Chemistry 11.5

Objectives


Explain what a hydrate is and how its name reflects is composition.
Determine the formula for a hydrate.


Hydrate
Key Terms

Naming Hydrates




A compound that has a specific number of water molecules bound to its atoms.
The number of water molecules is written following a dot.
Na
2
CO
3

• 10 H
2
O 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



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



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


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 BaCl
2
• x H
2
O.
After heating substance until water is driven off, mass is 4.26 g.

Formula for a Hydrate




5.00 g BaCl
= 0.74 g H
2
2 hydrate – 4.26 g anhydrous BaCl
O.
2
Now convert the grams to mol.
4.26 g BaCl
2 x 1 mol BaCl
2
= 0.0205 mol BaCl
2
208.23 g BaCl
2
0.74 g H
2
O x 1 mol H
2
O = 0.041 mol H
2
O
18.02 g H
2
O

Formula for a Hydrate


To determine the coefficient that precedes water in the formula for a hydrate:
(moles water/moles compound)

0.041 mol H
2
O = 2
0.0205 mol BaCl
2

BaCl
2
• 2 H
2
O


Pg 340
Practice Problems

Homework

Section 11.5 Problems 33 and 34 on page 877