Edexcel GCSE Chemistry Formulae and equations
This page covers the following topics:
1. Empirical formula
2. Molecular formula
3. Balancing equations
4. 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.
A balanced equation is a model of a chemical reaction that shows the formulae of the reactants and the products. In order for an equation to be balanced, the number of atoms must be equal on both sides of the equation.
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.
A compound has an empirical formula of H₃Cl₄ and a Mᵣ of 429. Calculate its molecular formula.
An unknown compound has an empirical formula of SO₃ and a Mᵣ of 320. What is its molecular formula?
14.5 g of ammonia are reacted with 7.2 g of oxygen to produce nitric oxide and water. Which reactant is in excess?
Balance the equation S₈ + F₂ → SF₆.
2.5 g of zinc are reacted with 9.8 g of hydrogen chloride. Which is the limiting reactant?
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