Page 45 - Q & A STPM 2022 Chemistry

Chemistry STPM Answers
When the pressure on the gas is high, the molecules are pushed very close
to one another and the volume occupied by the gas is small. In such case
the total volume of the gas molecules cannot be ignored when compared
to the volume of the container where the gas is placed. However, at very
low pressure, the volume occupied by the gas is so large that the total
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volume of the gas molecules is negligible.
At low temperatures, the kinetic energy of the gas molecules is low
and the intermolecular forces between the molecules become significant.
However, at high temperatures, the kinetic energy of the gas molecules is
very high and the intermolecular forces between them can be ignored.
(b) (i) Composition by mass of nitrogen = 100 – 12.5
= 87.5%
87.5 12.5
Mol ratio of N : H = :
14 1
= 6.25 : 12.5
= 1 : 2
Empirical formula of X is NH .
2
Let the molecular formula be (NH )n.
2
Equation of decomposition:
n
(NH )n ➝ N + nH 2
2
2
2
From the equation:
(
12.5 cm of X would produce 12.5 n + n ) cm product
3
3
2
(
12.5 n + n ) = 37.5
2
n = 2
Molecular formula of X is N H .
2
4
(ii) The oxidation number of H in X = +1.
Let the oxidation number of N = y.
2y + (1 × 4) = 0
y = –2
(iii) Lewis diagram of X is:
H H
H N N H
(c) Toluene is a hydrocarbon. The intermolecular forces are the van der
Waal’s forces.
Ethanol on the other hand contains the highly polar O—H bond.
CH — CH — O — H
3 2
The intermolecular forces are the van der Waal’s forces and hydrogen
bonds.

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08 Ans STPM Q&A Chem T1 Layout.indd 540 4/19/19 10:48 AM