Lecture 2

Empirical and Molecular Formulae

IB Chemistry Power Points

Topic 1

Quantitative Chemistrywww.pedagogics.ca

Percent Composition by Mass

Recall: the relative molecular mass of H2SO4 can be determined as follows:

Hydrogen 2 atoms × 1.008 = 2.01Sulfur 1 atom × 32.07 = 32.07Oxygen 4 atoms × 16.000 =64.00

MR 98.09

The percent composition by mass states the fraction of the total mass of each element in the compound.

For example % sulfur

32.07% 100 32.7%

98.09S

Similarly:

2.016% 100 2.1%

98.09H

64.00% 100 65.2%

98.09O

Note the sum of these values should add to 100%

Empirical Formula Determination

That is “empirical” not Empire you fool!

Definition

The empirical formula is the formula that gives

the simplest whole number ratio of atoms in

a compound

An example

Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?

49.32 14.107

12.01

g C mol Cmol C

g C

Come to the dark

side

1. Assume 100g of the compound

2. So, in 100 g of adipic acid, there would be 49.32 g of C, 43.84 g of O and 6.85 g of H.

3. Determine moles of each element. For example: carbon

An example continued

H mol 78.6mol g 1.01

H g 85.61-

Similarly:

I’m your father!

O mol 74.2mol g 16.00

O g 84.431-

Step 4. Divide each value of moles by the smallest of the values.

1 2.74 mol 74.2 :

2.5 2.74 mol 78.6 :

1.5 2.74 mol .124 :

Oxygen

Hydrogen

Carbon

Step 5. If necessary, multiply each number by an integer to obtain all whole numbers.

Carbon: 1.5 Hydrogen: 2.5 Oxygen: 1x 2 x 2 x 2

3 5 2

Empirical formula:C3H5O

2

Alternatively, the empirical formula can be determined from mass composition data.

Using the same example: a 21.6 g sample of adipic acid is decomposed yielding some carbon, 9.5 g of oxygen gas and 1.5 g of hydrogen gas. What is the empirical formula of adipic acid?

First find the number of MOLES of each elementThen determine the lowest whole number ratio for the mole values

A MOLECULAR FORMULA is the actual chemical formula of a compound. The molecular formula will contain elements in the same ratio as the empirical formula.

Molecular formulae are whole number multiples of empirical formulae.

We have determined that the empirical formula for adipic acid is C3H5O2. If the molar mass of adipic acid is 146 g/mol, what is the molecular formula of adipic acid?

1. Find the molar mass of C3H5O2

3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g

2. Divide the molecular mass by the mass given by the empirical formula.

146 ÷ 73.08 = 2

3. Multiply the empirical formula by this number to get the molecular formula.

(C3H5O2) x 2 = C6H10O4