1. Calculation of Molecular Formula
If we know the composition of a compound in terms of the mass percentages of the elements, we can determine the empirical formula, but not the molecular formula.
In most mass spectrometry analysis, the mass percentages of the elements present, as well as an estimated molar mass is given as a result of the test.
This information can be used to determined the empirical and molecular formula of a substance.

2. Calculation of Molecular Formula
Example: A compound used as an additive for gasoline to help prevent engine knock (Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder does not start off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front… in case you were wondering)
shows the following percent composition: 71.65% Cl, 24.27% C, 4.07% H. The molar mass is approximately grams. Determine the molecular formula.

3. Calculation of Molecular Formula
Example: A compound used as an additive for gasoline to help prevent engine knock shows the following percent composition: 71.65% Cl, 24.27% C, 4.07% H. The molar mass is approximately grams. Determine the molecular formula.
Equation: (Empirical Formula)n = molecular formula
n = molar mass / empirical formula mass
Approach: a) Determine the empirical formula (sing the song!)
b) Divide the molar mass (given) by the empirical formula mass to find “n”
c) multiply the empirical formula by “n”

4. Calculation of Molecular Formula
Example: A compound used as an additive for gasoline to help prevent engine knock shows the following percent composition: 71.65% Cl, 24.27% C, 4.07% H. The molar mass is approximately grams. Determine the molecular formula.
Equation: (Empirical Formula)n = molecular formula
n = molar mass / empirical formula mass
Approach: a) Determine the empirical formula (sing the song!)
- Cl2C2H4
b) Divide the molar mass (given) by the empirical formula mass to find “n”
divided by = 2
c) multiply the empirical formula by “n”
- Cl4C4H8

5. What is a Hydrated Compound or Hydrate?
Solid ionic compound with associated water molecules
Many ionic compounds have one or more water molecules complexed in their crystalline structure
Copper(II) sulfate is a hydrate.
Hydrated copper(II) sulfate is deep blue in color.

6. The chemical formula for a hydrate includes the formula unit for the ionic compound and the number of complexed water molecules separated by a dot:
a) CaSO4 • 2 H2O
calcium sulfate dihydrate
b) CoCl2 • 6 H2O
cobalt II chloride hexahydrate

7. What does the Chemical Formula of A Hydrate Look Like?
BaCl2•2H2O
barium chloride dihydrate
FeSO4•6H2O
iron(II) sulfate hexahydrate
Na2CO3•10H2O
sodium carbonate decahydrate
CuSO4•5H2O
copper (II) sulfate pentahydrate

8. The water molecule is highly polar

9. The positive end of the water molecule can interact with
the negative ion or anion of an ionic compound

10. The negative end of the water molecule can interact with
the positive or cation of an ionic compound

11. The number of water molecules depends on the size and
structure of the crystal structure formed by the ionic
compound

12. Purpose of lab tomorrow:
1. How do you separate the water from the ionic
compound?
Heating is usually the easiest and most effective.
Water is turned into steam (a gas)

13. Analysis of an unknown hydrate:
There is no simple way to predict the number of water molecules found in a hydrate compound.
It must be determined empirically by heating the compound and evaporating the water
Example: A 14 gram sample of hydrated cobalt II chloride is heated thoroughly to remove all the water. The resulting anhydrous cobalt II chloride weighs g. What is the chemical formula of the hydrated cobalt II chloride

14. Example: A 14 gram sample of hydrated cobalt II
chloride is heated thoroughly to remove all the water.
The resulting anhydrous cobalt II chloride weighs
7.65 g. What is the chemical formula of the hydrated
cobalt II chloride ?
To know the chemical formula, you must determine the number of moles of water present per mole of cobalt II chloride
Approach: a) the 7.65 g is only anhydrous cobalt II chloride
b) the difference between 7.65 g and the original 14 g is the amount of water present in the original sample

15. Approach: a) the 7.65 g is only anhydrous cobalt II
chloride
b) the difference between 7.65 g and
the original 14 g is the amount of
water present in the original sample
7.65 g 1 mole = mol CoCl2
130 g
= 6.35 g H2O 1 mole = mol H2O
18 g
0.353 mol H2O = 6 mol H2O → CoCl2 • 6 H2O
mol CoCl mol CoCl2

16. A 0.234 g sample of hydrated nickel II chloride is heated
thoroughly to remove all the water. The anhydrous
result weighs g. What is the chemical formula of
the hydrated nickel II chloride
0.128 g 1 mole = mol NiCl2
127 g
= g H2O 1 mole = mol H2O
18 g
mol H2O = mol H2O → NiCl2 • 6 H2O
mol NiCl mol NiCl2

17. Dry-rite is used to dehumidify drawers and closets
Dry-rite is used to dehumidify drawers and closets. Anhydrous calcium chloride is added to an area and left for several days where it attracts water from the air. An gram sample of anhydrous calcium chloride is added to an enclosed area and left for several days. The mass of the sample increases to 11.0 grams. Determine the name and chemical formula of the hydrated calcium choride.

18. Dry-rite is used to dehumidify drawers and closets
Dry-rite is used to dehumidify drawers and closets. Anhydrous calcium chloride is added to an area and left for several days where it attracts water from the air. An gram sample of anhydrous calcium chloride is added to an enclosed area and left for several days. The mass of the sample increases to 11.0 grams. Determine the name and chemical formula of the hydrated calcium chloride.
8.33 g CaCl mole = mole
111 g
11.0 – 8.33 = 2.67 g H2O 1 mole =
18 g
0.148 = 1.98 ≈ CaCl2 · 2 H2O
0.075
Calcium chloride dihydrate

19. After being heated thoroughly, a 1.023 g sample of
hydrated copper II sulfate is reduced to a weight of
0.653 g. Determine the chemical formula of hydrated
copper II sulfate

20. After being heated thoroughly, a 1.023 g sample of
hydrated copper II sulfate is reduced to a weight of
0.653 g. Determine the chemical formula of hydrated
copper II sulfate.
0.653 g 1 mole = mol CuSO4
160 g
= g H2O 1 mole = mol H2O
18 g
mol H2O = mol H2O → CuSO4 • 5 H2O
mol CuSO mol CuSO4