Percent Composition Formula

Last Updated : 30 Dec, 2023

Percent composition refers to the ratio of the amount of an element to the total amount multiplied by 100. The quantity used in the percent composition formula is generally measured in grams. Their application is found in chemistry to find the percentage composition of an element in a compound. It also helps in the chemical analysis of that compound. A compound is defined as a mixture of two or more elements.

Percentage composition of an element is given by:

$\frac{Mass\ of\ an\ element}{Total\ mass\ of\ a\ compound} \times 100$

% C = gE/gC

Where,

gE is the mass of an element E in grams

gC is the mass of the compound C in grams

The mass of the elements in the periodic table is generally described in terms of gram per mole (g/mol).

For example,

To find the percent composition of element A in a compound ABC

If the mass of A = 30

B = 40

C = 50

To find the percentage composition of an element, we first need to find the mass of the element and the total mass of each element in a compound.

Mass of an element A = 30

For total mass, we need to add the mass of element A, B, and C.

Total mass = mass of A + mass of B + mass of C

Total mass = 30 + 40 + 50

Total mass = 120

After adding, we get:

Total mass = 120

Percentage composition = 30/120 × 100

=1/4 × 100

= 25%

Thus, the percentage composition of element A in a compound is 25%.

Sample Questions

Question 1: Find the percent composition of Calcium (Ca) in the compound Ca(OH)₂?

Solution:

The above compound has three elements calcium, oxygen, and hydrogen. We need to divide the mass of calcium with the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of Oxygen = 16 g/mol

Molar mass of hydrogen = 2 g/mol

Total mass = mass of calcium + mass of oxygen + mass of hydrogen

Total mass = 40 + 2×16 + 2×1

Total mass = 40 + 32 + 2

Total mass = 74 g/mol

Percent composition of Calcium = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of Calcium = $\frac{40}{74} \times 100$

Percent composition of Calcium = 54%

Question 2: Find the percent composition of Oxygen (O) in the compound Ca(OH)₂?

Solution:

The above compound has three elements calcium, oxygen, and hydrogen. We need to divide the mass of Oxygen with the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of Oxygen = 16 g/mol

Molar mass of hydrogen = 2 g/mol

Total mass = mass of calcium + mass of oxygen + mass of hydrogen

Total mass = 40 + 2×16 + 2×1

Total mass = 40 + 32 + 2

Total mass = 74 g/mol

Percent composition of oxygen = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of oxygen = $\frac{32}{74} \times 100$

Percent composition of oxygen = 43.24%

It is because there are two atoms of oxygen in the compound.

Question 3: Find the percent composition of Calcium (Ca) in the compound Ca(Cl)₂?

Solution:

The above compound has two elements calcium and chloride. We need to divide the mass of calcium with the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of chloride = 35.5 g/mol

Total mass = mass of calcium + mass of chloride

Total mass = 40 + 2×35.5

Total mass = 40 + 71

Total mass = 111 g/mol

Percent composition of oxygen = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of oxygen = $\frac{40}{111} \times 100$

Percent composition of oxygen = 36.03%

Question 4: Find the percent composition of chlorine (Cl) in the compound Ca(Cl)₂?

Solution:

The above compound has two elements calcium and chlorine. We need to divide the mass of chlorine by the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of chlorine = 35.5 g/mol

Total mass = mass of calcium + mass of chlorine

Total mass = 40 + 2×35.5

Total mass = 40 + 71

Total mass = 111 g/mol

Percent composition of chlorine = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of chlorine = $\frac{71}{111} \times 100$

Percent composition of chlorine = 63.96%

Question 5: Find the percent composition of calcium (Ca) in the compound CaCO₃?

Solution:

The above compound has three elements calcium, carbon, and oxygen. We need to divide the mass of calcium with the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of carbon = 12 g/mol

Molar mass of oxygen = 16 g/mol

Total mass = mass of calcium + mass of carbon + mass of oxygen

Total mass = 40 + 12 + 3×16

Total mass = 40 + 12 + 48

Total mass = 100 g/mol

Percent composition of calcium = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of calcium = $\frac{40}{100} \times 100$

Percent composition of calcium = 40%

Question 6: Find the percent composition of carbon (C) in the compound CaCO₃?

Solution:

The above compound has three elements calcium, carbon, and oxygen. We need to divide the mass of carbon by the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of carbon = 12 g/mol

Molar mass of oxygen = 16 g/mol

Total mass = mass of calcium + mass of carbon + mass of oxygen

Total mass = 40 + 12 + 3×16

Total mass = 40 + 12 + 48

Total mass = 100 g/mol

Percent composition of carbon = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of carbon = $\frac{12}{100} \times 100$

Percent composition of carbon = 12%

Question 7: Find the percent composition of Oxygen (O) in the compound CaCO₃?

Solution:

The above compound has three elements calcium, carbon, and oxygen. We need to divide the mass of oxygen by the total mass of the compound.

Molar mass of Calcium = 40 g/mol

Molar mass of carbon = 12 g/mol

Molar mass of oxygen = 16 g/mol

Total mass = mass of calcium + mass of carbon + mass of oxygen

Total mass = 40 + 12 + 3×16

Total mass = 40 + 12 + 48

Total mass = 100 g/mol

Percent composition of oxygen = $\frac{mass\ of\ calcium}{total\ mass} \times 100$

Percent composition of oxygen = $\frac{48}{100} \times 100$

Percent composition of oxygen = 48%

The total percentage of each component in a substance is always 100%.