1. Empirical & Molecular Formulas

2. Empirical Formula: a formula which has been reduced to the lowest terms.
Molecular formula: a formula of a compound in which the subscripts give the actual number of each element in the formula

3. Here are the four formulas being used as examples:
Molecular Formula
Empirical Formula
H2O
C2H4O2
CH2O
C6H12O6
Notice two things:
1. The molecular formula and the empirical formula can be identical.
2. You scale up from the empirical formula to the molecular formula by a whole number factor.

4. Why chemists use it?
After a compound has been analyzed for percent composition, the formula can be calculated- the empirical formula. If the actual weight of a single molecule is known then the molecular formula can be calculated as well.
% comp.  Empirical Formula
Actual weight  Molecular formula

5. How to Find Molecular Formulas
1.Calculate the mass of the empirical formula (which you have already found or it will be given to you )
2.Divide the known molar mass by the mass of the empirical formula.
3.Multiply that number by the subscripts of the empirical formula to get the subscripts for the molecular formula.

6. I DO: The formula mass of a compound is 181
I DO: The formula mass of a compound is g/mol and the empirical formula is C2HCl. What is the molecular formula?

7. WE DO: The empirical formula of a compound is C4H9
WE DO: The empirical formula of a compound is C4H9. If the formula mass is 114 g/mol, what is the molecular formula of this compound?

8. YOU DO:
Find the molecular formula of Nutrasweet.  (The molar mass of NutraSweet is g/mol)
(HINT: You have already found the empirical formula of Nutrasweet)

9. Reminder
To determine the molecular formula, you must know the compound’s empirical formula AND the molar mass of the molecular compound.