Obtaining Formulae Experimentally

Obtaining Formulae Experimentally

Metal Oxides

Hydrogen can be passed over metal oxides to reduce it to the metal. To find the formula of copper oxide, the experimental steps are as follows:

1. Measure the mass of the empty combustion tube.
2. Use a spatula to put copper oxide into the tube. Weigh the tube.
3. Set up the apparatus as shown. Turn the gas at the jet to light the excess gas.
4. Heat the copper oxide until it has all turned into red copper.
5. Stop heating but leave gas passing through until everything has cooled.
6. Weigh the combustion tube.
7. Put masses in a table and calculate empirical formula from there.

In the Case of Water of Crystallisation

When substances crystallise from a solution, water becomes chemically bounded with the salt. This is called water of crystallisation and the salt is said to be hydrated.

Suppose you had to find the formula of a BaCl₂•nH₂O (a barium chloride crystal), to find n:

1. Weight the mass of an empty crucible.
2. Add barium chloride crystals and reweigh.
3. Heat the crucible gently (so the barium chloride won’t decompose), and reweigh.
4. Put masses into a table and calculate the formula from there.