Page 15 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 12

m
The mass spectrum of dichloromethane consists of peaks at — values of 84, 86
e
and 88. Only H, C, Cl and Cl isotopes are present in the molecule.
1
35
12
37
(a) Write the formula of ions responsible for the above peaks.
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(b) Calculate the percent abundance of the above peaks if the natural abundance
ratio of Cl and Cl is 3 : 1.
35
37
(c) Sketch and label a mass spectrum for 1,1-dichloroethane showing the above
peaks.
Answer:
Term
1 Exam Tips
Exam Tips
Write all the possible combination of chlorine isotopes in the dichloromethane
mass
molecule, then add up their respective atomic mass to get the —–—– ratio,
m charge
—–.
e
m 12 1 35 + m 12 1 35 37 + m 12 1 37 +
e
e
e
(a) — 84: C H 2 Cl 2 , — 86: C H 2 Cl Cl , — 88: C H 2 Cl 2
m 3 3
(b) — 84: — × — = 0.5625
e
4
4
m
1
3
— 86: 2 × — × — = 0.375
e
4
4
m
1
1
— 88: — × — = 0.0625
e
4
4
m
m
m
— 84: — 86: — 88 = 56.25% : 37.50% : 6.25%
e
e
e
(c) Percent abundance
56.25
37.50
6.25
84 86 88 m/e

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01 STPM Q&A Chem T1 layout.indd 10 4/19/19 9:28 AM