1. Percent Composition, Empirical Formulas, Molecular Formulas

2. So… Percent Composition Part _______ Percent = x 100% Whole
Percent Composition – the percentage by mass of each element in a compound (*note: not the ratio of elements in the compound)
Law of Definite Proportions – compounds always contain the same proportions of elements by mass.
Part
_______
Percent =
x 100%
Whole
So…
Percent composition
of a compound or =
molecule
Mass of element in 1 mol
____________________
x 100%
Mass of 1 mol of compound

3. Formulas
Percent composition allow you to calculate the simplest ratio among the atoms found in compound.
Empirical Formula – formula of a compound that expresses lowest whole number ratio of atoms.
Molecular Formula – actual formula of a compound showing the number of atoms present
Examples:
C6H12O6
- molecular
C4H10
- molecular
C2H5
- empirical
CH2O
- empirical

4. Formulas Is H2O2 an empirical or molecular formula?
Molecular, it can be reduced to HO
HO = empirical formula

5. Calculating Molecular Formula
A white powder is analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of g/mol. What is the compound’s molecular formula?
Step 3: Multiply
Step 1: Molar Mass
P = 2 x g = 61.94g
O = 5 x 16.00g = g g
(P2O5)2 =
P4O10
Step 2: Divide MM by
Empirical Formula Mass g
= 2
141.94g

6. Calculating Molecular Formula
A compound has an experimental molar mass of 78 g/mol. Its % composition is found to be 92.24% carbon and 7.76% hydrogen. What is its molecular formula of this compound?
First, determine the empirical formula using the % composition.
Empirical formula of this compound is CH.
Determine the multiple
C = g/mol
H = g/mol
13.02 g/mol
(CH)6 =
C6H6
78 g/mol
= 6
13.02 g/mol

7. Homework:
Page 298 #1
Page 300 #2