This document provides examples and explanations for calculating percent composition, empirical formulas, and molecular formulas. It begins with examples of calculating percent composition of elements in compounds and mixtures. It then defines empirical and molecular formulas, and provides steps for calculating empirical formulas from mass percentages of elements or experimental data. Several examples are worked through. The document emphasizes that empirical formulas show the simplest whole number ratio of elements in a compound, while molecular formulas indicate the actual number of each type of atom in a molecule.

1. BY THE END OF THE CLASS PERIOD
TODAY I WILL BE ABLE TO…
Define and Calculate
‘percent composition’ of molecules
and compounds.

2. OUR PROBLEM FOR THE DAY…
What is the
percentage
composition of
ibuprofen??

3. FIRST THINGS FIRST…
Chex Mix!
Your bag of Chex mix contains 15 pretzels and 3 rye crackers
and a whole lot of Chex. What is the percent of pretzels in the
mixture?

4. WHAT IS THE PERCENTAGE
COMPOSITION OF PRETZELS IN MY
MIXTURE??
Mass of Pretzels
Total Mass of Chex Mix
X 100 = mass % of Pretzels
We can use this same idea to calculate the
mass % of elements in a
compound…consider CaCl2

5. IMPORTANT NOTE:
Percent composition: the
percentage by mass of
each element in a
compound.

6. PERCENT COMPOSITION
The mass percentage of an
element in a compound is the
same regardless of the
sample’s size

7. PERCENT COMPOSITION
Mass of element in 1 mole of compound
Molar Mass of Compound
X 100 =
% element
in
compound

8. CLASS EXAMPLE: WHAT I WRITE…YOU
WRITE
Calculate the percent
composition by mass of
each element in a sample
of calcium chloride.

9. Find the percent composition of
ibuprofen, C13H18O2. Record answers
to the nearest hundredth.

10. MOLYMPICS COMPETITION #2: SWIM
RELAY

11. WHAT’S GOING TO GO DOWN…
1. Each member/pair of the team will solve their
problem (all members of the team are solving their
problem at the same time
2. The first leg of the relay will pour the correct amount
of water into their graduated cylinder and pass it to
the next second leg.
3. Speed and Accuracy…mostly ACCURACY! (I will
judge your team on how close you were to the correct
number of mL in your graduated cylinder)
1g H2O = 1

12. SWIMMING RELAY COMPETITION
EXPECTATIONS
1. $5 footlong voices
2. Stay in your
assigned
area…movement
out of area may
result in
disqualification

13. EMPIRICAL AND
MOLECULAR
FORMULAS

14. BY THE END OF THE CLASS
PERIOD TODAY I WILL BE ABLE
TO…
Define empirical and molecular
formulas and calculate empirical
formulas.

15. Molecular and empirical formulas
A molecular formula
tells exactly how
many of each atom
are in the molecule.
molecular formula of
sugar = C6H12O6
C C C C C C O
HH H
HH HH
H
H
H
H
H
OO
OO
O
An empirical formula
tells the ratio of
atoms in a molecule
C
H
H
O: :
empirical formula of
sugar = CH2O

16. EMPIRICAL FORMULA
“simplest ratio of elements in a
chemical formula”

17. How do we find a substance’s
empirical formula?
By dividing all subscripts by the
same number.
What is the empirical
formula of H4S2O8? SH
O
SOO
O
OO
OO
HH
H
Divide all subscripts by 2
to get H2SO4

18. Ethanol is made up of carbon, hydrogen, and
oxygen. An 11.51 g sample of ethanol was
found to contain 6.00 g of carbon and 1.51 g of
hydrogen. Find the empirical formula of
ethanol.

20. A pure sample was analyzed and was
found to contain 6.25 g of lithium, 5.40 g of
carbon, & 21.60 g of oxygen. Calculate the
empirical formula.

