NumericalModerate1 mark
5(b)
In sodium aluminium fluoride [Na₃AlF₆], [Atomic Mass: Na = 23, Al= 27, F= 19] Calculate the percentage to the nearest whole number of:
Aluminium
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Na₃AlF₆, molar mass = 210 g mol⁻¹ (from part (i)).
Mass of aluminium in the formula = 1 × 27 = 27
Percentage of aluminium = (27 ÷ 210) × 100 = 12.85 %
Percentage of aluminium ≈ 13 %
Explanation
There is only one aluminium atom, so the numerator is simply 27.
Now add the three answers: 54 + 33 + 13 = 100. That check takes five seconds and catches almost every arithmetic slip in a percentage-composition question. If your three do not total 100 (allowing a unit either way for rounding), one of them is wrong.
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