O-LevelChemistryAcids, Bases & Salts5070/21 · Oct/Nov 2016 · Q1010 Marks

A student prepared some crystals of hydrated lithium nitrate by reacting aqueous lithium hydroxide with dilute nitric acid. LIOH + HNO3 + 2H2O → LINO3.3H2O

(a)Explain how you would carry out this procedure to obtain pure dry crystals of hydrated lithium nitrate.[4]
(b)The student used 20.0 cm³ of 0.500mol/dm³ of lithium hydroxide to prepare the crystals. Calculate the maximum mass of hydrated lithium nitrate crystals that could be made.[3]
(c)A sample of hydrated lithium nitrate is heated in a test-tube. 2LINO3.3H2O(s) → Li2O(s) + 2NO2(g) + ½O2(g) + 6H2O(l) What is observed during this reaction?[1]
(d)Explain why lithium oxide conducts electricity when molten.[1]
(e)Predict the products of electrolysis of molten lithium oxide at the anode (positive electrode), the cathode (negative electrode).[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2016). The mark or mark type is shown in brackets.

[1] add acid to alkali in flask with indicator and record volume of acid needed (to neutralise alkali) (1)
[1] repeat without indicator adding the recorded volume of acid (1)
[1] evaporate solution to point of crystallisation/leave to crystallise (1)
[1] filter off crystals and dry with filter paper/leave in a drying oven (1)
[1] mol LiOH = 0.500 × 20 / 1000 OR 0.01 mol (1)
[1] molar mass of hydrated lithium nitrate = 123 (1)
[1] mass = 123 x 0.01 = 1.23 g (1)
[1] brown fumes/colourless liquid condensing at mouth of tube/white solid forming (1)
[1] ions (are free to) move (1)
[1] anode: oxygen/O2 AND cathode lithium/Li (1)

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