CIE iGCSE Co-Ordinated Science C3.2 Relative masses of atoms and molecules Exam Style Questions Paper 2
▶️ Answer/Explanation
Relative atomic mass is defined as the average mass of an atom of an element compared to one-twelfth of the mass of an atom of carbon-12. The correct wording is “the mass of \(\frac{1}{12}\) the mass of one atom of \(^{12}\text{C}\)”. Option C is correct.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
From the balanced equation, 1 mole of PbO produces 1 mole of PbSO₄. The molar masses are: PbO = 207 + 16 = 223 g/mol, and PbSO₄ = 207 + 32 + (4 × 16) = 303 g/mol. To find the mass of PbO needed for 22.6 g of PbSO₄: (223/303) × 22.6 = 16.6 g. The stoichiometry is 1:1, so the same number of moles is involved.
✅ Answer: (B)
✅ Answer: (B)
▶️ Answer/Explanation
The molar mass of \( C_9H_{20} \) is \( (9\times12)+(20\times1)=128 \text{ g/mol} \), so 64 g is \( \frac{64}{128}=0.5 \) mol of nonane.
From the equation, each mole of nonane needs 14 mol of \( O_2 \), so 0.5 mol needs \( 0.5\times14=7 \) mol of \( O_2 \).
Mass of oxygen \( = 7 \times 32 = 224 \text{ g} \) (using \( M_r(O_2)=32 \)).
✅ Answer: (B)
From the equation, each mole of nonane needs 14 mol of \( O_2 \), so 0.5 mol needs \( 0.5\times14=7 \) mol of \( O_2 \).
Mass of oxygen \( = 7 \times 32 = 224 \text{ g} \) (using \( M_r(O_2)=32 \)).
✅ Answer: (B)
▶️ Answer/Explanation
Hydrogen has a relative atomic mass of 1, so 1 g of hydrogen (= 1 mole) contains \(6 \times 10^{23}\) atoms, which is the Avogadro constant.
Helium has a relative atomic mass of 4, meaning each helium atom is 4 times heavier than a hydrogen atom.
So 1 g of helium contains 4 times fewer atoms than 1 g of hydrogen: \(\frac{6 \times 10^{23}}{4} = 1.5 \times 10^{23}\).
✅ Answer: (A)
Helium has a relative atomic mass of 4, meaning each helium atom is 4 times heavier than a hydrogen atom.
So 1 g of helium contains 4 times fewer atoms than 1 g of hydrogen: \(\frac{6 \times 10^{23}}{4} = 1.5 \times 10^{23}\).
✅ Answer: (A)
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