Mole fraction is the total number of moles of a particular substance that are present in a sample, usually a solution, divided by the combined number of moles of all of the components of the same sample. If a solution contains 8 moles of material total and 3 moles of the total are carbon molecules, the mole fraction would be 0.375 if expressed as a decimal, which is more common than representing it as an actual fraction.
A mole is defined as the number of molecules present in 12 grams of pure carbon-12. This number, also known as Avogadro's number, is 6.02 x 1023. A mole of any substance is contains this same number of molecules. This means that a mole of any substance has a mass of the combined atomic numbers of all the atoms present in one molecule of that material, in grams. This is useful in writing chemical equations and for other calculations in chemistry.
In order to determine the mole fraction of any material in a solution, it is only necessary to know the number of moles of that particular component and the total number of moles of everything else in the solution. The number of moles of the other individual components is not needed as long as the total number of moles is known. If the number of moles of each constituent is known, however, the mole fraction for each may also be found by dividing the number of moles of one by the number of moles of all the constituents combined.
Finding the mole fraction of a particular constituent of a solution is useful for a number of reasons. Using the definition of a mole, it is possible to calculate several other figures based on the mole fraction. By applying knowledge of the definition of a mole and how it relates to the mass of a particular pure substance, if the number of moles of each constituent of a solution is known, one can determine through simple calculations the mass of each substance, even if it is dissolved in the solution. Mole fractions are also used for many other things in chemistry, such as determining how much of a particular substance to dissolve into a solvent to obtain a solution of a particular concentration.