Empirical and Molecular
Formulas
Chemistry GT 3/20/15
• 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=142.05 g/mol
• 32.37% Na, 22.58% S, 45.05% O
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
Agenda
•
•
•
•
Drill
HW Review
Molecular Formula Notes
Empirical and Molecular Formula Practice
Problems
• Closure
HW Review
• Let’s go over the answers—any questions??
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?
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.
Molecular Formula
Example
• STEP 1:
– The empirical formula is CH2O and the molar mass is
180.18 g.
• STEP 2:
– The empirical mass is 12.01g + 2(1.01g) + 16.00g = 30.03 g
• STEP 3:
– 180.18 g/ 30.03 g = 6
• STEP 4:
– CH2O becomes C6H12O6
Practice
With a group of 3-4; one sheet
to turn in
Calculate the empirical
formula:
1.
2.
3.
4.
5.
6.
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:
1. P 56.4%, O 43.7%,
molar mass = 220 g
2. P 43.6%, O 56.4%,
molar mass = 284 g
Answers
• Empirical Formula
1. FeS2
2. FeS
3. MnS
4. K2Cr2O7
5. Na2SO3
6. Na2SO4
• Molecular Formula
1. P4O6
2. P4O10
Closure – Empirical
Formula Practice
• Calculate the empirical formula for a
compound with a percent composition of
53.73% Fe and 46.27% S.
Closure – Practice
• The formula mass of a compound is 92 u.
Analysis shows that there are 0.608 g N and
1.388 g O. What is the molecular formula?
• N2O4