Empirical and Molecular Formula

Percent Composition

The percent composition of an element in a compound is calculated using the formula.

Example 1

What is the percent composition of FeSO₄?

The total percent composition adds up to 100% (36.7% + 21.1 % + 42.2% = 100%)

The empirical formula is used to show the relative ratios of various atoms of a compound.

The masses of each element in a compound can be measured using experiments with that the empirical formula is derived as shown in the example below.

Find the empirical formula of a compound that contains 38.7% C, 16.2% H, and 45.1% N.

Step 1:

Assume 100 g of the compound then the number of moles of each element can be calculated as follows.

Step 2: Determining mole ratio

C_3.22H_16.2N_3.22

Step 3 : Divide all the moles by the smallest one found

Step 4: The empirical formula now can be written as follows

C₁H₅N₁ = CH₅N

Ethylene glycol has 38.7% C, 9.75% H, and Oxygen. Its molecular weight is 62.07g. Find the molecular formula of ethylene glycol.

Step 1

The amount of oxygen in the compound can be found using the following method since the total percent composition is equal to 100%

O = 100% - (38.7% + 9.75%) = 51.55%

Step 2

Assume 100 g of the compound then the number of moles of each element can be calculated as follows.

Step 3

Divide all the moles by the smallest one found

Then the empirical formula becomes CH₃O

Step 4

Calculating the ratio of molecular weight to empirical forumal weight

Molecular weight = 62.07g

Empirical formula weight = (1*12.0g)+(3*1.0g)+(1*16.0g) = 31.0g

Step 5

Multiply the mols of empirical formula by the ratio calculated in step 4

Then the molecular formula becomes C₂H₆O₂