1.23 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation

metal oxides

•        Weigh a sample of metal in a crucible

•        Heat is strongly so it oxidises. Have a lid on the crucible, but briefly lift it periodically to let in air.

•        Weigh it again. It should be heavier.

•        The increase in mass is the mass of oxygen that has added to the metal.

•        Work out the moles of the metal and the oxygen to deduce the formula (see Section 1.25)

water

•        Electrolyse a sample of water (acidified to increase conductivity) in a Hoffmann Voltameter (below).

•        Measure the volume of hydrogen and oxygen produced at each electrode

•        Since one mole of any gas occupies the same volume, the ratio of the two volumes is the same as the mole ratio, which should be 2H : 1O. This gives the formula H2O.

water of crystallisation

•        Weigh a sample of the hydrated salt

•        Heat the salt in a crucible to drive off the water until there is no further change in mass

•        Record the final mass and subtract it from the initial mass. This gives the mass of water.

•        Work out the moles of the anhydrous salt (final mass) and the moles of the water (the drop in mass). The ratio of these moles gives the formula of the hydrated salt.

e.g.     2.50 g of hydrated copper sulphate is heated strongly until there is no further change in mass, The final mass is 1.60 g. What is the formula of the hydrated salt?

moles of anhydrous CuSO4          = 1.60 g ÷ (64 + 32 + 4x16) = 0.010 mol

moles of water                          = (2.50 - 1.60) g ÷ (16 + 2x1) = 0.050 mol

ratio water : CuSO4                         = 0.050 ÷ 0.010 = 5 : 1

therefore hydrated salt formula = CuSO4.5H2O