Atomic Theory

The molecular formula must therefore be equal to C2H2O4.

Finding the empirical formula of a compound experimentally

For example, find the empirical formula of an oxide of copper. The empirical formulae of oxides of lead and iron could also be found by this method.

Procedure: Weigh a porcelain boat and record its weight. Put some of the oxide whose empirical formula is to be found in the porcelain boat and weigh again. Record this mass.

Arrange the apparatus as shown in the diagram below:

Pass dry hydrogen or carbon monoxide (two very powerful reducing agents) over the heated oxide whose empirical formula is to be found. At intervals, take the boat and contents out of the combustion tube and weigh. It is noted that the mass decreases, as the reducing agents ‘steal’ oxygen from the metal oxide as they reduce it to the metal. Keep on passing either of the reducing agents indicated over the heated metal oxide, until the boat and contents reach constant mass, that is, until the weight of the boat and contents no longer changes. At this point all the metal oxide would have been reduced to the metal.

Precaution

Allow the boat and contents to cool in a stream of the reducing agent by switching off the heat but allowing the flow of hydrogen or carbon monoxide to pass over the hot metal. This is done in order to prevent the hot metal from being reoxidized by the incoming oxygen.

On cooling, take the boat and contents out of the combustion tube, weigh and record the mass.

Results

Mass of porcelain boat and metal oxide before heating = (a) g

Mass of porcelain boat                                                    = (b) g

Mass of metal oxide                                                        = (a-b) g = (c) g