AQA GCSE Chemistry

Revision Notes

3.2.3 Using Moles to Balance Equations

Higher Tier Only

Using Moles to Balance Equations

  • Stoichiometry refers to the numbers in front of the reactants and products in an equation, which must be adjusted to make sure that the equation is balanced
  • These numbers are called coefficients (or multipliers) and if we know the masses of reactants and products, the balanced chemical equation for a given reaction can be found by determining the coefficients
  • First, convert the masses of each reactant and product in to moles by dividing by the molar masses using the periodic table
  • If the result yields uneven numbers, then multiply all of the numbers by the same number, to find the smallest whole number for the coefficient of each species
    • For example, if the resulting numbers initially were 1, 2 and 2.5, then you would multiply all of the numbers by 2, to give the whole numbers 2, 4 and 5
  • Then, use the molar ratio to write out the balanced equation

Worked Example

64 g of methanol, CH3OH, reacts with 96 g of oxygen gas to produce 88 g of carbon dioxide and 72 g of water. Deduce the balanced equation for the reaction.

(C = 12, H = 1, O = 16)

Answer

    • Calculate the molar masses of the substances in the equation

CH3OH = 32 g mol-1         O2 = 32 g mol-1        

CO2 =  44 g mol-1             H2O =  18 g mol-1  

    • Divide the masses present by the molar mass to obtain the number of moles

CH3OH = 64 g ÷ 32 g mol-1  =  2 mol 

O2 = 96 g ÷ 32 g mol-1          =  3 mol   

CO2 =  88 g ÷ 44 g mol-1      =  2 mol

H2O =  72 g ÷ 18 gmol-1       =  4 mol

    • The mole ratios are the same as the coefficients in the balanced equation

2CH3OH  +  3O⟶ 2CO2 + 4H2O

Exam Tip

The molar ratio of a balanced equation gives you the ratio of the amounts of each substance in the reaction.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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