1. Empirical and Molecular Formulas
Honors Chemistry
Mrs. Coyle

2. Empirical Formula
A chemical formula that represents the simplest whole number ratio of atoms in a compound.

3. Problem # 1
Determine the empirical formula of C4H10 .
Answer: C2H5

4. Molecular Formula
The exact formula of a molecule.
(Empirical Formula)n
n= Molar Mass
Empirical Formula Mass

5. Problem # 2
Determine the empirical and molecular formulas of a compound that has a percent composition by mass of
43.64% phosphorous and 56.36% oxygen and a formula mass of g.

6. Problem # 2 Solution Assume 100g of the compound.
#mol P= g P x 1 mol P=1.409 mol P
30.97g P
#mol O= 56.36g O x 1 mol O= mol O
16.00 g O

7. Problem # 2 Solution Cont’d
Find the mole ratio of the larger to the smaller number of moles.
3.523 mol O = 2.5 mol O
1.409 mol P mol P
Simplest whole number ratio= 5 mol O
2 mol P
So, empirical formula: P2O5

8. Problem # 2 Solution Cont’d
Molecular formula calculation:
n=Molar Mass______ = g = 2
Empirical Formula Mass g
So, Molecular Formula = (P2O5)2
P4O10