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AQA A-level Chemistry Formulae and equations 1. Empirical formula
2. Molecular formula
3. Limiting reactants

The empirical formula gives the simplest ratio of atoms for each element within a chemical compound. To calculate the empirical formula, the percentage by mass of each component is divided by the relative atomic mass of the element. The molecular formula gives the total number of atoms of each element within a chemical compound. It can be calculated using the empirical formula and the relative molecular mass of the compound: the relative molecular mass is divided by the mass of the atoms in the empirical formula. The numbers in the empirical formula are then multiplied by the resulting number. The limiting reactant is the first reactant to be used up in a chemical reaction. When the limiting reactant is used up, the reaction stops and no more of the product can be formed. Therefore, the mass of the product is dependent on the mass of the limiting reactant. The left-over reactant is referred to as the reactant in excess. 1

A compound has an empirical formula of H₃Cl₄ and a Mᵣ of 429. Calculate its molecular formula. 2

An unknown compound has an empirical formula of SO₃ and a Mᵣ of 320. What is its molecular formula? 3

14.5 g of ammonia are reacted with 7.2 g of oxygen to produce nitric oxide and water. Which reactant is in excess? 4

2.5 g of zinc are reacted with 9.8 g of hydrogen chloride. Which is the limiting reactant? 5

Which is the limiting reactant in the reaction of 9.6 g of nitrogen with 6 g of hydrogen? End of page