Page 23 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 12

1 dm sample of air containing carbon dioxide is passed through aqueous
3
calcium hydroxide, Ca(OH) 2 . A white precipitate is formed.
(a) (i) Name the precipitate.
(ii) Write a balanced equation for the reaction.
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(b) If 0.45 g of precipitate is formed, determine
(i) the number of moles of CO 2 present in the sample
(ii) the percentage by volume of CO 2 in the sample at s.t.p.
[Relative molecular mass of CaCO 3 = 100; Molar volume of gas = 22 400 cm
3
at s.t.p.]
Term
1 Answer:
(a) (i) Calcium carbonate
(ii) Ca(OH) 2 + CO 2 : CaCO 3 + H 2 O
(b) (i) Number of moles of CO 2 = Number of moles of CaCO 3
0.45
= —— = 4.5 × 10
–3
100
(ii) Volume of CO 2 = 4.5 × 10 × 22 400 = 100.8 cm 3
–3
100.8
Percentage of CO 2 = ——– × 100% = 10.08%
1000
Question 13
In the presence of H ions, hydroxyammonium ions NH 3 OH reduce Fe ions
+
+
3+
2+
to Fe ions. In the reaction, NH 3 OH is oxidised to dinitrogen oxide, N 2 O.
+
In an experiment, an aqueous solution, labelled as A is prepared by boiling
5.00 g of hydroxyammonium sulphate, (NH 3 OH) 2 SO 4 per dm with excess iron(III)
3
ammonium sulphate and dilute sulphuric acid. An aqueous solution labelled as
B, contains 3.0 g KMnO 4 per dm solution. 25.0 cm of H 2 SO 4 is added to 25.0 cm
3
3
3
3
of solution A. The mixture completely reacts with 31.60 cm solution B.
[Relative atomic mass: H = 1, N = 14, O = 16, S = 32, K = 39, Mn = 55, Fe = 56]
(a) Calculate the molarity of manganate(VII) ions in solution B.
(b) Calculate the concentration of iron(II) ions in solution A.
(c) Determine the number of moles of iron(III) ions required to oxidise 1 mol
of hydroxyammonium ions.
(d) Write a balanced redox equation between NH 3 OH ions and Fe ions.
3+
+
Answer:
(a) Molar mass of KMnO 4 = 39 + 55 + 4(16) = 158
Mass 3.0
The number of moles of KMnO 4 = ——–——– = —— = 0.019
Molar mass 158
Concentration of KMnO 4 = 0.019 mol dm –3
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01 STPM Q&A Chem T1 layout.indd 18 4/19/19 9:28 AM