7.3: “Formulas and Percentage Composition”
1. The percentage composition is the percentage by ___________ of each
_______________ in a compound.
a. Percentage composition helps verify a substance’s _______________.
b. Percentage composition can also be used to compare the ___________ of masses
contributed by the _________________ in two different substances.
2. An actual formula shows the _____________ ratio of elements or ions in a single unit of
a compound.
a. Empirical formula: a chemical formula that shows the _______________ ratio
for the relative numbers and kinds of ____________ in a compound.
b. For example, the _______________ formula for hydrogen peroxide is HO, while
the actual formula is _________________.
3. Rules for Determining Empirical Formulas
1.
Change the percentage of each _____________ in the compound to ___________.
%
grams
2. Use the __________ __________ to change grams to _____________,
grams
moles
3. Compare the amounts in ___________ to find the simplest whole-number
________.

To find the simplest whole-number ratio, divide each amount of ____________ by
the ______________ number of moles you found.

This will give a _______________ of 1 for the atoms present in the smallest amount.

Finally, you may need to ________________ all of the amounts of moles by a number
to convert all subscripts to small, _________ numbers.

The _____________ numbers you get are the ____________ in the empirical
formula.
4. Determining an Empirical Formula from Percentage Composition (Sample Prob. G, pg 242)
Chemical analysis of a liquid shows that it is 60.0% carbon, 13.4% hydrogen, and 26.6%
oxygen by mass. Calculate the empirical formula of this substance.
1.
Assume that you have a 100 g sample so that each percentage is the same as the
amount in grams:

for carbon: 60.0% = 60.0 grams C

for hydrogen: 13.4% = 13.4 grams H

for oxygen: 26.6% = 26.6 grams O
2. Use the molar mass to change grams

moles:
60.0 g C X 1 mole C = 5.00 mole carbon
12.01 g C

13.4 g H X 1 mole H = 13.3 mole hydrogen
1.01 g H

26.6 g O X 1 mole O = 1.66 mol oxygen
16.0
O
3. Divide each number of moles found by the smallest number of moles found
(1.66)

Carbon: 5.00 mol = 3.01 mol carbon
1.66 mol

Hydrogen: 13.3 mol = 8.01 mol hydrogen
1.66 mol

Oxygen: 1.66 mol
= 1 mole oxygen
1.66 mol
 These numbers are within experimental error to be considered whole numbers so
the empirical formula is: C3H8O
5. One More Example
Find the empirical formula given the following composition: 32.37% Na, 22.58% S, and
45.05% 0.
6. Additional Practice.
Find the empirical formula given the following composition: 26.58% K, 35.35% Cr, and
38.07% O
7. Molecular Formulas are ________________of Empirical Formulas
a. Molecular Formula: a chemical formula that shown the _____________ and kinds
of _____________ in a molecule, but not the arrangement of atoms.
b. A molecular formula is a whole-number multiple of the ______________ formula.
c. The ____________ ___________ of any compound is equal to the molar mass of
the empirical formula times a whole number, n.
n (empirical formula) = molecular formula OR
n=
exp.molar mass of compound
molar mass of empirical formula
8. Rules for Determining a Molecular Formula from an Empirical Formula
1.
Use the _____________ _____________ to find the molar mass of the empirical
formula.
2. Divide the experimental molar mass by the empirical molar mass. (Hint: the
____________ number goes on __________) This number is the multiplier, n.
3. Finally, _____________ the empirical formula by the n value to get the molecular
formula.
9. Determining a Molecular Formula from an Empirical Formula (Sample Problem H, pg 245)
The empirical formula for a compound is P2O5. Its experimental molar mass is 284 g/mol.
Determine the molecular formula of the compound.
1.
Use the periodic table to find the molar mass of the empirical formula:

P: 2(30.97) = 61.94 g/mol

O: 5(16.00) = 80.00 g/mol
Molar Mass of P2O5 = 141.94 g/mol
2. n = molar mass of compound
HINT: the bigger # always goes on TOP!
molar mass of empirical formula
n = 284 g/mol
= 2
141.94 g/mol
3. To find the molecular formula, simply multiply the empirical formula by 2:
2 (P2O5) = P4O10
10. Additional Practice:
Determine the molecular formula for the following:
Molar mass: 232.41 g/mol
Empirical formula: OCNCl
11. One More Example.
Determine the molecular formula from the following information:
Experimental molar mass: 120.12 g/mol
Empirical Formula: CH2O
12.
Rules to Determine Percentage Composition from Chemical Formulas
1. Use the periodic table to find the _________ ________ of the compound.
2. Divide the mass contributed by each __________ by the molar mass of the
____________.
3. Multiply by ____ to find the percentage composition of that element in the compound.
13. Using a Chemical Formula to Determine the % Composition (Sample Prob I), pg 247
Calculate the percentage composition of copper (I) sulfide.
 The formula for copper (I) sulfide is: Cu2S because you need 2 Cu
1+
to balance out
the S 21.
Find the molar mass of Cu2S:

Cu: 2(63.55 g/mol) = 127.1 g/mol

S: 32.07 g/mol
Molar mass of Cu2S = 159.17 g/mol
2. Divide the mass of each element in the compound by the molar mass of the
compound.

For Cu: 2(63.55) X 100%
= 79.85%
159.17

For S: 32.07 X 100% = 20.15%
159.17
 Check your work, when added together, the sum should be near 100%
 These are the same calculations we did in the jelly bean lab!
14. Additional Practice. Determine the percentage composition of H2SO3.
15. One More Example. Determine the percentage composition of C2H5COOH.