O-LevelChemistryStoichiometry5070/21 · May/June 2023 · Q38 Marks

This question is about the preparation of salts.

(a)(i)Zinc chloride is a soluble salt. It is prepared by the reaction of an insoluble base with a dilute acid. The equation for this reaction is shown. ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l) A sample of 3.50g of zinc oxide is added to 50.0cm³ of 1.20mol/dm³ hydrochloric acid. Show by calculation that the zinc oxide is in excess.[3]
(a)(ii)State why it is important to use an excess of zinc oxide in this preparation.[1]
(a)(iii)Suggest how the excess zinc oxide is removed from the reaction mixture to leave only aqueous zinc chloride.[1]
(b)Barium sulfate is an insoluble salt. It is prepared using a precipitation reaction. Name two aqueous solutions that react together to give a barium sulfate precipitate. and[1]
(c)(i)Sodium nitrate is a soluble salt. It is prepared by the reaction of an acid and an alkali. Name the acid and the alkali used. acid alkali[1]
(c)(ii)Name the experimental technique used to make neutral aqueous sodium nitrate.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

3(a)(i) (moles of zinc oxide =) 3.50 /81 OR 0.0432 (1) (moles of HCl =) 0.0600 (1) idea that need 0.0864 moles of HCl/ only 0.0300 moles of ZnO react (1)
3(a)(ii) so all the acid is used up (1)
3(a)(iii) filtration (1)
3(b) barium nitrate or barium chloride AND any soluble sulfate (1)
3(c)(i) nitric acid and sodium hydroxide (1)
3(c)(ii) titration (1)

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