I/GCSE Chemistry - Calculating Masses in Reactions

Three Important Steps

1. Write out the balanced equation
2. For the two bits you want, work out the Relative Formula Mass and multiply them by the balancing numbers in the equation
3. Apply the rule - Divide to get one, then multiply to get all (but you have to apply this first to the substance they give information about, and then the other one)

I/GCSE Chemistry Style Example

What mass of magnesium oxide is produced when 60g is produced when 60g of magnesium is burnt in air?

1. Write out the balanced equation:  
   2Mg + O2 = 2MgO
2. Work out the Relative Formula Masses of the two bits you want and multiply them by the balancing numbers in the equation
   2Mg: 2 x24 =48       2MgO: 2x(24 + 16) = 80
3. Apply the rule: Divide to get one, then multiply to get all.

The two numbers, 48 and 80, tells us the 48g of Mg react to give 80g of MgO

Here's the tricky bit. We need to find out what happens when 60g of Mg is burnt in air

48g of Mg .............. 80g of MgO

Divide by 48 to get 1g                                                    Divide by 48 to get the mass of MgO at 1g                                   

                                      1g of Mg.................1.67g of MgO

Multiply by 60                                                                       Multiply by 60 to get MgO at 60g

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

This tells us that 60g of magnesuim will produce 100g of magnesium oxide.

The mass of product ( in this case Magnesium Oxide) is called the yield of a reaction. Masses you calculate in this way are called theoretical yields.

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Drafted by Catrina (Chemistry)