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The mole fraction of a substance is calculated by taking the ratio of the number of moles of that substance to the total number of moles of all substances present in the solution. This relationship can be mathematically represented as:

Mole Fraction = (Moles of the substance) / (Total moles of all substances in the solution).

This approach provides a dimensionless quantity that indicates the proportion of that particular substance within the mixture. By focusing on the moles, we account for the actual quantity of particles present, which is particularly significant in reactions and solutions where concentration and volume may not relate linearly due to different molar masses.

The other options provided do not accurately represent the concept of mole fraction. The calculation involving mass would not correctly account for substances with different molar masses and thus could lead to misleading concentrations. Volume alone may give variable results depending on the densities of the substances involved, and simply taking the ratio of concentrations does not convey the actual proportion of the particles in the solution. Hence, only the method involving moles accurately represents the mole fraction.