Reacting Masses (Cambridge O Level Chemistry)

• Chemical equations can be used to calculate the moles or masses of reactants and products
• To do this, information given in the question is used to find the amount in moles of the substances being considered
• Then, the ratio between the substances is identified using the balanced chemical equation
• Once the moles have been determined they can then be converted into grams using the relative atomic or relative formula masses

Limiting Reactants

• A chemical reaction stops when one of the reactants is used up
• The reactant that is used up first is the limiting reactant, as it limits the duration and hence the amount of product that a reaction can produce
• The amount of product is therefore directly proportional to the amount of the limiting reactant added at the beginning of a reaction
• The limiting reactant is the reactant which is not present in excess in a reaction
• In order to determine which reactant is the limiting reactant in a reaction, we have to consider the ratios of each reactant in the balanced equation
• When performing reacting mass calculations, the limiting reactant is always the number that should be used as it indicates the maximum possible amount of product
• The steps are:
  1. Write the balanced equation for the reaction
  2. Calculate the moles of each reactant
  3. Compare the moles & deduce the limiting reactant