In chemistry, percentage composition is the mass percentage of each component element present in any compound. The percentage composition (or mass percentage) of each element is equal to the mass of that element divided by the total mass present in the compound, multiplied by 100%.
Thus, we can calculate the percentage composition of each element by the following formula:
Percentage composition of element = (Mass of element / Molar mass of compound) x 100
In short, % Composition = (X / M) x 100
Here, X is the mass of an element in the compound or molecule and M is the molar mass of a compound.
Steps to Calculate Percentage Composition of a Compound
We can obtain the percentage composition of a compound from the molecular formula of the compound in the following steps:
(a) First write down the molecular formula of the compound.
(b) Find out the mass of each element present in the compound by multiplying the atomic mass of that element by the number of atoms in the compound or molecule.
(c) Calculate the molar mass of the compound by adding atomic masses of all atoms present in its molecule.
(d) Use the above formula to calculate the percentage composition of each element.
Let us take a compound water (H2O) as an example to understand the percentage composition or mass percent. As water contains hydrogen and oxygen, we can calculate the percentage composition of both these elements as follows:
% composition of an element = mass of that element in the compound * 100 / molar mass of the compound
Molecular formula of water is H2O
Mass of hydrogen = 2 * 1.008 = 2.016g
Mass of oxygen = 1 * 16.00 = 16.00g
Molar mass of water = 2.016 + 16.00 = 18.02 g
Mass % of hydrogen = 2.016 * 100 / 18.02 = 11.18% (Ans)
Mass % of oxygen = 16.00 * 100 / 18.02 = 88.79% (Ans)
Examples of Percentage Composition
Example 1:
Determine the percentage composition of calcium (Ca), nitrogen (N), and oxygen (O) in calcium nitrate.
Solution:
Molecular formula of calcium nitrate is Ca(NO3)2.
Mass of calcium (Ca) = Atomic mass of Ca = 40g
Mass of nitrogen (N) = 2 x Atomic mass of N = 2 x 14 = 28g
Mass of oxygen (O) = 6 x Atomic mass of oxygen = 6 x 16 = 96g
Molar mass of calcium nitrate (Ca(NO3)2) = 40 + 28 + 96 = 164g
Percentage composition of Ca = (Mass of element / Molar mass) * 100 = (40 / 164) * 100 = 24%
Percentage composition of N = (28 / 164) * 100 = 17%
Percentage composition of O = 100 – (24 + 17) = 59%
Example 2:
Find the mass percentage of water of crystallisation, iron, sulphur and oxygen in pure ferrous sulphate (FeSO4·7H2O ).
Solution:
Molar mass of ferrous sulphate = At. mass of Fe + At. mass of S + 4 x At. mass of oxygen + 7 x Molar mass of H2O
Malar mass of FeSO4·7H2O = 56.0 + 32.0 + 4 x 16.0 + 7 x 18.0 = 278.0g
Thus,
Mass percent of water of crystallisation = (126 / 278) x 100 = 45.32% (Ans)
Similarly, mass percent of iron = (56 / 278) x 100 = 20.14% (Ans)
Mass percent of sulphur = (32 / 278) x 100 = 11.51% (Ans)
Mass percent of oxygen = (32 / 278) x 100 = 23.02% (Ans)
Example 3:
Calculate the percentage composition of carbon, hydrogen and oxygen in ethanol?
Solution:
Molecular formula of ethanol is C2H5OH
Molar mass of ethanol = (2 × 12.01 + 6 × 1.008 + 16.00) g = 46.068g
Mass percent of carbon = (24.02g / 46.068g) * 100 = 52.14%
Mass percent of hydrogen = (6.008g / 46.068g) * 100 = 13.13%
Mass percent of oxygen = (16.00g / 46.068g) * 100 = 34.73%
Example 4:
On mixing 16.5 g of a metal with oxygen, 35.60 g of metal oxide is obtained. What is the percentage of metal and oxygen in the compound.
Solution:
Mass of oxygen in metal oxide = (35.60 – 16.50) = 19.10g
% composition of metal = (16.50 / 35.60) * 100 = 46.30%
% composition of oxygen = (19.10 / 35.60) * 100 = 53.7%
Example 5:
Hydrogen and oxygen are combined in the proportion 1:16 by mass in hydrogen peroxide. What is the percentage composition of hydrogen and oxygen in hydrogen peroxide?
Solution:
Mass percent of hydrogen = (1 / 17) * 100 = 5.88%
Mass percent of oxygen = 100 – 5.88 = 94.12%
In this tutorial, we have discussed percentage composition in chemistry with a lot of various examples. Hope that you will have understood the basic concepts of mass percentage formula and practiced all examples.
Thanks for reading!!!
