1.24 calculate empirical and moleculer formulae from experimental data

Empirical (and molecular) formula can also be calculated from experimental data. This is how its done...

- List all the elements in the compound
- Underneath, write their experimental masses or percentages
- Divide each mass by the Ar (relative atomic mass) of that particular element
- Turn the numbers into a ratio
- Simplify the ratio


For example: In an experiment, some iron oxide powder is reduced to pure metallic iron. Use the following experimental data to find the empirical formula of the iron oxide used.

Mass of empty container = 32.0g
Mass of container + mass of iron oxide = 96.0g
Mass of container + iron = 76.8g

METHOD - 
During this experiment, oxygen is lost. To find the mass of oxygen lost, minus the 'mass of container + iron' from 'mass of container + mass of iron oxide'...

96 - 76.8 = 19.2g

The mass of iron made is 'mass of container + iron' minus 'mass of container'...

76.8 - 32 = 44.8g

Now, list the elements in iron oxide...                       
    Fe                                O

write their experimental masses                          
   44.8                            19.2

divide by their Ar                                   
   44.8 / 56 = 0.8         19.2 / 16 = 1.2

multiply by 10 to put into ratio          
    8              :               12

cancel down                                            
   2              :                3

This means that the simplest formula is 2 Fe atoms to every 3 O atoms, so the empirical formula is Fe2O3