Empirical and Molecular Formulas

1
Empirical and Molecular Formulas

• Section 11.4
• Chemistry

2
Objectives

• Explain what is meant by the percent composition
  of a compound.
• Determine the empirical and molecular formulas
  for a compound from mass percent and actual mass
  data.

3
Key Terms

• Percent composition
• Empirical formula
• Molecular formula

4
Percent Composition

• Percent by mass of each element in a compound.
• Mass of Element x 100
• Mass of Compound

5
Example

• Calculate percent composition of H in H2O
• Molar mass of water 18.02 g/mol
• Determine mass of H in 1 mol of H2O

6
Mass of H

• 1.01 x 2 2.02 g H in 1 mol water

Atomic mass of H from periodic table
Number of H in H2O
7
Percent Composition of H

• 2.02 g of H x 100 11.2 H
• 18.02 g of H2O

8
Empirical Formula

• Formula for a compound with the smallest
  whole-number ratio of elements.
• Percent composition can be used to find the
  chemical formula.

9
Empirical Formula

• When given percent composition, assume
• The total mass of the compound is 100 g
• The percent composition of the element is equal
  to the mass in grams of the element.

10
Example

• A compound has a percent composition of 40.05 S
  and 59.95 O.
• So in 100 g of the compound, 40.05 g are S and
  59.95 g are O.
• Find the amount of mol for each element.

11
Empirical Formula

• 40.05g S x 1 mol S 1.249 mol S
• 32.07g S
• 59.95g O x 1 mol O 3.747 mol O
• 16.00g O

12
Empirical Formula

• The element with the smallest number of mol gets
  the subscript 1.

13
Empirical Formula

• S has a subscript 1
• Then divide the mol O by the mol S
• 3.747 mol O/ 1.249 mol S 3 mol O
• Then write your empirical formula using your
  subscripts SO3

14
Practice Problems

• Pg. 333

15
Molecular Formula

• Formula that specifies the actual number of atoms
  of each element in one molecule or formula unit
  of a substance.
• n molecular formula mass
• empirical formula mass

16
Example

• Compound is composed of 40.68 carbon, 5.08
  hydrogen, and 54.24 oxygen and has a mass of
  118.1 g/mol. Determine the empirical and
  molecular formulas for succinic acid.

17
Example

• 40.68 g C x 1 mol C 3.387 mol C
• 12.01 g C
• 5.08 g H x 1 mol H 5.04 mol H
• 1.008 g H
• 54.24 g O x 1 mol O 3.390 mol O
• 16.00 g O

18
Example

• 3.387 mol C/ 3.387 1 mol C
• 5.040 mol H/ 3.387 1.49 1.5 mol H
• 3.390 mol O/ 3.387 1.001 1 mol O
• Ratio of C H O 1 1.5 1

19
Example

• Empirical Formula C2H3O2

20
Example

• To find molecular formula, calculate n.
• n molecular formula mass
• empirical formula mass
• Molecular mass is in the problem!
• Calculate molar mass of empirical

21
Example

• n 118.1 2
• 59.04
• Multiply the subscripts of the empirical by n to
  find the molecular formula.
• Molecular Formula C4H6O4

22
Practice Problems

• Pg 335

23
Homework

• Section 11.4 Problems 27-29 on page 877