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# Ch. 10: The Mole - PowerPoint PPT Presentation

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.

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### Ch. 10: The Mole

Sec. 10.5: The Formula for a Hydrate

• Determine the formula for a hydrate from laboratory data.

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

• 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.

• 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”.

• 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.

• 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.

• 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

• 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

• 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.

• 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?