By Jack Brubaker
Updated Aug 30, 2022

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In physical chemistry, the mole fraction—or mole percent—of a component tells you how many moles of that substance exist relative to the total moles in a mixture. One mole equals 6.022 x 10²³ atoms or molecules. To find the mole percent, first determine the moles of each component by dividing its mass by its molecular weight, then divide that number by the sum of all moles and multiply by 100.

1. Determine Mass

Record the mass of every species in the mixture. If you’ve already prepared the solution, the mass should be listed in the protocol. If you’re creating the mixture, note the grams of each compound. For instance, a solution of 10 g NaCl with 100 g H₂O. If you work in ounces, convert to grams with a reliable online calculator.

2. Calculate Molecular Weight

Compute the formula weight of each component by multiplying the number of atoms of each element by its atomic weight (from a trusted periodic table, such as the NIST database). Example: NaCl = (1 × 22.99) + (1 × 35.45) = 58.44 g/mol; H₂O = (2 × 1.01) + (1 × 16.00) = 18.02 g/mol.

3. Divide Mass by Molecular Weight

Determine the moles of each substance:

• NaCl: 10 g ÷ 58.44 g/mol = 0.171 mol
• H₂O: 100 g ÷ 18.02 g/mol = 5.55 mol

4. Calculate Mole Percent

Find the mole percent of each component:

• NaCl: (0.171 ÷ (0.171 + 5.55)) × 100 = 2.99 %
• H₂O: (5.55 ÷ (5.55 + 0.171)) × 100 = 97.01 %

Things Needed

• Reliable periodic table (e.g., NIST or IUPAC)
• Scientific calculator or spreadsheet software

TL;DR (Too Long; Didn’t Read)

For any mixture, mole fractions add to 100 %. In our example of 10 g NaCl + 100 g water, the mole percent of NaCl is 2.99 % and that of water is 97.01 %.

Although the terms "molecular weight" and "formula weight" technically distinguish covalent from ionic compounds, the calculation method is identical in practice.