O Levels Chemistry (5070)•5070/11/M/J/24
Explanation
Calculate relative formula mass of Al₂O₃ using atomic masses
Steps:
- Formula of aluminium oxide is Al₂O₃.
- Relative atomic mass of Al is 27; O is 16.
- Multiply: 2 × 27 = 54 for Al; 3 × 16 = 48 for O.
- Add: 54 + 48 = 102.
Why C is correct:
- Relative formula mass is the sum of relative atomic masses of all atoms in the formula unit, yielding 102 for Al₂O₃.
Why the others are wrong:
- A: 43 undercounts atoms, like Al + O only.
- B: 75 mistakes ratios, like 3Al ignoring oxygen.
- D: 113 uses incorrect atomic masses, like Al=29.
Final answer: C
Topic: Relative masses of atoms and molecules
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