Page 9 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Answer: A
M 2 O 3 + 2Al : 2M + Al 2 O 3
Mass of oxygen in the oxide = 2.28 – 1.56
= 0.72
0.72
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The number of moles of oxygen = ——
16
= 0.045
From the chemical formula,
2
the number of moles of M = — × the number of moles of oxygen
3
Term
2
= — × 0.045
1 3
= 0.03
Mass
Relative atomic mass of M = ———————–
Number of moles
1.56
= ——
0.03
= 52

Common Errors
Students wrongly calculate the mole ratio of M in the oxide.

Question 5
The density of a gaseous compound WCl 4 is r g cm at room conditions. What is
–3
the relative atomic mass of W if the volume of one mol of gas at room conditions
is 24.0 dm ? [Relative atomic mass: Cl = 35.5]
3
A 24 000r – 142
B 12 000r – 142
C 24 000r – 71
D 6000r – 142

Answer: A
The mass of 1 mol of gas (24 dm ) = 24 000r
3
Relative molecular mass of WCl 4 = 24 000r
Relative atomic mass of W = 24 000r – 142

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01 STPM Q&A Chem T1 layout.indd 4 4/19/19 9:28 AM