1 of 32

PERCENT COMPOSITION, EMPIRICAL FORMULAS, AND MOLECULAR FORMULAS

Curiosity, Jeff Jordan, 2003.

2 of 32

AP Topics

Topic 1.3: Elemental Composition of Pure Substances

Learning Objective: Explain the quantitative relationship between the elemental composition by mass and the empirical formula of a pure substance.

Essential Knowledge:

Some pure substances are composed of individual molecules, while others consist of atoms or ions held together in fixed proportions as described by a formula unit.
According to the law of definite proportions, the ratio of the masses of the constituent elements in any pure sample of that compound is always the same.
The chemical formula that lists the lowest whole number ratio of atoms of the elements in a compound is the empirical formula.

3 of 32

PERCENT COMPOSITION

4 of 32

EXAMPLE

A 45.70 g sample of an unknown substance is decomposed and found to contain 39.94 g of nitrogen and 5.76 g of hydrogen. What is the percent composition of the unknown compound?

Answer: 87.40% N and 12.6% H

5 of 32

EXAMPLE

What is the percent composition of lithium carbonate?

Answer: 18.78% Li, 16.25% C, and 64.96% O

6 of 32

WHAT IS AN EMPIRICAL FORMULA?

A molecular formula where the subscripts have been reduced to the smallest whole number ratio.

Ex: C₆H₁₂O₆ becomes:

CH₂O

Ex: CO₂ becomes:

CO₂

Ex: N₂H₄ becomes:

NH₂

7 of 32

Which of the following is NOT a possible molecular formula for the empirical formula, C₂H₅?

C₄H₁₀

C₃H₈

C₆H₁₅

C₁₀H₂₅

8 of 32

WHY DO WE NEED TO KNOW EMPIRICAL FORMULAS

Empirical formulas are one of the experimental steps needed in determining the identity of unknown substances.
Once you identify the ratio of each element in a substance, you can use mass spectrometry to determine the molar mass of the substance.
Then using the empirical formula and the molar mass, you can determine the molecular formula of the unknown substance.

9 of 32

HOW DO YOU DETERMINE THE EMPIRICAL FORMULA OF A COMPOUND?

Convert all grams or mass percentages to moles. (If you are given the percent composition of a compound, assume you have 100 g of the compound and then all the percentages are now the grams of each element (e.g., 18% carbon becomes 18 g of carbon)).
Divide all of the moles by the smallest of the number of moles to determine the mole ratios.
Use the mole ratios (as whole numbers*) as subscripts in the empirical formula.
*Multiply all mole ratios by 2 if one of the mole ratios ends in 0.5 or 3 if one of the mole ratios ends in 0.33.

14 of 32

EXAMPLE

/ 0.0998 = 1

/ 0.0998 ≈ 2

15 of 32

EXAMPLE

/ 0.0998 = 1

/ 0.0998 ≈ 2

Ca Br

16 of 32

EXAMPLE

/ 0.0998 = 1

/ 0.0998 ≈ 2

Ca Br2

17 of 32

EXAMPLE

/ 0.0998 = 1

/ 0.0998 ≈ 2

Ca Br2

18 of 32

EXAMPLE

A compound contains 48.63% carbon, 8.18% hydrogen, and 43.19% oxygen. What is its empirical formula?

19 of 32

EXAMPLE

A compound contains 48.63% carbon, 8.18% hydrogen, and 43.19% oxygen. What is its empirical formula?

Step 1:

20 of 32

EXAMPLE

A compound contains 48.63% carbon, 8.18% hydrogen, and 43.19% oxygen. What is its empirical formula?

Step 2:

21 of 32

EXAMPLE

A compound contains 48.63% carbon, 8.18% hydrogen, and 43.19% oxygen. What is its empirical formula?

Step 3:

22 of 32

EXAMPLE

A compound contains 48.63% carbon, 8.18% hydrogen, and 43.19% oxygen. What is its empirical formula?

Step 4:

C₃H₆O₂

23 of 32

WHAT IS A MOLECULAR FORMULA?

The formula of a compound that uses subscripts to show how many atoms of each element is in a molecule of the compound.

Ex: C₆H₁₂O₆

Ex: CO₂

Ex: N₂H₄

24 of 32

WHAT DO YOU NEED TO DETERMINE THE MOLECULAR FORMULA OF A COMPOUND?

The empirical formula of the compound.
The molar mass of the compound.

25 of 32

EXAMPLE 1

The empirical formula for a compound is NO₂. Its molar mass is 92.02 g/mol. What is its molecular formula?

26 of 32