Some of the content of this guide was modeled after a guide originally created by Openstax and has been adapted for the GPRC Learning Commons in April 2021.
This work is licensed under a Creative Commons BY NC SA 4.0 International License.
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
C3.22H16.2N3.22
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