Page 18 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Answer: C
Exam Tips
Exam Tips
Calculate the number of moles of Na ions in both solutions, then find the
+
total number of moles of Na ions.
+
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MV
Number of moles = ——–
1000
(0.13 × 200)
2(0.07 × 0.8)
The number of moles of Na ion = ————–– + ————–—
+
1000 1000
= 0.138 Term
Total volume of solution = 130 + 70 = 200 cm = 0.20 dm 3
3
Number of moles
Concentration of Na ion = ————–——– 1
+
Volume (dm )
–3
0.138
= ——–
0.20
= 0.69 mol dm –3
Common Errors
Students forget that each mole of sodium sulphate has two moles of sodium
ions.

Question 5
Hydrogen sulphide gas has the odour of rotten eggs. It burns in air to form
sulphur dioxide.
2H 2 S(g) + 3O 2 (g) : 2H 2 O(l) + 2SO 2 (g)
If 6 dm hydrogen sulphide is burnt in 12 dm oxygen gas, what is the final
3
3
volume of the gaseous mixture?
A 6 dm
3
B 9 dm
3
C 12 dm 3
D 21 dm 3
Answer: B
From the equation, 2 mol of H 2 S reacts with 3 mol of O 2 to produce 2 mol of
SO 2 .
Since the volume of gas is directly proportional to the amount of gas (V ∝ n),
6 dm of H 2 S reacts with 9 dm of O 2 to produce 6 dm of SO 2 .
3
3
3
Volume of excess O 2 gas = 12 – 9 = 3 dm
3
Total volume of gas = 3 + 6 = 9 dm 3
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01 STPM Q&A Chem T1 layout.indd 13 4/19/19 9:28 AM