CAIE A Level Chemistry 9701 · Atoms, molecules and stoichiometry

Atoms, molecules and stoichiometry: suggest question

Suggest6 marksChallenge

Benzene, $\ce{C6H6}$ , and ethyne, $\ce{C2H2}$ , have the same empirical formula, $\ce{CH}$ . A student is given a third hydrocarbon, X, and is told only that its empirical formula is also $\ce{CH}$ . The student also has samples of anhydrous iron(III) sulfate and hydrated magnesium sulfate.

(a) Suggest what is meant by the terms empirical formula and molecular formula.

(b) Suggest why benzene and ethyne have the same empirical formula even though they are different compounds.

(c) Suggest what further information is needed for the student to determine the molecular formula of X from its empirical formula.

(d) Iron forms ions with a charge of 3+ and the sulfate ion is $\ce{SO4^2-}$ . Suggest the formula of iron(III) sulfate.

(e) Suggest what is meant by the term water of crystallisation.

How this question is marked

6 marks, so the mark scheme is looking for about 6 separate creditworthy points. Making the same point twice in different words earns it once.

Suggest means: Apply what you know to an unfamiliar situation. There may be more than one acceptable answer. Reason from the principle you have learned to the new context, and say which principle you are using.

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