Balancing chemical equations
Deriving empirical and molecular formulae The percentage of an element in a compound can be calculated from the relative mass of the element in the formula and the relative formula mass of the compound.
The empirical formula of a compound can be calculated from the masses or percentages of the elements in a compound. Candidates should be able to calculate empirical formulae.
Experiment to find the formula of hydrated copper (II) sulfate (experiment and worked example calculation)
Law of conservation of mass
Total mass of products = Total mass of reactants
The masses of reactants and products can be calculated from balanced symbol equations. Candidates should be able to calculate the masses of individual products from a given mass of a reactant and the balanced symbol equation.
Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:
■ the reaction may not go to completion because it is reversible
■ some of the product may be lost when it is separated from the reaction mixture
■ not all reactants react to make product: some of the reactants may react in ways different from the expected reaction.
Percentage yield: the amount of a product obtained is known as the yield. When compared with the maximum theoretical amount, as predicted from the chemical equation, it is called the percentage yield. The chemical reaction must actually be carried in order to determine the yield of desired product.
Atom economy: the atom economy of a chemical reaction is a measure of the amount of starting materials that become useful products. It is a theoretical measure that is calculated by ONLY using the chemical equation.
Inefficient, wasteful processes have low atom economies. Efficient ‘greener’ processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste.
Equilibrium (reversible reaction) In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented:
Rules for answering questions relating to moles and masses in chemical equations