1. 7.3b Hydrates

2. POINT > Define what a hydrate is
POINT > Determine the formula and name of a hydrate

3. POINT > Define what a hydrate is
A hydrate is an ionic compound that contains water molecules as part of its crystalline structure
The water molecules are not random; they occupy specific positions in the hydrate crystal structure
The water in the hydrate is known as the water of hydration or the water of crystallization
The compound without the water is called the anhydrate

4. POINT > Define what a hydrate is
A substance is hygroscopic if it readily absorbs water from the atmosphere and forms a hydrate
Hygroscopic compounds are used as dessicants
Dessicants are compounds that absorb water vapor and are used as drying agents

5. WB CHECK:
Store-bought vitamins often contain silica gel to absorb water and prevent the vitamins from spoiling.
The silica gel that absorbed H2O could be described as
a) a dessicant
b) hygroscopic
c) anhydrous
d) a and b
e) all of the above

6. POINT > Determine the formula and name of a hydrate
Hydrates formulas are expressed by writing the anhydrous formula (without water) for the compound with a dot followed by the ratio of water molecules present
Example: BaCl2•2H2O
This shows that each BaCl2 formula unit contains one barium ion, two chloride ions and two water molecules
This also shows that 1 mole of BaCl2 combines with 2 moles of water

7. POINT > Determine the formula and name of a hydrate
Naming Hydrates:
1. Name the compound
2. Add the term “hydrate” with a prefix representing the number of water molecules in the formula
Example: BaCl2•2H2O is barium chloride dihydrate

8. WB CHECK:
What is the name of the hydrate?
Na2SO4 * 5H2O
AlPO3 * 3 H2O
Mg(NO3)2 * 6 H2O

9. POINT > Determine the formula and name of a hydrate
Example problem
A 25.0 g sample of a hydrate of iron(III) phosphate was heated until all the water was driven off. The mass of anhydrate after heating is 16.9 g. What is the formula & name of the hydrate?
1. How many moles of anhydrate?
2. How many moles of water?
3. Determine ratio by dividing both quantities by the number of moles of anhydrate

10. POINT > Determine the formula and name of a hydrate
FePO4 molar mass = 150.9g 16.9g FePO4 x 1 mol/150.9g = mol FePO4 8.1g H2O x 1 mol/18.0g = mol H2O mol FePO4 ÷ = 1.00 mol FePO mol H2O ÷ = 4.02 mol H2O
FePO4 . 4 H2O
Iron(III) phosphate tetrahydrate

11. WB CHECK:
A calcium chloride hydrate has a mass of 4.72 g. After heating for several minutes the mass of the anhydrate is found to be 3.56 g. Use this information to determine the formula for the hydrate.
CaCl2 * 2H2O calcium chloride dihydrate

12. Homework:
Hydrate Problem Set Pre-Read Hydrate Lab for tomorrow