Percent Composition, Empirical
Formulas, Molecular Formulas
Percent Composition
•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
Percent composition Mass of element in 1 mol
of a compound or = ____________________ x 100%
Mass of 1 mol of compound
molecule
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:
C4H10 - molecular
C2H5 - empirical
C6H12O6 - molecular
CH2O
- empirical
Formulas
Is H2O2 an empirical or molecular formula?
Molecular, it can be reduced to HO
HO = empirical formula
Calculating Molecular Formula
A white powder is analyzed and found to have an
empirical formula of P2O5. The compound has a molar
mass of 283.88g/mol. What is the compound’s molecular
formula?
Step 3: Multiply
Step 1: Molar Mass
P = 2 x 30.97 g = 61.94g
O = 5 x 16.00g = 80.00 g
141.94 g
Step 2: Divide MM by
Empirical Formula Mass
238.88 g
=2
141.94g
(P2O5)2 =
P4O10
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 = 12.01 g/mol
H = 1.01 g/mol
13.02 g/mol
78 g/mol
13.02 g/mol
=6
(CH)6 =
C6H6
Homework:
Page 298 #1
Page 300 #2