Reactions of metals with water

3 marks · Normal

When 7.2 grams of magnesium ribbon are heated in steam, magnesium oxide and hydrogen gas are produced. The equation for the reaction is:

Calculate the volume (in dm) of hydrogen gas produced at room temperature and pressure (RTP) from this reaction. (Molar mass of Mg = 24 g/mol; the molar volume of a gas at RTP = 24 dm/mol)

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7.2 dm^3 (±0.1)

Also accept: 7.20 dm^3

Common mistakes: Dividing by 2 when not required from the mole ratio; Using wrong molar mass; Omitting units

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Marking points

M1 · 1 mark · Calculates moles of magnesium: n = 7.2 g / 24 g/mol = 0.3 mol

M2 · 1 mark · Relates moles of magnesium to moles of hydrogen gas (1:1 ratio from the equation)

A1 · 1 mark · Calculates final volume: 0.3 mol 24 dm/mol = 7.2 dm; correct answer with units (follow-through)

Worked solution

Step 1: Calculate the number of moles of magnesium.

Step 2: Use the chemical equation to find the moles of hydrogen produced. 1 mol Mg gives 1 mol H, so 0.3 mol Mg gives 0.3 mol H.

Step 3: Calculate the volume of hydrogen gas at RTP:

Final answer: