Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

Percent Composition Notes

Percent Composition (also called % composition by mass)

When we analyze a compound, to figure out what elements are present and in which amount, the analysis does not give information about the mass of each element in one mole of the compound but instead it gives the mass of each element in 100 g of the compound. 

Percent composition (or percent composition by mass) is just that. The amount in grams of each element in a compound. 

When we calculate the % composition,  we actually get how many grams of each element make up 100 g of the compound. There is a formula for calculating % composition, but it is easier if we use proportions to do the calculation

Example:

In 18 g of water, there are 16 g of Oxygen and 2 g of Hydrogen.

The % of Oxygen  can be calculated as follows:

Solving for X ==>

% O = 16 g Oxygen  x 100 g water/ 18 g of water

         =  1600 g Oxygen/18 

         = 88.89 g of oxygen

         = 88.89%

The % of Hydrogen can be calculated as follows:

Solving for Y ==>

% H = 2 g Hydrogen  x 100 g water/ 18 g of water

        =  200 g Hydrogen/18 

        = 11.11 g of Hydrogen

        = 11.11%

In general, we can use the following formula for the % composition: 

 

Videos just in case you need to review what we saw in class:

% compositon #1

% Composition #2 (More complex)