1. Empirical and Molecular Formulas
Chemistry GT 3/20/15

2. Calculate the percent composition of Na2SO4.
Chem Joke of the Day
The photon checked into a hotel. What did it say when the bellhop asked for its bag?
“No bags—I’m travelling light!”
Drill
Calculate the percent composition of Na2SO4.
HW: Molecular Formula WS (pg. 4)
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. Agenda Drill HW Review Molecular Formula Notes
Empirical and Molecular Formula Practice Problems
Closure

5. HW Review
Let’s go over the answers—any questions??

6. 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?

7. Calculating Molecular Formula
STEP 1:
You will be given the molar mass of the compound and the empirical formula (or info to calculate 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.

8. 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

9. Practice With a group of 3-4; one sheet to turn in
Calculate the empirical formula:
Fe 46.56%, S 53.44%
Fe 63.53%, S 36.47%
Mn 63.1%, S 36.9%
K 26.6%, Cr 35.4%, O 38.0%
Na 36.5%, S 25.4%, O 38.1%
Na 32.4%, S 22.6%, O 45.0%
Calculate the molecular formula:
P 56.4%, O 43.7%, molar mass = 220 g
P 43.6%, O 56.4%, molar mass = 284 g

10. Answers Empirical Formula FeS2 FeS MnS K2Cr2O7 Na2SO3 Na2SO4
Molecular Formula
P4O6
P4O10

11. Closure – Empirical Formula Practice
Calculate the empirical formula for a compound with a percent composition of 53.73% Fe and 46.27% S.

12. Closure – 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