O-LevelChemistryAcids, Bases & Salts5070/22 · May/June 2015 · Q610 Marks

Ammonium carbonate, (NH4)2CO3, is a white solid that is a component of ‘smelling salts'. It decomposes when it is heated. (NH4)2CO3(s) → 2NH3(g) + H2O(g) + CO2(g)

(a)A sample of ammonium carbonate is heated strongly until it all decomposes. Suggest what you would observe during the experiment.[1]
(b)Describe how you would show that both ammonia and carbon dioxide are formed in this decomposition.[4]
(c)Ammonium carbonate is soluble in water but zinc carbonate is insoluble in water. Describe how you would prepare a sample of pure, dry zinc carbonate using a solution of ammonium carbonate.[3]
(d)Excess ammonium carbonate reacts with phosphoric acid, H3PO4. Construct an equation for this reaction.[2]

Mark scheme

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

[1] (a) white solid disappears/pungent smell/condensation/colourless droplets (1)
[4] (b) For ammonia: Test with (moist red) litmus (1)
turns blue (1)
OR
Test (with stopper/glass rod from) (concentrated) HCl (1)
white smoke/white fumes(1)
For carbon dioxide: Test with lime-water (1)
Goes milky/cloudy/white precipitate/goes white (1)
[3] (c) Add soluble zinc compound/zinc chloride/zinc sulfate/zinc nitrate/other named soluble zinc compound (1)
Filter (1)
NOTE: This mark can only be scored for filtration directly after mixing the reagents and implying that it is the solid that is on the filter paper
Wash and (air) dry residue (1)
[2] (d) 3(NH4)2CO3 + 2H3PO4 → 2(NH4)3PO4 + 3CO2 + 3H2O
Correct formulae (1)
Balancing - dependent on correct formulae (1)

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