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### Ch. 10: The Mole

Sec. 10.5: The Formula for a Hydrate

Formula for a Hydrate

- Determine the formula for a hydrate from laboratory data.

Formula for a Hydrate

- A hydrate is a compound that has a specific number of water molecules bound to its atoms.

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.

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”.

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.

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.

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

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 = 0.0209 mol CoCl2
129.9 g

- 2.28 g H2O x 1 mole = 0.127 mol H2O
18 g

- 2.72 g CoCl2 x 1 mole = 0.0209 mol CoCl2

Formula for a Hydrate

- Now the following formula must be used to find x in CoCl2•xH2O
x = molesH2O/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.

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?

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