Explain the percentage composition of a compound. Describe the method used to calculate the percentage of each element present in a compound with a suitable example.

📘 Percentage Composition of a Compound

The percentage composition of a compound gives the percentage by mass of each element present in the compound.

Percentage of an Element

= (Mass of the Element in 1 mole of Compound ÷ Molar Mass of Compound) × 100

🔵 What Does Percentage Composition Mean?

A compound contains two or more elements in a definite proportion. Percentage composition tells us how much percentage by mass of the compound is contributed by each element.

Total Percentage of All Elements = 100%

📝 Method to Calculate Percentage Composition

Step 1: Write the chemical formula of the compound.

Step 2: Calculate the molar mass of the compound.

Step 3: Calculate the total mass contributed by each element.

Step 4: Apply the percentage composition formula.

Step 5: Check that the sum of all percentages is approximately 100%.

🧮 Suitable Example: Percentage Composition of H₂O

Calculate the percentage of hydrogen and oxygen in water.

H₂O

Atomic Masses:
H = 1     O = 16

🔵 Step 1: Calculate Molar Mass of H₂O

Molar Mass of H₂O

= (2 × 1) + (1 × 16)

= 2 + 16

= 18 g mol⁻¹

🟢 Step 2: Mass of Hydrogen

There are two hydrogen atoms in H₂O.

Mass of Hydrogen

= 2 × 1

= 2 g

🟠 Step 3: Percentage of Hydrogen

% H = (2 ÷ 18) × 100

= 11.11%

✅ Percentage of Hydrogen = 11.11%

🟣 Step 4: Mass of Oxygen

There is one oxygen atom in H₂O.

Mass of Oxygen

= 1 × 16

= 16 g

🔴 Step 5: Percentage of Oxygen

% O = (16 ÷ 18) × 100

= 88.89%

✅ Percentage of Oxygen = 88.89%

🎬 3D-Style Molecular Animation

One molecule of water contains two hydrogen atoms and one oxygen atom.

H H O

H₂O → 18 g mol⁻¹

H = 2 g → 11.11%
O = 16 g → 88.89%

📊 Percentage Composition Table

Element	Number of Atoms	Atomic Mass	Total Mass	Percentage
H	2	1	2	11.11%
O	1	16	16	88.89%
Total			18	100%

📐 General Formula

% of Element = (Mass of Element in One Mole of Compound ÷ Molar Mass of Compound) × 100

For an element X present in a compound:

% X = (n × Atomic Mass of X ÷ Molar Mass of Compound) × 100

where n is the number of atoms of element X in the chemical formula.

🔬 Quick Example: CO₂

Molar Mass of CO₂ = 12 + (2 × 16) = 44 g mol⁻¹

% C = (12 ÷ 44) × 100 = 27.27%

% O = (32 ÷ 44) × 100 = 72.73%

27.27% + 72.73% = 100%

⭐ Important Points

• Always write the correct chemical formula first.
• Calculate the molar mass carefully.
• Multiply the atomic mass by the number of atoms of that element.
• Divide the element's mass by the molar mass.
• Multiply the result by 100.
• The sum of the percentages of all elements should be approximately 100%.

🎯 Quick Revision Flow

Chemical Formula
↓
Calculate Molar Mass
↓
Mass Contribution of Each Element
↓
% Element = (Element Mass ÷ Molar Mass) × 100
↓
Check Total ≈ 100%

✅ Final Answer

Percentage composition is the percentage by mass of each element present in a compound.

% Element = (Mass of Element in 1 mole of Compound ÷ Molar Mass of Compound) × 100

For H₂O:
H = 11.11%
O = 88.89%

Total = 100%