O-LevelChemistryStoichiometry5070/21 · May/June 2015 · Q610 Marks

An aqueous solution of ammonium nitrite, NH4NO2, decomposes when heated gently. NH4NO2(aq) → N2(g) + 2H2O(l)

(a)Describe how you could show that aqueous ammonium nitrite contains ammonium ions.[2]
(b)A sample of 25.0 cm³ of 0.500mol/dm³ aqueous ammonium nitrite is heated. Calculate the volume of nitrogen formed, measured at room temperature and pressure. volume of nitrogen =[3]
(c)Ammonium nitrate, NH4NO3, decomposes when heated, in a similar way to ammonium nitrite. Suggest the formulae of the two products made in this reaction.[1]
(d)Describe how a pure sample of aqueous ammonium nitrate can be prepared from dilute nitric acid.[4]

Mark scheme

Marking points from the official mark scheme (May/June 2015). The mark or mark type is shown in brackets.

[1] (a) Add sodium hydroxide (and warm) (1) Ammonia formed/gas that turns most red litmus paper blue (1)
[1] (b) Moles of NH4NO2 = 0.025 × 0.500 OR 0.0125 (1) Moles of N2 = 0.0125 (1) Volume of N2 = 0.3 dm³/300 cm³ (1)
[1] (c) N2O and H2O (1)
[1] (d) Use of ammonia/ammonium carbonate (1) Use titration/add acid or alkali via a burette to other chemical (1) Note volume of acid or alkali used / find reacting volume/find the end-point (1) Repeat without the use of an indicator (using the same volumes)/heat neutralised solution with carbon and then filter (1)

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