CIE iGCSE Co-Ordinated Science C3.2 Relative masses of atoms and molecules Exam Style Questions Paper 2
Question
What is the definition of the relative atomic mass, Ar, of an element?
(A) the average mass of atoms of the element on a scale in which an atom of $^{12}$C has a mass of exactly 12 units
(B) the average mass of atoms of the element on a scale in which an atom of $^{1}$H has a mass of exactly 1 unit
(C) the average mass of atoms of the element on a scale in which an atom of $^{12}$C has a mass of exactly 1 unit
(D) the mass in grams of one mole of atoms of the element
▶️Answer/Explanation
Ans: A
Question
1 g of hydrogen contains 6 × 10$^{23}$ atoms.
The relative atomic mass of helium is 4.
How many atoms does 1 g of helium contain?
(A) 1.5 × 10$^{23}$
(B) 3 × 10$^{23}$
(C) 6× 10$^{23}$
(D) 2.4 × 10$^{24}$
▶️Answer/Explanation
Ans: A
Question
Carbon dioxide is produced in a reaction. It is collected in a gas syringe. The readings on the gas syringe are shown:
What is the mass of carbon dioxide gas produced?
- A: 0.0015 g
- B: 0.0018 g
- C: 0.0660 g
- D: 0.0788 g
▶️Answer/Explanation
Answer: C
Explanation: The volume of CO₂ produced is 43 cm³ – 7 cm³ = 36 cm³. Using the molar volume of a gas at room temperature (24 dm³/mol), the number of moles of CO₂ is 36 cm³ / 24000 cm³/mol = 0.0015 mol. The molar mass of CO₂ is 44 g/mol, so the mass of CO₂ produced is 0.0015 mol × 44 g/mol = 0.0660 g.
Question
Which compounds have different relative molecular masses?
A C₂H₆ and NO
B CO₂ and N₂O
C H₂O₂ and H₂S
D NH₃ and C₂H₄
▶️Answer/Explanation
Answer: D
Explanation: NH₃ (ammonia) has a relative molecular mass of 17 (14 + 3), while C₂H₄ (ethene) has a relative molecular mass of 28 (2 × 12 + 4). These two compounds have different relative molecular masses.