Hydrated salts lose their water of crystallization when heated.
–CuSO4·5H2O(s)
à CuSO4 (s) +
5H2O(g)
The number of moles of water combined with each mole of the
anhydrous salt can be determined by gravimetric analysis.
It involves heating a known mass of the original hydrated
sample, driving off all the water by repeated heating and
measuring the mass until there is no change in the mass of the
anhydrous residue left behind.
–The
residue of course will be the anhydrous salt, and the difference in mass between
the original hydrated salt and that of the residue will give the mass of the water
in the sample.
Example: A 0.520 g of NiSO4·xH2O
after a couple of cycles of heating, cooling and measuring the
mass gave a residue of 0.306 g. Determine the formula of
the salt (ie determine x).
–NiSO4·xH2O(s)
à NiSO4 (s) +
xH2O(g)
–0.520
g 0.306 g