1. Empirical and Molecular Formulas

2. Empirical vs. Molecular Formulas
Empirical Formula – the simplest formula. A formula with the smallest whole-number ratio of the elements that make up a compound.
Molecular Formula – the true formula A formula that specifies the actual number of atoms of each element in one molecule of 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 (ratio) of the empirical formula
ex. H2O2
Empirical formula is HO
Molecular formula is TWO times the empirical formula

4. Copy Table Name Molecular Formula Empirical Formula
Lowest ratio of elements
Hydrogen peroxide
H2O2 ÷2 HO
1:1
Glucose
C6H12O6 ÷6
CH2O
1:2:1
Benzene
C6H6 ÷6 CH
Ethyne
C2H2 ÷2
Aniline
C6H7N
6:7:1
water
H2O
2:1

5. 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.

6. 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.

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

8. 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

9. 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.

10. Molecular Formula

11. 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?

12. 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.

13. Your ratio will give you a whole number ratio.
Step #3 in more detail
Ratio = Molar Mass (Actual)
Molar Mass (EF)
Your ratio will give you a whole number ratio.

14. Example
The empirical formula of a compound is found to be CH2O. After careful analysis the molar mass is found to be g. Determine the molecular formula of this compound.

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:
Ratio= g/ g = 6
STEP 4:
CH2O becomes C6H12O6