To find the hydrogen volume produced by magnesium and hydrochloric acid, identify which reactant runs out. Use that reactant to calculate the hydrogen formed. Then work out how much of the other reactant was consumed and subtract it from the starting amount.
The 2027 SEC G3 Chemistry K324 syllabus includes limiting-reactant calculations and uses 24 dm³ per mole of gas at room temperature and pressure. This example begins with a mass of magnesium and a volume of acid solution.
- Balance the equation.
- Convert the starting quantities to moles.
- Use the limiting reactant to calculate hydrogen volume.
- Subtract the amount consumed from the starting amount.
CHAPTER 1 OF 4 · Back to contents
Put both starting amounts in moles
The balanced equation is Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Every mole of magnesium consumes two moles of hydrochloric acid and produces one mole of hydrogen.
Suppose 0.60 g of magnesium reacts with 40.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid. Using 24.0 g mol⁻¹ for magnesium, the starting amount is 0.60 ÷ 24.0 = 0.025 mol. The acid volume is 0.0400 dm³, so it contains 1.00 × 0.0400 = 0.0400 mol of HCl.
CHAPTER 2 OF 4 · Back to contents
Worked example | Find the gas and solid remaining
To react with all 0.025 mol of magnesium, you would need 0.050 mol of acid. Only 0.0400 mol is available, so the acid is limiting. It can consume 0.0400 ÷ 2 = 0.0200 mol of magnesium.
The reaction produces 0.0200 mol of hydrogen, which occupies about 0.480 dm³ or 480 cm³ at room temperature and pressure. The magnesium remaining is 0.025 − 0.0200 = 0.005 mol, or approximately 0.12 g.
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CHAPTER 3 OF 4 · Back to contents
Try a larger volume of acid
At the same concentration, 50.0 cm³ supplies exactly enough acid to consume all the magnesium in this example. The ideal yield is 0.025 mol of hydrogen, or 600 cm³.
At 60.0 cm³, magnesium limits the reaction. The hydrogen volume stays at 600 cm³ and 0.0100 mol of acid remains. The additional acid cannot produce more hydrogen once the magnesium runs out.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Why is acid limiting even though it starts with more moles?
The reaction requires twice as many moles of acid as magnesium. Compare the acid available with the amount the equation requires.
What can a parent ask during revision?
Ask the student what 0.0200 mol represents in each line. It is both the magnesium consumed and the hydrogen produced because their reacting ratio is one to one.