Percent Composition
Mass percentage of an element in a compound.
Formula
Variables
Example
In water (18 g/mol), oxygen is 16/18 × 100% ≈ 89% by mass.
How it works
Percent composition tells you how much of a compound’s total mass comes from one particular element. It works because you compare the mass contributed by that element to the compound’s whole molar mass, then convert the fraction to a percentage by multiplying by 100. This gives a mass-based share, not a count of atoms.
When to use it
Use this when you need the percentage by mass of an element in a compound, such as in lab reports, chemical formulas, or composition questions in chemistry. It is also useful for checking which element makes up the largest part of a compound.
Worked example, step by step
Find the percent composition of nitrogen in ammonia, NH3. Step 1: Find the molar mass of NH3. N = 14 g/mol, H = 1 g/mol each, so NH3 = 14 + (3 × 1) = 17 g/mol Step 2: Use the formula. % nitrogen = (mass of nitrogen / molar mass of compound) × 100 % nitrogen = (14 / 17) × 100 Step 3: Calculate. % nitrogen ≈ 82.35% Final answer: Nitrogen makes up about 82.4% of ammonia by mass.
Common mistakes to avoid
- Using the number of atoms instead of the element’s mass
- Forgetting to include every atom when finding the compound’s molar mass
- Leaving the answer as a decimal fraction instead of converting to percent
Did You Know?
Percent composition reveals what fraction of a compound's mass each element contributes.
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