1. Chapter 3 Percent Compositions and Empirical Formulas
AP Chemistry

2. Calculating Percent Composition of a Compound
The following general formula is used to determine the percent composition of each element in a compound:
Mass of element
(in 1 mole of compound) X 100
Molar Mass of Compound

3. What is the percent composition of each element in Ca(NO3)2 ?
Step 1: Find the molar mass of Ca(NO3).
Ca grams/mole = g/mol
N 2 x (14.0) grams/mole = 28.0g/mol
O 6 x (16.0) grams/mole = g/mol
164.1 g/mole

4. Step 2: Divide the mass of each element by the molar mass and multiply by 100 to get the percent.
% Ca = g x 100% = % Calcium
164.1g
% N = g x 100% = % Nitrogen
164.1 g
% O = g x 100% = % Oxygen

5. *Note percent composition can also be done given masses of a compound and each element in the compound.
Calculate the percent composition of a compound that is made of 29.0 grams of Ag with 4.30 grams of S.
29.0 g Ag
X 100 = 87.1 % Ag
33.3 g total
Total = 100 %
4.30 g S
X 100 = 12.9 % S
33.3 g total

6. Formulas
Empirical formula: the lowest whole number ratio of atoms in a compound.
Molecular formula: the true number of atoms of each element in the formula of a compound.
The empirical formula for C3H15N3 is CH5N.
you are basically dividing by the greatest common factor , which in this case is 3, for each element subscript.

7. Formulas (continued)
Formulas for ionic compounds are ALWAYS empirical (the lowest whole number ratio = cannot be reduced).
Examples:
NaCl
MgCl2
Al2(SO4)3
K2CO3

8. Formulas (continued)
Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio).
Molecular:
H2O
C6H12O6
C12H22O11
(Correct formula)
Empirical:
H2O
CH2O
C12H22O11
(Lowest whole number ratio)

9. Calculating Empirical
We can get a ratio from the percent composition.
Assume you have a 100 g sample
- the percentage become grams (75.1% = 75.1 grams)
Convert grams to moles.
Find lowest whole number ratio by dividing each number of moles by the smallest value.

10. Example
Calculate the empirical formula of a compound composed of % C, % H, and %N.
Step 1 : Assume 100gram sample
Step 2 : Convert grams to moles for each element
38.67 g C x 1mol C = 3.22 mole C g C
16.22 g H x 1mol H = mole H g H
45.11 g N x 1mol N = 3.22 mole N g N
Now divide each value by the smallest value

11. Example The ratio is 3.22 mol C = 1 mol C 3.22 mol N 1 mol N
The ratio is mol H = 5 mol H mol N mol N
= C1H5N1 which is = CH5N

12. Empirical to molecular
Since the empirical formula is the lowest ratio, the actual molecule would weigh more.
By a whole number multiple.
Divide the actual molar mass by the empirical formula mass – you get a whole number to increase each coefficient in the empirical formula

13. Caffeine has a molar mass of 194 g
Caffeine has a molar mass of 194 g. and its empirical formula is C4H5N2O, what is its molecular formula?
Find the empirical formula molar mass.
97 g/mole
Set up ratio g = 2
97g
Multiply each subscript in the empirical formula by 2.
C8H10N4O2 is the molecular formula.

14. Empirical formula from combustion reactions
Example: grams of an organic compound containing carbon, hydrogen, and oxygen was analyzed by combustion. The amount of carbon dioxide produced was grams and the amount of water produced was grams. Determine the empirical formula of the compound.
Combustion reaction looks like the following:
CHO + O2  CO H2O

15. Assume all of the Carbon in the compound turned into carbon dioxide, and determine the mass percent of carbon in the compound
g CO2 1mole mol C g C
44g CO2 1 mol CO mol
= grams Carbon
g x = % Carbon
7.321 g

16. Assume all of the hydrogen in the compound went to water, and determine the mass percent of hydrogen in the compound
7.316 g H2O 1mol H2O 2 mols H 1.0 g H 18g H2O 1mol H2O 1 mol H = grams Hydrogen x 100 = 11.10% Hydrogen 7.321

17. With the mass percent of Carbon and Hydrogen, you can figure out the mass percent of oxygen in the compound.
(66.58% C % H) =
22.32 % Oxygen
Now use these percents to find the empirical formula!
C4H8O