When chemicals react, atoms inside them swap places to make new compounds which are the **products** of the reaction. The atoms are the same as before in number, but they have been rearranged. The overall mass stays the same. It is possible to predict the **yield **of a reaction using the chemical equation.

__ Example__: Calcium carbonate (

**CaCO**) is thermally decomposed into calcium oxide (

_{3}**CaO**) and carbon dioxide (

**CO**). The symbol equation is as follows:

_{2}**CaCO _{3}**

_{ }

**CaO**+

**CO**

_{2}_{ }

How much carbon dioxide is made when 1000 g of calcium carbonate decomposes?

__ Answer__: In order to calculate this without knowing the amount of calcium oxide made from 1000 g of calcium carbonate, we need to calculate the relative formula masses of the compounds involved. In this case, calcium carbonate and carbon dioxide.

Calcium carbonate has 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen, and we need to add the relative atomic masses of all those atoms to find the relative formula mass. The **relative atomic mass **of calcium is 40, of carbon is 12 and of oxygen is 16:

**relative formula mass of ****calcium carbonate**: 40 + 12 + 16 + 16 + 16 = **100**

Carbon dioxide has 1 atom of carbon and 2 atoms of oxygen, so:

**relative formula mass of ****carbon dioxide**: 12 + 16 + 16 = **44**

Using the relative formula masses and the equation for the reaction, we know that:

100 g of calcium carbonate gives 44 g of carbon dioxide

1000 g is 10 times more than 100 g, so 1000 g of calcium carbonate gives 10 x 44 = **440 g of carbon dioxide**.

Make sure you're confident with how that answer was obtained and then move on to have a go at some questions.