Question detail
Calculate the relative formula mass of aluminium sulphate, Al₂(SO₄)₃.
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Amount of substance
Question
- A. 342.20 g/mol
- B. 342.15 g/mol (A-level cue 06b6b26b distractor 1)
- C. 342.25 g/mol (A-level cue 06b6b26b distractor 2)
- D. 342.30 g/mol (A-level cue 06b6b26b distractor 3)
Answer
342.20 g/mol
Explanation
The correct option is 342.20 g/mol. 342.20 g/mol is correct because it supports the objective: Use relative formula mass for ionic compounds.. The reasoning stays within Relative atomic mass and relative molecular mass and avoids drifting into a similar A-Level Chemistry idea. This item is treated as conceptual revision rather than a formal calculation item because the validated answer is an explanation or option choice, not a worked numerical response.
Common mistake
Misunderstanding Ionic Compounds
Students often confuse the relative formula mass of ionic compounds with that of covalent compounds, leading to incorrect calculations.
To calculate the relative formula mass of an ionic compound, use the formula: Mr = sum(Ar of all ions in the formula). For example, for NaCl: Mr = Ar(Na) + Ar(Cl) = 23 + 35.5 = 58.5 g/mol. Thus, the relative formula mass of NaCl is 58.5 g/mol.
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