1. 85.7% C Percent Composition 24g C 28g CH4 mass of element x 100 x 100
The relative amounts of elements in a compound.
% composition =
mass of element
x 100
mass of compound
24g C
85.7% C
x 100 =
28g CH4

2. Percent Composition from Formula
C2H4
2 moles of C x 12g/mol = 24 g
4 moles of H x 1 g/mol = 4 g
28 g
24g/28g x 100 = 85.7% C
4g/ 28g x 100 = 14.3% H

3. You try one:
NaC2H3O2

4. Empirical Formula
The lowest whole number ratio of elements in a compound

5. Calculate empirical formula from percent composition:
1st - Begin by pretending to have 100 g sample & convert the % to
grams.
2nd - Convert grams to the moles by dividing by molar mass.
3rd - Obtain whole number ratios of moles by:
1) dividing each number by the smallest number
2) multiplying to get rid of any fractions
4th - Write the empirical formula using the number of moles as
subscripts.

6. Example 1
Calculate the empirical formula of a compound composed of % C, % H, and %N.
Assume 100 g so
38.67 g C / 12 g/mol C = mole C
16.22 g H / 1.07g/mol H = mole H
45.11 g N / 14 g/mol N = mole N
Divide each by the smallest value

7. Example 1 cont. 3.220 mole C / 3.22 = 1 16.09 mole H / 3.22 = 5
3.22 mole N / = 1
The empirical formula is CH5N

8. Example 2
A compound is % P and % O. What is the empirical formula?
43.6 g P /31 g/mol = 1.4 mole P /1.4 = 1
56.4 g O /16 g/mol = 3.5 mole O /1.4 = 2.5
Divide by smallest number
Multiply the result to get rid of any fractions.
P1O2.5
= P2O5
2 X

9. Molecular Formulas Are whole number multiples of empirical formulas.
Divide molar mass of compound by molar mass of empirical formula.
Multiply subscripts in compound by the number from step 1.
Ex. An empirical formula is CH5N.
The molar mass is 93 g/mol.
93 g/mol / 31 g/mol = 3
3 x CH5N = C3H15N3