How to Calculate Mole Percent

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In the physical sciences, some properties of mixtures relate to mole fraction, or mole percent, of one of the mixture’s components. The mole represents a quantity of substance, but relates to the number of atoms or molecules rather than mass or volume. Specifically, 1 mole represents 6.022 x 10^23 atoms or molecules of substance. You calculate the number of moles by dividing the mass of substance by the substance’s atomic or molecular weight. You then find the mole fraction by dividing the moles of one substance in a mixture by the total number of moles of all substances in the mixture.

  1. Determine Mass

  2. Determine the mass of all chemical species in a mixture. If the mixture is already prepared, this information should be provided. If you prepare the mixture yourself, record the mass in grams of each substance. For example, consider a solution prepared by combining 10 grams of sodium chloride (NaCl) and 100 milliliters or 100 grams of water (H2O). If a mixture is prepared using non-metric measurements, such as ounces, convert to units of grams using an online calculator.

  3. Calculate Molecular Weight

  4. Calculate the formula weights or molecular weights of all components of the mixture by multiplying the number of atoms of each type in the formula by its corresponding atomic weight. Check the periodic table of the elements for atomic weight. For example, NaCl contains one sodium atom and one chloride atom with atomic weights of 22.99 and 35.45, respectively. The formula weight of NaCl is therefore (1 x 22.99) + (1 x 35.45) = 58.44. H2O contains one hydrogen and one oxygen atom with atomic weights of 1.01 and 16.00, respectively. This gives water a molecular weight of (2 x 1.01) + (1 x 16.00) = 18.02.

  5. Divide Mass by Molecular Weight

  6. Determine the number of moles of each substance by dividing its mass in grams by its formula or molecular weight. In this case, 10 grams of NaCl represents 10 ÷ 58.44 = 0.171 moles of NaCl, and 100 grams of water represents 100 ÷ 18.02 = 5.55 moles of H2O.

  7. Calculate Mole Percent

  8. Calculate the mole percent of one of the components by dividing its number of moles by the total number of moles of all substances and multiplying the result by 100. In the case of 0.171 moles of NaCl and 5.55 moles of H2O, the mole fraction of NaCl becomes 0.171 ÷ (0.171 + 5.55) x 100 = 2.99 percent. The mole fraction of water becomes 5.55 ÷ (5.55 + 0.171) = 97.01 percent.

    Tips

    • The mole fractions for all substances in a mixture should add up to 100 percent. In the example of 10 grams of NaCl combined with 100 grams of water, the mole fraction of NaCl and H2O were 2.99 and 97.01 = 100.

      The difference between "molecular weight" or "formula weight" represents a formality that differentiates between compounds with covalent and ionic bonds. However in practical terms, you calculate molecular and formula weights identically.

References

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