Hydrates are compounds that incorporate water molecules into their 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 with two water molecules present for every formula unit of CaSO₄. The chemical formula for gypsum is CaSO₄ • 2H₂O 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 - LiClO₄ • 3H₂O; magnesium carbonate pentahydrate - MgCO₃ • 5H₂O; and copper(II) sulfate pentahydrate - CuSO₄• 5 H₂O.

The 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.

    CuSO₄• 5 H₂O(s) + HEAT ---> CuSO₄ (s) + 5 H₂O (g)
     hydrate                                       anhydrate

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.
 

EXAMPLE 1 When a 1.000 g sample of CuSO4• 5 H2O(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?      CuSO4• 5 H2O(s) + HEAT ----> CuSO4 (s) + 5 H2O (g)           1.000 g                                      0.6390 g 1.   The difference between the hydrate mass and anhydrate mass is the mass of water lost.           1.000 g - 0.6390 g = 0.3610 g 2.   Divide the mass of the water lost by the mass of hydrate and multiply by 100.           (0.3610 g /1.000 g)(100) = 36.10%

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 by 100.
 

EXAMPLE 2 What is the percent water in copper(II) sulfate pentahydrate, CuSO4• 5 H2O? 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/mol) + (1 S)(32.07 g/mol) + (4 O)(16.00 g/mol) = 159.62 g/mol      Formula mass = 159.62 g/mol + (5 H20)( 18.02 g H20/mol) = 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%

PROCEDURE
1.  Accurately weigh a clean, dry crucible. Record this mass to +0.01 g.
2.  Transfer approximately 3 grams of barium chloride dihydrate, BaCl₂• 2 H₂O, into the weighed crucible and weigh the
     crucible and its contents. Record this mass to +0.01 g.
3.  Place the crucible on a ring stand using a ring and clay triangle and heat gently for 10 minutes.  Then heat the sample
     more strongly for 10 more minutes by bringing the flame of the bunsen burner directly under the dish.  The residue
     should be almost pure white.  Allow the crucible to cool, then weigh it.  Record this mass to +0.01 g.
4.  Heat the crucible for another 5 minutes, cool, then weigh.  If all the water has been driven off, the two masses should
     agree.  Record this mass to +0.01 g.
5.  Dispose of the barium chloride in the container provided.

DATA
1.  Mass of empty crucible _______________g

2.  Mass of crucible & BaCl₂• 2 H₂O _______________g

3.  Mass of BaCl₂• 2 H₂O _______________g (#2 - #1)

4.  Mass of crucible & BaCl₂ after first heating _______________g

5.  Mass of BaCl₂ after first heating _______________g (#4 - #1)

6.  Mass of crucible & BaCl₂ after second heating _______________g

7.  Mass of BaCl₂ after second heating _______________g (#6 - #1)

8.  Mass of water lost _______________g H₂O

9.  Percent hydration _______________% H₂O

10.  Theoretical value _______________% H₂O

11.  Percent error _______________%

Atomic masses: H = 1.01; O = 16.00; Cl = 35.45; Ba = 137.33

Reminders: 1.  Barium chloride is toxic. Use care when handling. 2.  The used barium chloride should be put in the waste container provided.       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. 3.  The ring stand, ring, and crucible are hot.   BE CAREFUL!!!!!