Transcript Section 11.5

The Formula for a
Hydrate
Chemistry 11.5
Objectives
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Explain what a hydrate is and how its name
reflects is composition.
Determine the formula for a hydrate.
Key Terms
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Hydrate
Naming Hydrates
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A compound that has a specific number of water
molecules bound to its atoms.
The number of water molecules is written
following a dot.
Na2CO3 • 10 H2O
 sodium carbonate decahydrate (Table 11-1)
Note: When calculating molar mass, the mass of
water associated with the formula must be
included.
Analyzing a Hydrate
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To analyze a hydrate, the water associated with
the compound must be driven off.
Usually this is done by heating the material.
The remaining substance is termed anhydrous:
“without water.”
Uses of Hydrates
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Absorb water when in the anhydrous form.
Added to fuel to keep water out the of mixture.
Keeps electronics safe from humid air
conditions.
Formula for a Hydrate
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Determine the number of moles of water
associated with one mole of the hydrate.
Example: Suppose you have a 5.00 g sample of
a hydrate of barium chloride.
 Formula is BaCl2 • xH2O.
 After heating substance until water is driven
off, mass is 4.26 g.
Formula for a Hydrate
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5.00 g BaCl2 hydrate – 4.26 g anhydrous BaCl2
= 0.74 g H2O.
Now convert the grams to mol.
4.26 g BaCl2 x 1 mol BaCl2 = 0.0205 mol BaCl2
208.23 g BaCl2
0.74 g H2O x 1 mol H2O = 0.041 mol H2O
18.02 g H2O
Formula for a Hydrate
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To determine the coefficient that precedes
water in the formula for a hydrate:
(moles water/moles compound)
 0.041 mol H2O
=2
0.0205 mol BaCl2
 BaCl2
• 2 H2O
Practice Problems
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Pg 340
Homework
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Section 11.5 Problems 33 and 34 on page 877