21. • All the percentages of the
elements in a compound add
up to 100%
• Assume a 100g sample, then the
weight of each element is the
same as the percent
EMPIRICAL FORMULAS FROM %
COMPOSITION

22. Cisplatin is a compound that was once used to
treat tumors. Use the percent by mass of each
element in the compound to calculate the
empirical formula…
65.02% Pt
9.34% N
2.02% H
23.63% Cl
The ratio of each element in
a compound is always the
same – no matter the size of
your sample!

23. “the actual number of elements in a
compound”
MOLECULAR FORMULA
Whole number multiple of the empirical
formula

24. Whole number multiple of the empirical
formula
MOLECULAR FORMULA
H4S2O8
H8S4O16
H2SO
4

25. The empirical formula of a compound of
phosphorus and oxygen was found to be
P2O5. Experimentation shows that the
molar mass of this compound is
283.89g/mol. What is the compound’s
molecular formula?

26. A sample of compound containing
140.1 g of nitrogen and 80.0 g of
oxygen has a molar mass of 220.1
g/mol. Calculate the empirical and
molecular formulas.

27. Determine the percentage
composition of calcium carbonate
CaCO3. Use four significant figures
in expressing percentages.
B OARDWORK

28. A mole of an organic compound has a
molecular mass of 46.08 grams. If it is
consisted of carbon, hydrogen, and
oxygen atoms only, find the number of
oxygen atoms given that the percentage
composition of carbon and hydrogen
atoms are 52.13% and 13.15%
respectively.

29. Cholesterol is an organic compound
consisted of carbon, hydrogen, and
oxygen atoms and has a molecular
mass of 386.73 grams per mole. If
Hydrogen and Oxygen are 12.01% and
4.14% of the molecule, how many
carbon atoms are present in a molecule
of cholesterol? What is the molecular
formula of cholesterol?

30. An unknown compound is found to
contain 40.0% carbon, 6.7%
hydrogen and 53.3% oxygen with a
molecular mass of 60.0 g/mol. What
is the molecular formula of the
unknown compound?
C2 H4O2

31. A piece of iron ore is found to
contain a compound containing
72.3% iron and 27.7% oxygen with
a molecular mass of 231.4 g/mol.
What is the molecular formula of the
compound?
Fe3O4

32. A compound containing 40.0%
carbon, 5.7% hydrogen and 53.3%
oxygen has an atomic mass of 175
g/mol. What is the molecular
formula?
C6H12O6

33. A gas sample is found to contain
39.10% carbon, 7.67% hydrogen,
26.11% oxygen, 16.82%
phosphorus, and 10.30% fluorine. If
the molecular mass is 184.1 g/mol,
what is the molecular formula?
C6H14O3PF

34. A compound is contains 87.4%
nitrogen and 12.6% hydrogen. If
the molecular mass of the
compound is 32.05 g/mol, what is
the molecular formula?
N2H4

35. A compound with molecular mass
of 60.0 g/mol is found to contain
40.0% carbon, 6.7% hydrogen and
53.3% oxygen. What is the
molecular formula?
C2H4O2

36. A compound with molecular
mass of 74.1 g/mol is found to
contain 64.8% carbon, 13.5%
hydrogen and 21.7% oxygen. What
is the molecular formula?
C4H10O

37. A compound is found to contain
24.8% carbon, 2.0% hydrogen and
73.2% chlorine with a molecular
mass of 96.9 g/mol. What is the
molecular formula?
C2H2Cl2

38. Freon 12, a gas, was widely used
as an aerosol can propellant until
scientists found that it was
damaging our ozone layer. Its
percentage composition is 9.9%
carbon, 58.7% chlorine and 31.4%
fluorine. What is the empirical
formula of this compound?
CCl2F2

39. A compound known to have a
molecular weight of 88 amu. If the
simple formula of this compound is
C2H4O, find its molecular formula.
C4H8O2

40. A compound has 88.8% oxygen and
11.2% hydrogen. Find its empirical
formula.
H2O

41. On analysis, a compound has 75%
carbon, and 25% hydrogen. If its
molecular mass is 6 amu, what is its
empirical formula? Molecular
formula?
CH4