O-LevelChemistryStoichiometry5070/22 · May/June 2019 · Q710 Marks

Ammonium iodide, NH4I, is a white solid which decomposes when heated. NH4I(s) → NH3(g) + HI(g)

(a)A small sample of ammonium iodide is heated in a test-tube. Describe how you would know when all the ammonium iodide has decomposed.[1]
(b)Calculate the volume of gas, measured at room temperature and pressure, formed when 2.90 g of ammonium iodide is completely decomposed. volume of gas ...........................................................[3]
(c)Describe a chemical test for the iodide ion. test ............................................................................................................................................ observation ...............................................................................................................................[2]
(d)Aqueous ammonium iodide reacts with aqueous bromine. Construct the ionic equation, including state symbols, for this reaction.[2]
(e)Solid ammonium iodide does not conduct electricity. Aqueous ammonium iodide conducts electricity. Explain these two observations.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] 7(a) ammonium iodide disappears / solid disappears / sample disappears / nothing left in tube
[3] 7(b) moles of ammonium iodide = 2.9/145 OR 0.02(00) (1) moles of gas = 2 × 0.02 OR 0.04(0) (1) volume of gas = (0.04 × 24) = 0.96 dm³ OR 960 cm³ (1)
[2] 7(c) (acidified aqueous) silver nitrate (1) (pale) yellow ppt (1)
[2] 7(d) 2I-(aq) + Br2(aq) → I2(aq) + 2Br¯(aq) correct formulae and balanced (1) correct state symbols – dependent on correct formulae (1)
[2] 7(e) in solid ions cannot move (1) in aqueous / solution ions can move (1)

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.