Page 22 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 10

In an experiment, 19.6 g of concentrated sulphuric acid is found to react
completely with 17.0 g of sodium nitrate, NaNO 3 .
[Relative atomic mass: H = 1, N = 14, O = 16, Na = 23, S = 32]
(a) Calculate the number of moles of sulphuric acid and sodium nitrate used.
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(b) Deduce a chemical equation for the reaction.
Answer:
(a) Relative molecular mass of H 2 SO 4 = [2(1) + 32 + 4(16)] = 98 Term
Mass 19.6
The number of moles of H 2 SO 4 = = = 0.20
Relative molecular mass 98
Relative formula mass of NaNO 3 = [23 + 14 + 3(16)] = 85 1
Mass 17.0
The number of moles of NaNO 3 = = = 0.20
Relative molecular mass 85
(b) Since 0.20 mol H 2 SO 4 reacts with 0.20 mol NaNO 3 ,
1 mol H 2 SO 4 reacts with 1 mol NaNO 3 .
H 2 SO 4 + NaNO 3 : NaHSO 4 + HNO 3
Question 11
A student was told that the white solid in a beaker is a mixture of barium oxide
and calcium oxide. The mass of the white solid is 10.0 g. The student found that
the white solid dissolves in exactly 100.0 cm of 2.0 mol dm hydrochloric acid.
3
–3
[Relative atomic mass: O = 16, Ca = 40, Ba = 137]
(a) Write two separate chemical equations for the reaction of both oxides with
hydrochloric acid.
(b) Determine the percentage of barium oxide in the mixture.

Answer:
(a) BaO + 2HCl : BaCl 2 + H 2 O
CaO + 2HCl : CaCl 2 + H 2 O
(b) Let the mass of BaO = x g; mass of CaO = 10.0 – x
BaO + 2HCl : BaCl 2 + H 2 O
x —–
2x
—–
153 153
CaO + 2HCl : CaCl 2 + H 2 O
2(10 – x)
10 – x —–——–
—–—
56 56
2.0 × 100.0
Total number of moles of HCl = ——–——
1000
2x
2(10 – x)
—– + ——–—– = 0.20
153 56
Solving, x = 6.94
6.94
Percentage of BaO = —— × 100% = 69.4%
10
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01 STPM Q&A Chem T1 layout.indd 17 4/19/19 9:28 AM