Relative formula mass: In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

The idea

This statement links conservation of mass to relative formula mass (Mr). In a balanced equation, the total Mr of all reactant species, each multiplied by its balancing number, equals the total Mr of all product species, each multiplied by its balancing number. For example, 2H₂ + O₂ → 2H₂O: reactants give 2 × 2 = 4 for H₂ plus 1 × 32 = 32 for O₂, totalling 36; products give 2 × 18 = 36. The balancing numbers are essential because they represent the quantities shown. This works because atoms are conserved in a chemical reaction, so the total mass of atoms on each side is unchanged. The skill is assessed by checking that you multiply each Mr by its balancing number before summing, and that you compare the two totals correctly.

What earns the marks

• State that relative formula mass is found by adding the relative atomic masses of all atoms in the formula.
• Multiply each species' Mr by its balancing number in the equation before adding.
• Sum the reactant values and sum the product values separately.
• Compare the two totals and conclude that they are equal for a correctly balanced equation.
• Use a worked example such as 2H₂ + O₂ → 2H₂O to show both totals equal 36.

Common mistakes and how to put them right

• Adding Mr values without multiplying by balancing numbers.

  Put it right by multiplying each species' Mr by its coefficient first.

• Using relative atomic masses of individual atoms instead of the full relative formula mass of each species.

  Put it right by summing all atoms within each formula.

• Forgetting that balancing numbers apply to the whole species.

  Put it right by treating 2H₂O as two lots of Mr 18, giving 36.

Exam tips

• Write the Mr calculation for each species clearly before applying balancing numbers.
• Show the multiplication step explicitly so the examiner can follow your reasoning.
• Check that your equation is balanced before comparing the two totals.