The sum is over all components, including the solvent in the case of a chemical solution. As indicated above, the same ratio is obtained using the number of molecules of , , and the total number of molecules of all kinds, .
Mole fractions are one way of representing the concentrations of the various chemical species. They are an ideal-mixture approximation to the effect of concentration on the equilbrium or rate of a reaction. In practice (except for very dilute solutions or for gasses at atmospheric pressure), all measures of concentration must be multiplied by correction factors called activity coefficients in order to yield accurate results.
Mole fractions are dimensionless numbers. Other ways of representing concentrations, e.g., molarity and molality, yield dimensional quantities (per litre, per kilogram, etc.). When chemical formulas seem to be taking the logarithms of dimensional quantities, there is an implied ratio, and such expressions can always be rearranged so that the arguments of the logarithms are dimensionless numbers, as they must be.
As an example, if a mixture is obtained by dissolving 10 moles of NaCl in 90 moles of water, the mole fraction of NaCl in that mixture is 0.1.
For mixtures of molecules of differing sizes, see: volume fraction.