The masses of reactants and products can be calculated from balanced…

The idea

A balanced symbol equation fixes the ratio in which particles react, so it also fixes the ratio of amounts in moles. To calculate masses, first balance the equation, then read the coefficients as the number of moles of each substance. Convert any given mass to moles using n = m ÷ Mᵣ, apply the mole ratio from the coefficients, and convert back to mass using m = n × Mᵣ. For example, in 2Mg + O₂ → 2MgO, burning 4.8 g of Mg (Mᵣ = 24) gives n(Mg) = 4.8 ÷ 24 = 0.20 mol; the ratio Mg : MgO is 2 : 2, so n(MgO) = 0.20 mol and m(MgO) = 0.20 × 40 = 8.0 g. The same method works for any reactant or product once the equation is balanced.

What earns the marks

• A correct balanced symbol equation is required before any mass calculation; coefficients give the reacting mole ratio.
• Converting a given mass to moles uses n = m ÷ Mᵣ, with Mᵣ values taken from the periodic table and summed correctly for compounds.
• The mole ratio from the balanced equation is applied to the substance whose mass is required, not assumed to be 1 : 1.
• The final answer is converted from moles back to mass using m = n × Mᵣ and given with a suitable unit such as g or kg.
• Where a reactant is in excess, the limiting reactant determines the amount of product formed.
• Answers should be checked for sensible magnitude and, where appropriate, given to a reasonable number of significant figures.

Common mistakes and how to put them right

• Using an unbalanced equation.

  Put it right: Balance the equation first and use the final coefficients.

• Assuming the mole ratio is always 1.

  Put it right: Read the coefficients directly from the balanced equation.

• Multiplying the given mass by Mᵣ instead of dividing.

  Put it right: Use n = m ÷ Mᵣ to find moles, then m = n × Mᵣ to return to mass.

• Forgetting to multiply by the coefficient when finding the mass of a product.

  Put it right: Include the coefficient in the mole ratio before converting to mass.

Exam tips

• Write the balanced equation and label the relevant mole ratio before doing any arithmetic; this makes method marks easier to award.
• Show each step: mass → moles → mole ratio → moles → mass, with units at every stage.
• Check that the relative formula mass you use matches the formula in the equation, but remember that coefficients do not affect the Mᵣ calculation to avoid double-counting.
• If the question gives two reactant masses, identify the limiting reactant before calculating product mass.