Using Moles to Balance Equations (AQA GCSE Chemistry: Combined Science)

Higher Tier Only

• 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

• For the reaction of elements forming compounds, it can be necessary to determine the formula of one of the products in order to balance the equation
• This involves calculating the empirical formula
  1. Write each element involved
  2. Determine the mass of each element
  3. Write the atomic mass of each element 
  4. Calculate the number of moles of each element (divide by the atomic mass)
  5. Calculate the ratio of elements (divide by the smallest answer)
  6.  Write the final empirical formula
• With the formula of the product, it is then possible to write a balanced symbol equation