In the analysis of Compounds, it is important to know the percentage of each of the elements present in the compound.

Importance

Analysis of the compound is important for determining:
• Empirical Formula
• Molecular Formula

1- Qualitative Analysis

“All the elements present in the compound are first identified. This is called qualitative analysis.”

2- Quantitative Analysis

“After the identification of elements, the compound is subjected to quantitative analysis in which the mass of each element in a sample of the compound is determined.”
From the quantitative analysis, we determine the percentage by mass of each element.

• The percentage of an element

The percentage of an element in the compound is the number of grams of that element present in 100 grams of the compound.

Percentage of an element = Mass of the element in the compound × 100
Formula Mass of the compound

Determination

8.657g of a compound were decomposed into its elements and gave 5.217g of Carbon, 0.962g of Hydrogen, 2.478g of oxygen. Now calculate the percentage composition of the compound.
Applying the formula

% of Carbon = Mass of Carbon × 100 = 5.217g × 100 = 60.28 amu
Mass of the compound 8.657g

% of Hydrogen = Mass of Oxygen × 100 = 0.962g × 100 = 11.11amu
Mass of the compound 8.657g

% of Oxygen = Mass of Hydrogen × 100 = 2.478g × 100 = 28.62 amu

Mass of the compound 8.657g

The above results tell us that in 100 grams of the given compound, there are 60.26 grams of Carbon, 11.11 grams of hydrogen and 28.62 grams of Oxygen.
The percentage composition of a compound can also be determined theoretically if we know the formula mass of the compound.

The following equation can be used for this purpose.
Percentage of an element = Mass of the element in the compound × 100
Formula Mass of the compound