1. Hydrates
Percent Composition
Empirical Formula
Naming
Chapter 8

2. What is a hydrate?
An ionic compound that contains water inside its crystal structure.
These compounds do not look or feel wet.
The water molecules are loosely attached and can be removed by heating.
Generic Formula
MxNmy • nH2O
MxNmy • nH2O hydrate
MxNmy • nH2O anhydrate
MxNmy • nH2O water

3. How do we analyze a hydrate?
The water molecules are loosely attached and can be removed by heating.
MxNmy • nH2O → MxNmy + nH2O
The remaining anhydrate (anhydrous crystals) may look the same, may have a different texture or may even be a different color
Mx – Metal Ion; Nmy – Nonmetal Ion; n = prefix # ∆

4. Law of Constant Composition
Determining the mass ratio of unknown “SG”
SG was a hydrate, G was the water S was the anhydrate
Look at this Lab Data
SG was actually
Potassium aluminum sulfate dodecahydrate
KAl(SO4)2 • 12H2O
Calculate the theoretical % of water in this compound
water
hydrate
45.6% water
Per 5 SG
14.505 S
7.773
G (water)
6.732
% water
46.4
12(18.02) [216.2]
x100 =
(32.07)+8(16)+12(18) [474.4]

5. Lets test a copper(II) sulfate hydrate
Measure the mass of the hydrate
8.986 g hydrate to start with
Heat the compound to drive off the water
Measure the mass of the remaining anhydrate
5.746 g anhydrate
Calculate the mass of water removed, H2O
= g water removed
Calculate moles of water
3.243 * 1mol/18g = mol H2O
Calculate moles of anhydrate, formula
5.764 * 1mol/159.6 = mol CuSO4
Determine the mole ratio water / anhydrate
0.180 / = 4.999/1
Thus CuSO4 • 5 H2O

6. Lets test barium chloride hydrate
Measure the mass of the hydrate
2.11 g hydrate to start with
Heat the compound to drive off the water
Measure the mass of the remaining anhydrate
1.78 g anhydrate
Calculate the mass of water removed, H2O
2.11 – 1.78 = 0.33 g water removed
Calculate moles of water
0.33 * 1mol/18g = mol H2O
Calculate moles of anhydrate, formula
1.78 * 1mol/208.2 = mol BaCl2
Determine the mole ratio water / anhydrate
0.0183mol H2O/ = 2.1
Thus BaCl2 • 2 H2O

7. A 344 gram sample of hydrated calcium sulfate and the sample is heated to evaporate the water. The dry sample of calcium sulfate has a mass of 272 grams. What is the mole ratio between the calcium sulfate, CaSO4 and water, H2O? What is the formula of the hydrate?
Step 1: Calculate the difference between the hydrated sample and the dry sample.
344 grams (hydrate) grams (dry sample) = 72.0 grams of water
Step 2: Convert mass to moles for each sample.
272g CaSO4(1 mol/136.14g) = mol CaSO4
72.0g H2O(1mol/18.02g) = mol H2O
Step 3: Divide by lowest molar value
1.99/1.99 = 1 mol CaSO4
3.99/1.99 = 2 mol H2O
CaSO4 • 2 H2O

8. A 87.2 gram sample of a hydrate of MgI2 was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 57.4 grams of the anhydrous compound remained. What is the formula of the hydrate?
Step 1: Calculate the difference between the hydrated sample and the dry sample.
87.2 grams (hydrate) – 57.4 grams (dry sample) = 29.8 grams of water
Step 2: Convert mass to moles for each sample.
57.4g MgI2(1 mol/278.11g) = mol MgI2
29.8g H2O(1mol/18.02g) = mol H2O
Step 3: Divide by lowest molar value
1.99/1.99 = 1 mol MgI2
3.99/1.99 = 2 mol H2O
MgI2 • 2 H2O 8

9. A hydrate was analyzed and determined to be 34% water
A hydrate was analyzed and determined to be 34% water. Further the anhydrate was analyzed to be 23.96% nickel, 17.16% nitrogen, and 58.88% oxygen. Determine the formula for this hydrate

10. A hydrate was analyzed and determined to be 34% water
A hydrate was analyzed and determined to be 34% water. Further the anhydrate was analyzed to be 23.96% nickel, 17.16% nitrogen, and 58.88% oxygen. Determine the formula for this hydrate