1. Sec. 10.5: The Formula for a Hydrate
Ch. 10: The Mole
Sec. 10.5: The Formula for a Hydrate

2. Formula for a Hydrate
Determine the formula for a hydrate from laboratory data.

3. Formula for a Hydrate
A hydrate is a compound that has a specific number of water molecules bound to its atoms.

4. Formula for a Hydrate
In the formula for a hydrate, the number of water molecules associated with each formula unit of the ionic compound is written following a dot
Na2CO3•10H2O
This is the same thing as saying that for every 1 mole of Na2CO3, there are 10 moles of water. There is a 1:10 mole ratio.

5. Formula for a Hydrate
We know that to analyze the amount of water in a hydrate you must drive off the water of hydration.
Often this is done by heating the compound.
The substance remaining is anhydrous or “without water”.

6. Formula for a Hydrate
Suppose you have a 5.00 g sample of a hydrate of cobalt chloride.
Its formula is CoCl2•xH2O
The value of x indicates the number of moles of water associated with 1 mole of cobalt chloride. X must be determined.

7. Formula for a Hydrate
The sample of the hydrate is heated to drive off the water of hydration.
The mass of the anhydrous cobalt chloride is determined. It is 2.72 g.

8. Formula for a Hydrate
The mass of the water of hydration is the difference between the mass of the hydrate (5.00 g) and the mass of the anhydrous compound (2.80 g).
5.00 g – 2.72 g = 2.28 g H2O

9. Formula for a Hydrate
This means there was 2.72 g of CoCl2 and 2.28 g of H2O in the hydrate.
Those masses must now be converted to moles.
2.72 g CoCl2 x 1 mole = mol CoCl2
129.9 g
2.28 g H2O x 1 mole = mol H2O
18 g

10. Formula for a Hydrate
Now the following formula must be used to find x in CoCl2•xH2O
x = moles H2O/moles anhydrous compound so, X = .127/.0209 = 6
This means 6 moles of water are associated with 1 mole of cobalt chloride.
The formula is CoCl2 • 6H2O
The name: cobalt(II) chloride hexahydrate.

11. Practice Problems
2.5 g of a copper sulfate hydrate (CuSO4 • xH2O) is heated. After heating, the sample is massed at 1.59 g. What is the formula and name of this hydrate?
A hydrate is found to have the following percent composition: 48.8% MgSO4 and 51.2% H2O. What is the formula and name for this hydrate?