Percent Composition Calculator

About Percent Composition Calculator

Percent composition shows what fraction of a compound's mass comes from each element.

How to use this calculator

1. Enter the chemical formula or the number of atoms for each element in the compound.
2. Use the element's atomic mass and the compound's total molar mass.
3. The calculator finds \(\frac{n \times M_{element}}{M_{compound}} \times 100\) for each element.
4. Read the result as the percent by mass of that element in the compound.

The formula explained

The formula computes the percent by mass of one element in a compound. It multiplies the number of atoms of that element, \(n\), by the element's atomic mass, \(M_{element}\), then divides by the compound's molar mass, \(M_{compound}\), and converts to a percent.

• n = the number of atoms of the element in one formula unit of the compound
• \(M_{element}\) = the atomic mass of the element
• \(M_{compound}\) = the molar mass of the entire compound

Step by step method

1. Find the number of atoms of the element in the formula, then multiply by the element's atomic mass to get that element's mass in the compound.
2. Add up the masses of all atoms in the formula to get the compound's molar mass.
3. Divide the element's mass by the compound's molar mass, then multiply by \(100\).
4. Repeat for each element if you want the full percent composition.

Worked example

Problem. Find the percent composition of oxygen in water, \(H_2O\). Use \(M_H = 1.008\) and \(M_O = 16.00\).

1. In \(H_2O\), oxygen has \(n = 1\), so its mass in the compound is \(1 \times 16.00 = 16.00\).
2. The molar mass of \(H_2O\) is \(2(1.008) + 16.00 = 18.016\).
3. Compute \(\frac{16.00}{18.016} \times 100 \approx 88.81\).

Tips and common mistakes

• Make sure you use the total molar mass of the whole compound, not just one element.
• If your percentages do not add up to about \(100\\%\), check your subscripts and atomic masses.

Frequently asked questions