1. Journal #10
A compound is found to contain 63.25% Iron and 36.48% sulfur. Find its empirical formula

2. Today we will calculate molecular formulas

3. Calculations of Molecular Formulas
Remember that the empirical formula contains the smallest possible whole numbers that describe the atomic ratio.
The molecular formula is the actual formula of a molecular compound.
An empirical formula may or not be a correct molecular formula.
Example: Ethene C2H4 and cyclopropane C3H6 have a different molecular formula but the same empirical formula

4. The relationship between a compound’s empirical formula and its molecular formula can be written as:
x(empirical formula mass) = molecular formula mass
To calculate the molecular formula of a compound, you must know the compounds formula mass.
Example: experimentation shows the formula mass of diborane to be amu. The formula mass for the empirical formula of BH3 is Dividing the formula mass by the empirical formula mass gives the value of x for diborane.
X= amu / amu = 2.000
The molecular formula of diborane is B2H6

5. Practice
The empirical formula of a compound was found to be P2O5. Experimentation shows that the molar mass of this compound is g/mol. What is the compounds molecular formula?
(Calculate empirical formula mass)
Mass of phosphorus=30.97 amu
Mass of oxygen= amu
P2O5= 2 (30.97) + 5 (16.00) = amu

6. Continued..
Divide the expermental formula mass by the empirical formula mass to get the value of x
/ = 2.000
The molecular formula is therefore P4O10