1. 6.9 – NOTES Formula of a Hydrate

2. C. The formula of a hydrate
Hydrate: - salt crystal with water trapped inside
In order to determine the formula for a hydrate, you need to determine the mass of water in a sample by performing an experiment. From the masses of the anhydrous compound and the water, the formula can be found from the mole ratio.

3. Examples:
1. Borax, hydrated sodium tetraborate, Na2B4O7·? H2O, is 52.8% anhydrous sodium tetraborate and 47.2% water. Determine the formula and name the hydrate.
Na2B4O7 – 52.8 g (1 mol/ g) = / = 1
H2O – 47.2 g (1 mol/ 18.0 g) = / = 10
Na2B4O7  10 H2O

4. 2. A hydrate of cobalt(II) chloride was analyzed in the lab
2. A hydrate of cobalt(II) chloride was analyzed in the lab. The sample had a mass of 3.73 grams, and 1.69 grams of mass was lost. What is the formula of the hydrate?
**when calculating the formula of a hydrate, must start with the mass of the ANHYDROUS and water
CoCl2 – 3.73 – = 2.04g (1 mol/ g) = / = 1
(hydrate – water = anhydrous)
H2O – 1.69 g (1 mol/ 18.0 g) = / = 6
CoCl2  6H2O

5. 3. From the data below, determine the formula of a hydrate of iron (III) nitrate:
Mass of crucible and cover g
Mass of crucible, cover and Fe(NO3)3 hydrate g
Mass of crucible, cover and anhydrous Fe(NO3) g
Mass of anhydrous – – = 2.53 g
Mass of water – – = 1.70 g
Fe(NO3)3 – 2.53 g (1 mol / g) = / = 1
H2O – 1.70 g (1 mol/ 18.0 g) = / = 9
Fe(NO3)3  9 H2O