CONCEPT:
A HYDRATE is a compound that incorporates water molecules into its fundamental solid structure. In a hydrate (which usually has a specific crystalline form), a defined number of water molecules are associated with each formula unit of the primary material. 
Gypsum is a hydrate in which two water molecules are present for every formula unit of CaSO_{4} in the solid. The chemical formula for gypsum is CaSO_{4 }·2 H_{2}O and the chemical name is calcium sulfate dihydrate. Note that the dot in the formula (or multiplication sign) indicates that the waters are there. 
Other examples of hydrates are: lithium perchlorate trihydrate  LiClO_{4} ·3H_{2}O; magnesium carbonate pentahydrate  MgCO_{3} ·5 H_{2}O; copper(II) sulfate pentahydrate  CuSO_{4} ·5 H_{2}O; Nickel(II) sulfate hexahydrate  NiSO_{4} ·6 H_{2}O; and aluminum potassium sulfate dodecahydrate  AlK(SO_{4})_{2} ·12 H_{2}O 
This water in the hydrate (referred to as "water of hydration") can be removed by heating the hydrate. When all hydrating water is removed, the material is said to be anhydrous and is referred to as an anhydrate. 
CuSO_{4} ·5 H_{2}O(s)  + HEAT >  CuSO_{4} (s) + 5 H_{2}O (g) 
Hydrate  Anhydrate 
EXPERIMENTAL MEASUREMENT OF PERCENT HYDRATION:
Experimentally measuring the percent water in a hydrate involves first heating a known mass of the hydrate to remove the waters of hydration and then measuring the mass of the anhydrate remaining. The difference between the two masses is the mass of water lost. Dividing the mass of the water lost by the original mass of hydrate used is equal to the fraction of water in the compound. Multiplying this fraction by 100 gives the percent water in the hydrate. 
EXAMPLE 1: When a 1.000 g sample of CuSO_{4} ·5 H_{2}O(s) was heated so that the waters of hydration were driven off, the mass of the anhydrous salt remaining was found to be 0.6390 g. What is the experimental value of the percent water of hydration? 
CuSO_{4} ·5 H_{2}O(s)  + HEAT >  CuSO_{4} (s) + 5 H_{2}O (g) 
1.000 g  0.6390 g 
1. Find the difference between the mass of hydrate before heating and
the mass of the anhydrate after heating. The difference is the mass of
water last.
1.000 g  0.6390 g = 0.3610 g 
2. Dividing the mass of the water lost by the mass of hydrate used
is equal to the fraction of water in the compound. Multiplying this fraction
by 100 gives the percent water in the hydrate.
(0.3610 g /1.000 g)(100) = 36.10% 
CALCULATION OF PERCENT HYDRATION FROM THE CHEMICAL FORMULA
The theoretical (actual) percent hydration (percent water) can be calculated from the formula of the hydrate by dividing the mass of water in one mole of the hydrate by the molar mass of the hydrate and multiplying this fraction by 100. 
EXAMPLE 2: What is the percent water in copper(II) sulfate pentahydrate, CuSO_{4} ·5 H_{2}O? 
1. Calculate the formula mass. When determining the formula mass for a hydrate, the waters of hydration must be included. 
1 Cu @ 63.55 g =  63.55 g  
1 S @ 32.07 g =  32.07 g  2 H @ 1.01 =  2.02 g 
4 O @ 16.00 g =  64.00 g  1 O @ 16.00 =  16.00 g 
159.62 g/mol  18.02 g/mol  
Formula Mass = 159.62 + 5(18.02) = 249.72 g/mol 
2. Divide the mass of water in one mole of the hydrate by the molar
mass of the hydrate and multiply this fraction by 100.
Percent hydration = (90.10 g /249.72 g)(100) = 36.08% 
CALCULATION OF THE PERCENT ERROR
Percent error = ( theoretical  experimental / theoretical value) x 100 EXAMPLE: Calculate the percent error from the previous examples. Percent error = [( 36.08  36.10 )/ 36.08] x 100 = 0.06% 
EXPERIMENTAL PROCEDURE FOR PERCENT WATER IN A HYDRATE
1. Accurately weigh a clean, dry evaporating dish. Record this mass.


1. Remember that barium is toxic. The used barium chloride should be put in the waste container provided. 2. In determining the mass of water lost and the moles of anhydrous sample, the lower mass of the two should be used, this will usually be after the second heating, but if the mass after the first heating is lower, have them use that figure. 3. It is very important that the evaporating dish cools to room temperature before weighing. If it is not cool, convection currents will be set up that will lower the mass. 