1. Calculating Empirical and Molecular Formulas
What is an Empirical Formula?
The formula of a compound expressed as the smallest possible whole-number ratio of subscripts of the elements in the formula.
What is a Molecular Formula?
It is the formula of a compound in which the subscripts give the actual number of each element in the formula.

2. Calculating Empirical Formula
What if a percent composition is given. How do we find the empirical formula?
Example: Chemical analysis of a liquid shows that it is 60.00% C, % H, and 26.60% O by mass. Calculate the empirical formula of this substance.
Step 1: Convert to grams
Assume you have a g sample, and convert percentages to grams.
60.00% Carbon is the same as g of C
13.40% of Hydrogen is the same as g of H
26.60% of Oxygen is the same as g of O

3. Step 2: Convert grams to moles Step 3: Divide each by the smallest decimal (in moles) to get whole numbers. * These numbers will be the subscripts.
1 mole C
60.00 g C
= 5.00 mole C
÷ 1.66
= mol C
12.01 g C
Round those answers to the nearest whole number and use them as the Subscripts on the final Empirical Formula.
13.40 g H
1 mole H
= 13.27mole H
÷ 1.66
= mol H
1.01 g H
26.60 g O
1 mole O
= 1.66 mole O
÷ 1.66
= 1.00 mol C
16.00 g O
THE EMPIRICAL FORMULA WOULD BE:
C3H8O1  C3H8O
Of the three calculated numbers, this is the smallest decimal in moles, so you divide all the other decimals and itself by that number.

4. Example 2 for Empirical Formulas
A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What is the empirical formula?
Step 1: Convert to grams : % Mg = 72.2 g Mg % N = 27.8 g N
Step 2: Convert grams to moles
72.2 g Mg
1 mole Mg
= 2.97 mole Mg
= 1.49 mole Mg
24.31 g Mg
27.9 g N
1 mole N
= 1.99 mole N
÷ 1.99
14.01 g N
Step 3: Divide each by the smallest decimal (in moles) to get whole numbers.
Still not whole numbers. We have to get rid of the If we multiply by 1 we get the same number of course. So, lets multiply by 2 and see what happens. Remember whatever we do to one side we do it to the other.
1.00 mole N x 2 = 2 mole N
THE EMPIRICAL FORMULA WOULD BE:
Mg3N2

5. How do we determine the molecular formula from the empirical formula?
Example: The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound.
Step 1: Find the molar mass of the empirical formula.
2 mol P x g = g P
5 mol O x 16.00g = g O
g/mol

6. Step 2: Molar mass of a compound divided by the molar mass of empirical formula.
Experimental molar mass of compound Molar mass of empirical formula 284 g/mol g/ mol
= 2.00
YOUR MOLECULAR FORMULA IS:
P4O10

7. Assignment
Determine the Empirical Formula for each of the following. Use the calculated empirical formula to calculate the molecular formula.
A sample compound with a molar mass of 34.00g/mol is found to consist of 0.44g H and 6.92g O. Calculate both empirical and molecular formulas.
A compound has a molar mass of g/mol and consists of 3.0 g of Fe and 4.81g of S. Calculate both empirical and molecular formulas
A compound consists of 36.48% Na, 25.41% S, and 38.11% O. It
has a molar mass of g/mol. Calculate both empirical and
molecular formulas.