Ch 10 the mole
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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.

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Ch. 10: The Mole

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Ch 10 the mole

Ch. 10: The Mole

Sec. 10.5: The Formula for a Hydrate


Formula for a hydrate

Formula for a Hydrate

  • Determine the formula for a hydrate from laboratory data.


Formula for a hydrate1

Formula for a Hydrate

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


Formula for a hydrate2

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 hydrate3

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 hydrate4

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 hydrate5

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 hydrate6

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 hydrate7

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


Formula for a hydrate8

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

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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