This percent composition calculator is a simple tool to evaluate the percentage of each element by weight (or mass) in a compound from its chemical formula. This article will explore percent composition, what is needed to determine percent composition, how to calculate it using the percent composition formula, and some frequently asked questions.

What is percent composition?

The percent composition of a compound expresses the weight (or mass) of each element in the compound as a percentage of the total weight of the compound. The formula for percent composition is given by:

element wgt% = (element mass/compound mass) × 100

where:

  • element wgt%Percentage of the element present in the compound by mass;
  • element massMass of the element in one mole of the compound; and
  • compound massMass of one mole of the compound or its molar mass.

Before you learn how to find the percent composition of a substance in the next section, use our mole calculator and Avogadro's number calculator to understand the concept of moles as a unit of measurement.

How do I calculate the percent composition of a compound?

To determine the percent composition of a substance, follow these simple steps:

  1. Determine the molar mass of the substance either from its molecular weight or from its mass and number of moles.
  2. Compute the mass of each element in one mole of the compound by multiplying their atomic mass with the number of atoms in one molecule of the compound.
  3. Calculate percent composition of each element as (mass of the element in 1 mol of compound/molar mass of compound) × 100.
  4. Verify your calculations with our percent composition calculator.

For example, let's calculate the mass percent composition of sulfuric acid, H2SO4\text{H}_2\text{SO}_4, using the percent composition equation.

  1. Molar mass of sulfuric acid:

    • Start by finding the molecular weight of H2SO4\text{H}_2\text{SO}_4, given by multiplying the number of atoms of each element in the molecule by its atomic mass:
      2 × (1.008) + (32.06) + 4 × (15.999) = 98.07 amu.
    • Since the molar mass of a compound in g/mol is numerically equivalent to its molecular weight in amu, we get molar mass of sulfuric acid as 98.07 g/mol.
  2. Mass of each element in one mole of sulfuric acid:

    • Mass of hydrogen = 2 × (1.008) = 2.016 g/mol
    • Mass of sulfur = 1 × 32.06 = 32.06 g/mol
    • Mass of oxygen = 4 × 15.999 = 63.996 g/mol
  3. Percent composition of each element in sulfuric acid:

    • Hydrogen% = (2.016/98.07) × 100 = 2.0556%
    • Sulfur% = (32.06/98.07) × 100 = 32.69%
    • Oxygen% = (63.996/98.07) × 100 = 65.25%

Now, do you understand how to calculate the percentage of an element in a compound?

How to find the molar mass from the percent composition of a compound?

To calculate the molar mass of a compound from its percent composition, follow these simple steps:

  1. Divide the percent composition of an element in the compound by 100 to get the value in decimals.
  2. Divide the mass of the element in one mole of the compound by this decimal value to obtain the molar mass of the compound.
  3. You can verify your result yourself by repeating these steps with the percent composition of other elements too!

How to use this percent composition calculator?

Our percent composition calculator is a simple tool to use. It can support up to five elements in a compound.

  1. In the field labeled Number of atoms of first element, you can select the element from the units drop-down list (five of the most common elements in compounds are the default).
  2. Enter the number of atoms of the selected element present in one molecule of the compound. For example, if the compound is H2O\text{H}_2\text{O}, enter 2 atoms for hydrogen and 1 for oxygen.
  3. Repeat these steps for every element in the compound.
  4. The calculator will automatically calculate the percent composition by mass using the percent composition equation and display them in the corresponding fields in the Composition of each element group.
  5. For the element fields you don't use, let their value stay at the default value of 0. Empty fields halt the calculation of molecular weight.

We've put together a collection of related calculators for you to explore and learn further:

FAQ

How much carbon is present in 100 g of sugar?

There are 42.1 grams of carbon in 100 grams of sugar. The molecular formula for sugar is C₁₂H₂₂O₁₁. From this, we can determine that the percent composition of carbon is 42.11%, hydrogen is 6.479%, and oxygen is 51.41%. Therefore, in 100 grams of sugar, we shall find:

  • 42.1 grams of carbon;
  • Around 6.5 grams of hydrogen; and
  • 51.4 grams of oxygen.

What is the mass percentage of hydrogen in water?

11.19%. To replicate this result, follow these simple steps:

  1. Determine the molecular weight of water as the sum of the atomic mass of all atoms in one molecule. 2 × 1.008 + 15.999 = 18.015 amu.
  2. Obtain the molar mass of water as the numerical equivalent of its molecular weight, when expressed in g/mol. Molar mass of water = 18.015 g/mol.
  3. Compute the mass of hydrogen in one mole of water as the numerical equivalent of the atomic mass of hydrogen atoms in one water molecule, when expressed in g/mol. Mass of hydrogen in 1 mol water = 2 × 1.008 = 2.016 g/mol.
  4. Calculate the mass percent composition of hydrogen in water as (mass of hydrogen in 1 mol water/molar mass of water) × 100 = (2.016/18.015) × 100 = 11.19%.

How to find the molecular formula from the percent composition of a compound?

To find the molecular formula of a compound from its percent composition, you need to know its molar mass:

  1. Determine the molecular weight of the compound in amu from its molar mass in g/mol since they are numerical equivalent.
  2. Calculate the weight of each element in the compound from the percent composition, using mass of an element per molecule = (element percent composition element × molecular weight)/100.
  3. Find the number of atoms of each element in the compound using their atomic mass. Number of atoms of each element per molecule = mass of each element per molecule/element atomic mass.
  4. Formulate the molecular formula of the compound using the element symbols and the number of atoms in one molecule.

What is the percent composition of NaHCO₃?

Element

Percent Composition

Sodium (Na)

27.367%

Hydrogen (H)

1.2%

Carbon (C)

14.298%

Oxygen (O)

57.14%

Total molecular weight : 84 amu

Krishna Nelaturu
Number of atoms of each element in the compound
First element
H
Second element
C
Third element
O
Fourth element
N
Fifth element
S
Molecular weight
0
amu
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