A hydrate is a molecule that has a definite number of water molecules in its crystal structure. The crystals seem like a dry solid, but they can be decomposed into an anhydrous salt (without water) and water when heated. A colour change often happens when the hydrate is decomposed
ex. calcium chloride hexahydrate: CaCl2*6H2O
6mol H2O are chemically combined to 1mol CaCl2
What are hydrates used for?
Grains of anhydrous CaCl2 can be used to in damp rooms to absorb the moisture out of he air, thereby forming a hydrate.
Lab 4C: Formula of a hydrate
The mass of a dry crucible was found on a centigram balance & the hydrate used in this experiment, MgSO4*7H2O, was heated in a dry crucible over a Bunsen burner and weighed as well. Using this information, and mass of the anhydrous salt & water was calculated.
%H2O in hydrate: mass of H2O/mass of hydrate = 2.23g/5.02g x 100% = 44.4%
# of mol of MgSO4 in hydrate:
mass after heating-mass of empty crucible = mass of anhydrous salt
28.90g-26.11g = 2.79g
1mol anhydrous salt = mass anhydrous salt x mol/120.4g = 2.79g x mol/120.4g = 0.02mol
mass of hydrate = mass of crucible & hydrate - mass of crucible = 31.13g-26.11g = 5.02g
mass of H2O = mass hydrate - mass of salt = 5.02g - 2.79g = 2.23g
1mol H2O = mass of H2O x mol/18g = 2.23g x mol/18g = 0.1239mol
mass of salt/1mol salt = 2.79g/120.4g/mol = 0.023mol
mol H2O/mol salt = ratio of H2O to anhydrous salt = 0.1239mol/0.23mol = .539
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