Empirical Formula of a Compound

Molecular Formula (MF)

• actual formula

• shows the number of atoms of each element that make up a molecule or a formula unit

Empirical Formula (EF)

• simplest formula of a compound

• shows the lowest whole number ratio of the elements in a compound

• can be determined from the molecular formula

or

• percentage composition of the compound

Using Molecular Formula- Examples

Substance Molecular Formula

(MF)

Empirical Formula

(EF)

Butane C4H10 C2H5

Glucose C6H12O6

Benzene C6H6

Water H2O

Substance Molecular Formula

(MF)

Empirical Formula

(EF)

Butane C4H10 C2H5

Glucose C6H12O6 CH2O

Benzene C6H6 CH

Water H2O H2O

Using Molecular Formula

- Examples

Using Percentage Composition- Examples

1. Calculate the EF of a compound that is 85.6 % carbon and 14.4 % hydrogen.

Answer:

Assume that you are using a 100 g sample of the compound and complete the next calculations. Use a table for your data.

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C

H

Lowest Whole Number

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C 85.6 85.6 12.01

H 14.4 14.4 1.01

Lowest Whole Number

Recall: n = m M

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C 85.6 85.6 12.01 7.13

H 14.4 14.4 1.01 14.3

Lowest Whole Number

Recall: n = m M

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C 85.6 85.6 12.01 7.13

H 14.4 14.4 1.01 14.3

Lowest Whole Number

Calculate LWN ratio by dividing: molar amount lowest molar amount

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C 85.6 85.6 12.01 7.13 7.137.13

H 14.4 14.4 1.01 14.3 14.37.13

Lowest Whole Number

Calculate LWN ratio by dividing: molar amount lowest molar amount

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C 85.6 85.6 12.01 7.13 7.13 = 1.007.13

H 14.4 14.4 1.01 14.3 14.3 = 2.007.13

Lowest Whole Number

Calculate LWN ratio by dividing: molar amount lowest molar amount

Using Percentage Composition- Example #1

Element Mass%

Massm (g)

Molar MassM (g/mol)

Molesn (mol)

LWNRatio

EF

C 85.6 85.6 12.01 7.13 7.13 = 1.007.13

C

H 14.4 14.4 1.01 14.3 14.3 = 2.007.13

H2

Empirical Formula: CH2

Using Percentage Composition- Examples

2. Calculate the EF of a compound with the following % composition: 32.4 % Na, 22.6 % S and 45.0 % O

3. Calculate the EF of TNT, with the following % composition: 37.0 % C, 2.20 % H, 18.5% N and 42.3 % O

Homework

• Text p 273 & 275

• Inv 6B (p 284) Read, answer pre-lab questions and make prediction (Q3)