1. Empirical & Molecular Formula Notes
Chemistry 4/8/15

2. Drill Calculate the percent composition of Na2SO4.
HW: Finish pg. 3 (Empirical Formulas)
MM= g/mol
32.37% Na, 22.58% S, 45.05% O

3. Objectives
IWBAT
Calculate the empirical formula of a compound, given percent composition or mass composition.
Calculate the molecular formula of a compound, given:
Empirical formula and molar mass OR
Percent composition and molar mass

4. HW Review
I’ll go over the answers—see if we need to put any on the board!

5. Empirical and Molecular Formulas

6. Empirical Formula Empirical vs. Molecular Formulas
Molecular Formula – a formula that specifies the actual number of atoms of each element in one molecule of a compound.
Empirical Formula – a formula with the smallest whole-number mole ratio of the elements that make up a compound.

7. Empirical Formula Empirical Formula
May or may not be the same as the molecular formula
Molecular formula is always a simple multiple of the empirical formula
ex. H2O2
Empirical formula is HO
Molecular formula is TWO times the empirical formula

8. How to calculate an empirical formula
STEP 1: You will be given either masses or percent composition.
STEP 2: If you are given % composition, turn it into grams by assuming a g sample. NOTE: If you are given mass, you do not need to do this step.
STEP 3: Convert the masses to the number of moles of each element.

9. STEP 4: Figure out the proportion of moles of each element in the compound by dividing each by the smallest number of moles.
STEP 5: If step 4 resulted in whole numbers, you are done! However, if there were decimals, you will need to multiply by small, whole numbers until you have whole numbers.

10. An example: STEP 1: Compound is 40.05% S and 59.95% O STEP 2:
I assume 100 g of the compound, so I have:
40.05 g S and g O
STEP 3:
40.05 g S•(1 mol S/32.07 g S) = mol S
59.95 g O•(1 mol O/16.00 g O) = mol O

11. Continued… STEP 4: 1.249 mol S : 3.747 mol O
Divide each by (smallest number in ratio)
1 mol S : 3 mol O
STEP 5:
SO3
You are done! The compound is sulfur trioxide.

12. A way to remember those steps:
A Poem by Joel Thompson:
Percent to mass
Mass to mole
Divide by small
Multiply ‘til whole

13. Molecular Formula
Molecular Formula – this tells us how many atoms of each type there really are in the compound.
Can two substances have the same empirical formula but be different?
YES! Benzene vs. acetylene: C6H6 vs. C2H2
What is their empirical formula? How is this different from ionic compounds?

14. Calculating Molecular Formula
STEP 1:
You will be given the molar mass of the compound and the empirical formula.
STEP 2:
Calculate the empirical mass (mass of the empirical formula).
STEP 3:
Divide the given molar mass by the empirical mass. You should get a small whole number.
STEP 4:
Multiply the subscripts of the empirical formula with the number obtained.

15. Molecular Formula Example
STEP 1:
The empirical formula is CH2O and the molar mass is g.
STEP 2:
The empirical mass is 12.01g + 2(1.01g) g = g
STEP 3:
g/ g = 6
STEP 4:
CH2O becomes C6H12O6

16. Empirical Formula Practice
Calculate the empirical formula for a compound with a percent composition of % Fe and 46.27% S.
Fe2S3

17. Practice
The formula mass of a compound is 92 u. Analysis shows that there are g N and g O. What is the molecular formula?
N2O4

18. Closure
Acetylene (C2H2) is a gas that is used as a fuel for welding. Benzene (C6H6) is a liquid solvent.
How are they similar?
How are they different?
Why are they different?
Why is one a gas at room temperature and one a liquid?