Page 10 - Q & A STPM 2022 Chemistry
P. 10
Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 6
The following mass spectrum is obtained for an element X which exists as a
molecule X 2. X has two isotopes P and Q with relative atomic mass a and b.
Relative
Penerbitan Pelangi Sdn Bhd. All Rights Reserved.
abundance
4 28 49
2a a + b 2b m/e Term
What is the relative abundance of isotope P and Q?
A 1 : 2 C 2 : 3 1
B 1 : 3 D 2 : 7
Answer: D
Let the abundance ratio of P and Q is x : y.
y
x
Probability of obtaining P and Q is ——— : ———
m + a a (x + y) (x + y)
— of X 2 : P P = (a + a) = 2a
e
x
x
Probability = ——– × ——–
x + y
x + y
m + a b b a Common Errors
— of X 2 : P P or P P = (a + b)
e
m
x
x
e
Probability = ——– × ——– × 2 For the — value of (a + b), students
did not multiply the relative
x + y
x + y
m + b b abundance by two (There are two
— of X 2 : P P = (b + b) = 2b
possible combinations of P P and
a b
e
x
x
a
Probability = ——– × ——– b P P.).
x + y
x + y
From the diagram,
x
x
m
at — 2a, probability = ——– × ——– = 4
x + y
x + y
e
x
x
m
at — (a + b), probability = ——– × ——– × 2 = 28
x + y
e
x + y
x
x
m
at — 2b, probability = ——– × ——– = 49
x + y
x + y
e
Since the denominator is the same in all probabilities,
x = 4 and y = 49
2
2
Hence,
1
—
relative abundance of isotope P = (4) = 2
2
1
—
relative abundance of isotope Q = (49) = 7
2
5
01 STPM Q&A Chem T1 layout.indd 5 4/19/19 9:28 AM
