Chemical Formulae, Equations and Calculations Part 1
3 marks · NormalA student has 21 g of magnesium chloride (MgCl2). Calculate the number of moles of MgCl2 present.
(Ar: Mg = 24, Cl = 35.5)
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0.22 mol (±0.01)
Also accept: 0.215 mol
Common mistakes: using mass instead of Mr; not using correct Mr
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Marking points
M1 · 1 mark · Calculates Mr of MgCl2: 24 + (2×35.5) = 95
M2 · 1 mark · Uses n = mass ÷ Mr
A1 · 1 mark · Correct final answer: 0.22 mol (accept answers within 0.01 of this) (follow-through)
Worked solution
Mr of MgCl2 = 24 + (2 × 35.5) = 24 + 71 = 95
Number of moles = mass / Mr = 21 g / 95 = 0.221 (rounded to 0.22 mol)