The combustion of elements in oxygen

3 marks · Normal

When magnesium burns in oxygen, it forms magnesium oxide. Student A burns 1.2 g of magnesium in excess oxygen. The equation is:

Calculate the mass of magnesium oxide formed. (Relative atomic masses: Mg = 24, O = 16)

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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2.0 g (±0.1)

Also accept: 1.98 g; 2 g

Common mistakes: Using wrong molar masses; Calculating for 1 mole not 1.2 g; Not converting between moles and mass

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Calculates moles of Mg: mol or shows correct working.

M2 · 1 mark · Uses 1:1 ratio from balanced equation to get moles of MgO = 0.05 mol (error carried forward allowed). (follow-through)

A1 · 1 mark · Correct final mass of MgO = 2.0 g (accept 1.98 g within 0.1 g). (follow-through)

Worked solution

Moles of Mg burned:

Balanced equation shows 2 mol Mg makes 2 mol MgO, so 0.05 mol Mg makes 0.05 mol MgO.

Mass of MgO = moles molar mass = (since MgO, 24+16=40).