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From grams to moles.
From moles to an answer.
Use a balanced equation to calculate a product's mass.
Captioned concept animation · No narration · Teacher-recorded version planned
Three ideas to take away
- Convert mass to amount: n = m ÷ M.
- Use the coefficients in the balanced equation as the mole ratio.
- Convert product moles back to mass: m = n × M.
This sample assumes complete reaction, pure substances and excess oxygen. Use the relative atomic masses given in each question.
What mass of magnesium oxide forms?
2.4 g of magnesium reacts completely with excess oxygen. Calculate the mass of magnesium oxide formed. Use Aᵣ(Mg) = 24 and Aᵣ(O) = 16.
Prepared AI-assisted explanation · Not a live model response or an official Cambridge mark scheme. Academic review is planned before classroom release.
n(Mg) = 2.4 g ÷ 24 g mol⁻¹ = 0.10 mol.
2 mol Mg forms 2 mol MgO, so Mg : MgO = 1 : 1. Therefore n(MgO) = 0.10 mol.
M(MgO) = 24 + 16 = 40 g mol⁻¹.
m(MgO) = 0.10 × 40 = 4.0 g (2 significant figures).
Watch the calculation
1-minute captioned walkthrough · Original question · No narration
Your turn. Take it step by step.
Use Aᵣ(Mg) = 24 and Aᵣ(O) = 16. Basic scoring checks the numerical answer; it does not assess written working or award official exam marks.
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Demo student · Stoichiometry
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