The percentage by mass of an element in a known compound is its share of the total formula mass, multiplied by 100. Remember to include every atom in the formula.
When magnesium reacts with oxygen, the final solid is heavier than the original magnesium. The extra mass comes from oxygen in the air. Compare readings using the same crucible and lid. In the school method, the sample is reheated, cooled and weighed until its mass stops changing.
Edexcel also asks you to find the simplest atom ratio from measured masses. Divide each element’s mass by its relative atomic mass (Ar). Divide all the results by the smallest and write the formula using whole-number subscripts.
The empirical formula gives the simplest ratio. If a substance exists as molecules and its molecular relative mass is known, divide that Mr by the mass of the empirical formula. Multiply every subscript by that number.
Ionic solids do not consist of separate molecules. Their chemical formulae describe the simplest ion ratio. Edexcel 1.44–1.46 covers these empirical-formula tasks at both tiers; AQA has a different stated GCSE scope.
Worked example
In magnesium oxide, MgO, Ar(Mg) = 24 and Ar(O) = 16. The percentage by mass of magnesium is 24 ÷ (24 + 16) × 100 = 60.0%.
A sample contains 5.4 g aluminium and 4.8 g oxygen. Divide by Ar: 5.4 ÷ 27 = 0.20 and 4.8 ÷ 16 = 0.30. The ratio is 2:3, giving Al2O3.
The empirical formula CH2 has a mass of 14. If the molecular Mr is 42, the factor is 42 ÷ 14 = 3. The molecular formula is C3H6.
Specification references
AQA 8462 4.3.1.2–4.3.1.3; Pearson Edexcel 1CH0 Issue 4 1.43–1.47. Edexcel 1.44–1.46 are unbolded, hence both tiers.
Copy these details if you'd like to report a problem. Your answers and history aren't included. Nothing is sent automatically.
Write an explanation
A student heats magnesium in a crucible. The mass of the oxide is greater than the mass of the magnesium originally weighed. They say this violates conservation of mass. Explain what they have missed.
Compare your explanation with the points below. The text you type isn't marked, saved or sent anywhere.
Compare with the example
Check your explanation for
Identify oxygen from the air as a reactant.
Explain why comparing the initial metal mass with the final oxide mass omits a reactant.
State why mass is still conserved when all reactants and products are included.
Example answer
Magnesium reacts with oxygen from the air. The oxide contains both magnesium and oxygen, so it has more mass than the original magnesium alone. There is no violation of conservation of mass: the total mass of the magnesium and oxygen reacted equals the mass of magnesium oxide formed, provided the product is retained.
An answer to improve
The product must be heavier because the reaction makes some extra mass.
Improve your answer
Include the oxygen taken from the air when you account for the mass of all the reactants.
Save your progress
Your progress stays in this browser. Turn on saving if you want to keep it when you leave the page.