Empirical formulae of metal oxides

Experimentally, there are two methods to determine the empirical formulae of metal oxides. The methods depend on the reactivity of the metal.


Method 1 : To determine the empirical formula of oxide of a more reactive metal than hydrogen (in the Reactivity series = metals that are positioned higher than hydrogen in the series).

The principle is that to react the reactive metal with oxygen to produce metal oxide. This is because the metal can react readily with oxygen. In the laboratory, the apparatus set up that is appropriate for the purpose is like the diagram shown below :



In determining empirical formulae of metal oxides, metals that are said to be more reactive than hydrogen include calcium, magnesium, aluminium and zinc.

Taking magnesium as an example, the chemical equation of the reaction is
2Mg + O2 --> 2MgO.



Method 2 : To determine the empirical formula of a less reactive metal than hydrogen (in the Reactivity series = metals that are positioned lower than hydrogen in the series).

For this method, the principle is that to react (reduced) the less reactive metal with hydrogen gas to give its pure metal and water. This method is used because it is more convenient rather than waiting for a slow reaction between the metal and oxygen to react completely.

Below is the apparatus set up used in the laboratory to determine the empirical formulae of oxides of less reactive metals:



These kind of metals include tin, lead and copper. For an example, the reaction between copper(II) oxide and hydrogen is described as follows :
CuO + H2O --> Cu + H2.