Page 20 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
mass of 1 mol gas m
m
For A, ————–——–— = —– — = 2N A
mass of 1 atom —––
2N A
mass of 1 mol gas m
m
For B, ————–——–—– = —–— = N A
mass of 1 molecule —–
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N A
volume of 1 mol gas V
V
For C, ————–——–—–– = —– — = 2N A
mass of 1 atom —––
2N A
For D, density of a substance does not depend on the number of particles. Term
1
Section B Structured Questions
Question 8
A 100 cm gaseous mixture of ethane, C 2 H 6 and butene C 4 H 8 is burnt in 500 cm 3
3
oxygen (excess) for complete combustion. After the mixture has been cooled,
the total volume of gas is 331 cm . Only 55 cm of gas remains when the gaseous
3
3
mixture is passed through concentrated NaOH solution.
(a) Write two separate equations for the combustion of both gases.
(b) Determine the volume of each gas in the mixture.

Answer:
7
(a) C 2 H 6 + —O 2 : 2CO 2 + 3H 2 O
2
C 4 H 8 + 6O 2 : 4CO 2 + 4H 2 O
(b) Assume that the volume of ethane gas = x cm ,
3
volume of butene gas = (100 – x) cm 3
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C 2 H 6 + —O 2 : 2CO 2 + 3H 2 O
2
x cm 3 2x cm 3
C 4 H 8 + 6O 2 : 4CO 2 + 4H 2 O
(100 – x) cm 3 4(100 – x) cm 3
Since NaOH absorbs CO 2 gas (acidic), volume of excess, unreacted O 2
= 55 cm
3
Total volume of CO 2 formed = 331 – 55 = 276 cm
3
Total volume of CO 2 formed = 2x + 4(100 – x) = 276
Volume of C 2 H 6 , x = 62 cm 3
Volume of C 4 H 8 = 38 cm
3

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01 STPM Q&A Chem T1 layout.indd 15 4/19/19 9:28 AM