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:
Sodium and
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 sodium in the formula = 3 × 23 = 69
Percentage of sodium = (69 ÷ 210) × 100 = 32.85 %
Percentage of sodium ≈ 33 %
Explanation
Same denominator, different numerator. There are three sodium atoms, so the numerator is 69, not 23 — using 23 gives 11 % and breaks the total.
The question says to the nearest whole number, so 32.85 rounds to 33.
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