Page 12 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 8

(a) The following mass spectrum was obtained when a gaseous sample of an
organic acid was analysed.

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Relative
abundance

12 15 16 17 28 43 60 61 m/e Term
m
(i) Suggest possible ions responsible for peaks with — values of 17, 28 1
e
and 43.
(ii) Draw a possible structural formula for this acid.
(iii) What is the relative molecular mass of the acid?

Answer:
+ m
m
+ m
16
1
16
1
12
16
12
(a) (i) — 17: O H , — 28: C O , — 43: C 2 H 3 O +
e
e
e
(ii) H O (iii) 60
& '
H9C9C9O9H
&
H
Question 9
(a) The relative atomic mass of X is 35.50.
(i) Why is the relative atomic mass of X not a whole number?
(ii) In nature, X contains two isotopes with nucleon numbers of 35 and 37.
Calculate the percent abundance of the isotopes X and X.
37
35
(iii) If X exists as a diatomic element, sketch the mass spectrum of molecule
X.
Answer:
(a) (i) X has more than one isotope.
(ii) Assume the relative abundance of X : X as x% and (100 – x)%.
37
35
35(x) + 37(100 – x)
Relative atomic mass of X = ————————–
100
35x + 3700 – 37x
35.50 = ———————–
100
7

01 STPM Q&A Chem T1 layout.indd 7 4/19/19 9:28 AM