O Levels Chemistry (5070)•5070/12/M/J/25
Explanation
Relative formula mass comparison via atomic masses
Steps:
- Identify formulas: A. CaCO₃, B. Li₂SO₄, C. MgCl₂, D. KNO₃.
- Use atomic masses: Ca=40, C=12, O=16, Li=7, S=32, Mg=24, Cl=35.5, K=39, N=14.
- Calculate Mr: A=40+12+(16×3)=100; B=(7×2)+32+(16×4)=110; C=24+(35.5×2)=95; D=39+14+(16×3)=101.
- Compare values: B has the highest at 110.
Why B is correct:
- Li₂SO₄'s Mr=110, the sum of atomic masses in its formula, exceeds others per the definition of relative formula mass.
Why the others are wrong:
- A: CaCO₃'s Mr=100, lower due to fewer heavy atoms.
- C: MgCl₂'s Mr=95, lowest from lighter elements.
- D: KNO₃'s Mr=101, slightly below B despite potassium's mass.
Final answer: B
Topic: Relative masses of atoms and molecules
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