Page 8 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
The formula of the compound indicates that the number of moles of the
compound is half that of nitrogen.
1
The number of moles of compound = —(0.04)
2
= 0.02
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5.22
Relative molecular mass of compound = ——
0.02
= 261
X + 2(14) + 6(16) = 261
Relative atomic mass of X = 137 Term

Question 3 1

121.7 g of an acid, HXO 4 is dissolved in distilled water and made up to 1 dm 3
solution. 25.0 cm of an aqueous solution of acid HXO 4 is exactly neutralised by
3
19.80 cm of 0.80 mol dm sodium hydroxide. What is the relative atomic mass
–3
3
of X? [Relative atomic mass: H = 1, O = 16]
A 77 C 108
B 91 D 127
Answer: D
Exam Tips
Exam Tips
The chemical formula of the acid indicates that it is a monoprotic acid. Hence,
the number of moles of acid is equal to the number of moles of the alkali.

HXO 4 + NaOH : NaXO 4 + H 2 O
Using the equation,
a
——– = — where M a, M b = molarity of acid and base respectively
M aV a
b
M b V b a, b = number of moles of acid and base in the equation
19.80(0.80)
Molarity of HXO 4 = ————— = 0.6336 mol dm
–3
25.0
121.7
Relative molecular mass of HXO 4 = ——— = 192
0.6336
Relative atomic mass of X = 192 – [1 + 4(16)] = 127
Question 4
The chemical formula of an oxide is M 2 O 3 . When 2.28 g of M 2 O 3 was reduced by
aluminium, the mass of the metal M left is 1.56 g. What is the relative atomic
mass of M? [Relative atomic mass: O = 16]
A 52 C 152
B 118 D 194

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01 STPM Q&A Chem T1 layout.indd 3 4/19/19 9:28 AM