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 rather than relating to mass or volume, it relates to the number of atoms or molecules. Specifically, 1 mole represents 6.022 x 10^23 atoms or molecules of substance. Scientists calculate the number of moles by dividing the mass of substance by the substance’s atomic or molecular weight, as determined by adding together all of the atomic masses for the atoms in a chemical formula as found on the periodic table of the elements. The scientist then calculates 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.
Determine the mass of all chemical species in a mixture. If the mixture has already been prepared, then this information would have to be provided to you by the person who prepared it. If you have prepared the mixture yourself, then this simply represents the grams of each substance. For example, consider a solution prepared by combining 10 grams of sodium chloride, or NaCl, and 100 milliliters or 100 grams of water, H2O. If a mixture has been prepared using nonmetric measurements, such as ounces, convert the measurements to units of grams using an online calculator, such as that provided in the Resources.
Calculate the formula weights or molecular weights of all of the components of the mixture by multiplying the number of atoms of each type in the formula by its corresponding atomic weight found in the periodic table of the elements. NaCl, for example, contains one sodium atom and one chloride atom with atomic weights of 22.99 and 35.45. The formula weight of NaCl is therefore (1 x 22.99) + (1 x 35.45) = 58.44. Water, or 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.
Determine the number of moles of each substance by dividing its mass in grams by its formula or molecular weight. Continuing the previous example, 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.
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.
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, and 2.99 + 97.01 = 100.
The difference between “molecular weight” or “formula weight” represents a formality that differentiates between compounds with covalent and ionic bonds. As a practical matter, molecular and formula weights are calculated identically.