1.25 calculate reacting masses using experimental data and chemical equations

1 - write out the balanced equation (you may be given this in an exam)

2 - for the two bits you want, work out the relative formula mass (Mr) and multiply them by the balancing numbers in the equation

3 - Apply the rule : divide to get one, them multiply to get all (but you have to apply the first to the substance they give information about, and then the other one)

For example: What mass of magnesium oxide is produced when 60g of magnesium is burnt is air?

Step 1: balanced equation...

2Mg + O2 ----->  2MgO

Step 2: work out the relative formula masses of each component and multiply them by the balancing numbers in the equation (sounds confusing, sorry)

2Mg: 2 x 24 = 48                                             2MgO: 2 x (24 x 16) = 80

Step 3: apply the rule 'divide to get one, multiply to get all'...

The two numbers, 48 and 80, tell us that 48g of Mg react to give 80g of MgO. However, the question asks us how much MgO is produced when 60g of Mg is reacted, so we need to work out how much MgO is produced when 60g of Mg is reacted...

48g of Mg........80g of MgO     (divide by the reactant, in this case, divide by 48)

1g of Mg..........1.67g of MgO           ( x by 60, as thats how much Mg we have)

60g of Mg........100g of MgO

Therefore, 60g of Mg reacts with air to produce 100g of MgO