1. Molecular Formulas

2. 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.

3. 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

4. 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.

5. 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.

6. An example: STEP 1: STEP 2: STEP 3: Compound is 40.05% S and 59.95% O
I assume 100 g of the compound, so it is:
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

7. Continued… STEP 4: STEP 5:
1.249 mol S : 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.

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

9. 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?

10. 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.

11. 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

12. Closure:
Acetylene is a gas that is used as a fuel for welding. Benzene 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?