Page 7 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Atom X has (18 – 2) = 16 electrons and hence, 16 protons. X is an element of
Group 16, which is sulphur. The nucleon number of X is (16 + 19) = 35. Since
X is a non-metal, it has low melting and boiling points.

Common Errors
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2–
For a charge of X , students wrongly minus two electrons from X instead of
adding two electrons. Hence, the proton number of X should be 16 and not 20.

1.2 Relative Atomic, Isotopic, Molecular and Formula Masses
Term
Section A Multiple-choice Questions
1
Question 1

When 2.40 g of metal X is added to an aqueous solution containing Y ions,
2+
3+
15.53 g metal Y is precipitated. In this reaction, X ions are produced. What is
the relative atomic mass of X if the relative atomic mass of Y is 207?
A 27.0
B 48.0
C 55.6
D 63.5

Answer: B
2X + 3Y : 3Y + 2X 3+
2+
Mass
15.53
The number of moles of Y = ————————– = ——– = 0.075
Relative atomic mass 207
2
The number of moles of X = — × 0.075 = 0.05 Exam Tips
Exam Tips
3
2.40
Relative atomic mass of X = —— = 48.0 Write a balanced ionic
0.05 equation for yhe reaction
Question 2
The mass of nitrogen in 5.22 g of a compound X(NO 3 ) 2 is 0.56 g. What is the
relative atomic mass of X? [Relative atomic mass: C = 12, N = 14, O = 16]
A 65 C 137
B 84 D 207

Answer: C
0.56
Exam Tips
The number of moles of nitrogen = —— Exam Tips
14
= 0.04 Determine the number of
moles of nitrogen in the
compound.

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01 STPM Q&A Chem T1 layout.indd 2 4/19/19 9:28 AM