&

Percent Composition

Indicates relative % of each element in a compound

Total % of the components ~ 100%

Examples

1. Determine the % of children in a group consisting of 10 men, 8 women, and 9 children.

10 + 8 + 9 = 27 total people

9 children x 100 = 33.3%

27 people

Examples

2. Find the % of aluminum in aluminum oxide.

3. Find the % of nitrogen in ammonium nitrate, a compound used in fertilizers.

4. Copper(II) sulfate pentahydrate is a blue compound used to make colored pigments, insecticides, and electric batteries. Determine the % water.

Empirical Formulas

Empirical formulas are the lowest possible ratio of components in the compound.

Ionic formulas are almost always empirical in nature.

Covalent compounds require more information

…..is the EF= MF?

Steps to determining the EF

1. Find the percent composition of the substance.

2. Assume you have a 100 gram sample

assuming 100g, you can remove the % sign and replace it with grams

3. Convert the grams found in step 2 into moles.

Steps to Determine the EF

4. Ratio the moles you found in step 3 5. Divide all moles by the smallest value

this determines the relative molar amount of each of the atoms— ~ whole number

6. If the numbers are not whole numbers multiply by a factor that will yield a whole number

Example: 1: 1.3 --- x 3 = 3:4

Steps to Determine the EFUse molar ratios obtained as subscripts in the

formula.Example 1What is the EF of a compound that contains

53.73% Fe and 46.27% S?

( Fe2S3)Example 2What is the EF of a compound that contains

90.7% Pb and 9.33% O?

( Pb3O4)

Molecular FormulasMolecular formulas can be found from the

EF

1. Find the empirical formula and mass

2. Find the relationship between the EF mass and the molecular mass

n = molecular formula mass

empirical formula mass

3. (Empirical formula)n = Molecular formula

EF = CH4 n = 2 C2H8

Example:

• A compound composed of hydrogen and oxygen is analyzed and a sample of the compound yields 0.59 g of hydrogen and 9.40 g of oxygen. The molecular mass of the compound is 34 g/mol. Find the empirical formula and molecular formula for this compound.

H = 0.59 gO = 9.40 g 9.99 g Total Sample

100% - 5.9% = 94.1% oxygen

Step 1: Find the percent composition

H%9.510099.9

59.0

g

g

Step 2: Assume a 100 g Sample

• Drop the percent sign and replace it with a “g” for grams.

Step 3: Convert grams to moles

5.9 g H 1 mole = 5.9 moles H

1.0 g

94.1 g 1 mole = 5.9 moles O

16 g

Step 4: Ratio the moles and reduce

Moles of H:Moles of O

5.9 moles : 5.9 moles

5.9 moles 5.9 moles

1:1

Step 5: Use mole ratio for empirical formula subscripts

• 1 hydrogen : 1 oxygen

Empirical Formula: HO

Step 6: Find the empirical formula mass

• Empirical Formula: HO

1 g + 16 g = 17 g/mol

Step 7: Divide Molecular Mass by the Empirical Formula Mass

Molecular mass (sometimes called molar mass or mass of the molecule) = 34.0 g/mol

Divide the molecular mass by the empirical formula mass to find “n”, a multiplier.

n = (molecular mass ) = 34 g/mol

(empirical formula mass) 17 g/mol

n = 2

Step 8: Multiply the subscripts in the empirical formula by “n”

Molecular Formula = (empirical formula)n

Molecular Formula = (HO)2

*distribute the 2 through the parentheses

Molecular Formula = H2O2