Amounts of Substances in Equations | Chemistry for Grade 10 PDF Download

Calculating Masses from Balanced Equations

• 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

Solved Example

Example 2: Calculate the mass of aluminium, in tonnes, that can be produced from 51 tonnes of aluminium oxide. The equation for the reaction is: 2Al₂O₃  ⟶  4Al +  3O₂

Balancing Equations using Reacting Masses

• If the masses of reactants and products of a reaction are known then we can use them to write a balanced equation for that reaction
• This is done by converting the masses to moles and simplifying to find the molar ratios

Example 3: A student reacts 1.2 g of carbon with 16.2 g of zinc oxide. The resulting products are 4.4 g of carbon dioxide and 13 g of zinc. Determine the balanced equation for the reaction.

Exam Tip

• These questions look hard but they are actually quite easy to do, as long as you follow the steps and organise your work neatly.
• Remember the molar ratio of a balanced equation gives you the ratio of the amounts of each substance in the reaction.