Percentage Composition by Mass Calculator
Calculate the percentage of an element in a compound.
Formula first
Overview
This formula is used to calculate the proportion of a specific element's mass relative to the total relative formula mass of a compound. It provides a quantitative measure of a substance's elemental makeup, which is essential for determining empirical formulas and assessing chemical purity.
Symbols
Variables
\% = Percentage, = Atomic Mass, n = Number of Atoms, = Formula Mass
Apply it well
When To Use
When to use: Use this equation when you need to calculate the theoretical mass contribution of an element within a known chemical formula. It is particularly useful in stoichiometry for converting between masses of compounds and masses of constituent elements during laboratory analysis.
Why it matters: This calculation is critical for quality control in industries such as agriculture, where the percentage of nitrogen in fertilizers must be exact for crop safety. It also allows forensic scientists to identify unknown substances by comparing their mass profiles to theoretical chemical standards.
Avoid these traps
Common Mistakes
- Forgetting to multiply by number of atoms.
- Using wrong Ar values.
- Not converting to percentage.
One free problem
Practice Problem
Calculate the mass percentage of Oxygen in water (H₂O). Use Ar = 16.00 for Oxygen and Mr = 18.02 for the total mass of water.
Solve for: percent
Hint: Oxygen only appears once in the chemical formula H₂O, so the value of n is 1.
The full worked solution stays in the interactive walkthrough.
References
Sources
- IUPAC Gold Book: Relative atomic mass
- IUPAC Gold Book: Relative molecular mass
- Chemistry: The Central Science, 14th ed. by Brown, LeMay, Bursten, Murphy, Woodward, Stoltzfus
- Wikipedia: Mass fraction (chemistry)
- IUPAC Gold Book
- Atkins' Physical Chemistry
- IUPAC Gold Book: Compound
- Chemistry: The Central Science by Brown, LeMay, Bursten, Murphy, Woodward