Hydrates are ionic salts that trap water molecules in their crystal lattice. This added mass must be used when making calculations therefore the ratio between molecules of ionic salt and water is given in the name.
For instance, calcium sulfate hexahydrate states that for every molecule of calcium sulfate there are 6 water molecules surround it. We represent a hydrate with a large dot then the number of water molecules. This dot is NOT a multiplication sign, it is actually a ratio.
To determine the molar mass of hydrate, determine the mass of the salt then add the mass of however many water molecules are attached to it.
1 Ca + 1 S + 4 O + 6(2 H + 1 O)
40.1 + 32.1 + 4(16.0) + 6(18.0)
244.2 g/mole
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