Page 14 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
(b) (i) 2Fe + 3Cl 2 : 2FeCl 3
(3.25 – 1.12)
(ii) The number of moles of Cl 2 = —————
71.0
= 0.03

(iii) The number of moles of Fe = 0.02
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1.12
Relative atomic mass of Fe = ——
0.02
= 56.0
(iv) The two samples have different isotopic compositions because the Term
relative atomic masses of the two samples are vary.

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Question 11

(a) Combustion of 1.18 g sample of an organic compound P yields 1.76 g CO 2
and 0.54 g H 2 O.
(i) Determine the empirical formula of P.
(ii) If the relative molecular mass of P is 118, what is the molecular formula
of P?

Answer:
12
(a) (i) Proportionate mass of carbon in CO 2 = —– × 1.76
44
= 0.48 g
2
Proportionate mass of hydrogen in H 2 O = —– × 0.54
18
= 0.06 g
Mass of oxygen = 1.18 – 0.48 – 0.06
= 0.64 g
0.48
0.06
0.64
Mole ratio of C : H : O = —— : —— : ——
12 1 16
= 1 : 1.5 : 1
= 2 : 3 : 2
Empirical formula of P is C 2 H 3 O 2 .
(ii) (C 2 H 3 O 2 ) n = 118
59n = 118
n = 2
Molecular formula of P is C 4H 6O 4.

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