of calcium ethanedioate, CaC₂O₄. CaC₂O₄ breaks down when heated to form calcium oxide, carbon dioxide and carbon monoxide. Construct an equation to represent the reaction of CaC₂O₄ when heated. Include state symbols.[2]
(b)(ii)Identify the type of reaction which occurs when CaC₂O₄ is heated.[1]
(b)(iii)Identify another compound containing calcium ions which will also produce carbon dioxide and calcium oxide when it is heated.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] M1 the mass of a molecule OR the (weighted) average / (weighted) mean mass of the molecule(s)
[1] M2 relative / compared to 1/12 (the mass) of an atom of carbon-12 OR on a scale in which a carbon–12 atom / isotope has a mass of (exactly) 12 (units)
[1] M1 mass of one mol of molecules
[1] M2 relative / compared to 1/12 (the mass) of 1 mol of C-12 OR which one mol C-12 (atom / isotope) has a mass of (exactly) 12 g
[1] CO2H
[1] M1 no mole ethanedioic acid 0.18/90 = 0.0020
[1] M2 no mole ethanedioic acid × 2 0.0020 × 2 = 0.0040
[1] M3 no mole ethanedioic acid × 6.02 × 1023 2.4 x 1021
[1] M1 correct formulae CaC2O4(s) → CaO(s) + CO2(g) + CO(g)
[1] M2 balancing equation AND state symbols.
[1] (thermal) decomposition OR disproportionation
[1] calcium carbonate / CaCO3

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