Formulae of Compounds

Formulae of Compounds

The formula of water is H2O. This is because 1 atom of oxygen combines with 2 atoms of hydrogen.

Alternatively, we can say that 1 mole of oxygen atoms combines with 2 moles of hydrogen atoms to form 1 mole of water molecules.

Moles can be changed to grams; therefore we can say 16 grams of oxygen combine with 2 grams of hydrogen to form 18 grams of water.

The formula of carbon dioxide is CO2. This is because 1 atom of carbon combines with 2 atoms of oxygen. Therefore, 12 grams of carbon combine with 32 grams of oxygen to form 44 grams of carbon dioxide.

Alternatively, 1 mole of carbon atoms combine with 2 moles of oxygen atoms to produce 1 mole of carbon dioxide.

To find the formula of a compound

  1. Start with the number of grams that combine
  2. Change the grams to moles
  3. This gives you the ratio which they combine
  4. So now you know the formula

A formula obtained in this way is called the Empirical Formula.


  1. 32 grams of sulphur react with 32 grams of oxygen
  2. 1 mole of sulphur reacts with 2 moles of oxygen
  3. Ratio of 1:2
  4. Formula SO2 - sulphur dioxide

The Reaction between Magnesium and Oxygen

Copyright S-cool

The apparatus below is used to calculate the mass of magnesium oxide, but firstly you must know the masses of the elements that combine.

Formulae of Compounds


  1. Weigh the mass of the crucible and lid
  2. Add a coil of magnesium ribbon and reweigh.
  3. Heat crucible strongly, lifting lid occasionally to allow oxygen in.
  4. When burning is complete allow the apparatus to cool.
  5. With the lid on reweigh the crucible and its contents.


mass of magnesium oxide - mass of magnesium = mass of oxygen

4.0g - 2.4g = 1.6g

2.4g of magnesium = 0.1 moles

1.6g of oxygen = 0.1 moles

Therefore the ratio of magnesium to oxygen used is 1:1


The formula of magnesium oxide is MgO.