Page 17 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
0.20(200)
MV
Number of moles of Cl ions from NaCl = ——– = ————– = 0.04
–
1000 1000
Total number of moles of Cl ions = (0.02 + 0.04) = 0.06 mol
–
0.06 mol
–
Concentration of Cl ions = ————– = 0.30 mol dm –3
0.20 dm 3
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Question 3
3
10 cm of a gaseous hydrocarbon X is mixed with 90 cm of oxygen and burned
3
completely. When the mixture is cooled, the total volume of gas is 70 cm .
3
After the gaseous mixture has been passed through concentrated potassium
hydroxide solution, 40 cm of gas remains. What is the molecular formula of W?
3
Term
A C 2H 6 C C 3H 8
1 B C 3H 6 D C 4H 8
Answer: C
y
y
C x H y + (x + — ) O 2 : xCO 2 + — H 2 O
10 cm 3 4 2
Volume of excess O 2 + volume of CO 2 formed = 70 cm 3
Since KOH absorbs an acidic gas, the residual gas is O 2 .
Volume of excess O 2 = 40 cm
3
Volume of CO 2 formed = 70 – 40 = 30 cm
3
Since 1 mole of C x H y produces x moles of CO 2 ,
30
x = —– = 3
10
y
Since 1 mole of C x H y reacts with (x + — ) moles of O 2 ,
4
Volume of O 2 used = 90 – 40 = 50 cm Exam Tips
3
Exam Tips
y 50
(x + — ) O 2 = —– = 5
4 10 Compare the volume ratio of
Substituting x = 3 into the equation, y = 8 gases to obtain the mole ratio.
Molecular formula of X is C 3 H 8 .

Question 4
What is the Na ion concentration in the solution formed by mixing 130 cm
3
+
of 0.20 mol dm NaCl solution with 70 cm of 0.80 mol dm Na 2 SO 4 solution?
3
–3
–3
A 0.41 mol dm –3
B 0.50 mol dm
–3
C 0.69 mol dm –3
–3
D 1.00 mol dm
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01 STPM Q&A Chem T1 layout.indd 12 4/19/19 9:28 AM