Question

What is percent composition by mass?

Answer 1

In a compound, we can calculate the mass of the elements present in the compound. For example, the percent composition of carbon, hydrogen, and oxygen is 40.0%, 6.7140%, and 53.282% respective in a molecule of ${{C}_{6}}{{H}_{12}}{{O}_{6}}$.

Complete answer:
We know that the compounds are made up of different elements in a specific ratio, and we can find the percentage of each element present in the compound. So, the percentage of the elements present in the compound in the form of mass is known as percent composition by mass. For example, let us calculate the percent composition of every element present in one molecule of glucose. The formula is ${{C}_{6}}{{H}_{12}}{{O}_{6}}$, in this there are three elements, i.e., carbon, hydrogen, and oxygen, and six atoms of carbon and oxygen are there while 12 atoms of hydrogen are present.
We know the mass of a carbon atom is 12, the mass of a hydrogen atom is 1, and the mass of an oxygen atom is 16.
So, the total mass of glucose molecule will be:
Molecular mass = 6 x 12 + 1 x 12 + 6 x 18 = 180
While the mass of carbon atoms will be:
6 x 12 = 72
The mass of hydrogen atoms will be:
1 x 12 = 12
The mass of oxygen atoms will be:
6 x 18 = 96
So, the percent composition of carbon in glucose by mass:
$\dfrac{72}{180}\text{ x 100 = 40}\text{.00}$%
The percent composition of hydrogen in glucose by mass:
$\dfrac{12}{180}\text{ x 100 = 6}\text{.7 }\\!\\!$
The percent composition of oxygen in glucose by mass:
$\dfrac{96}{180}\text{ x 100 = 53}\text{.28 }\\!\\!$

Note:
We can also calculate the percent composition by mass for the substances having more than one compound like solution having solute and solvent, and used for calculating the strength of the solution.