Portioning Particles Molality and Mole Fractions

Like the difference in their names, the practical difference between molarity and molality is subtle. Take a close look at their definitions, expressed next to one another in the following equations  

Molarity= Moles of solute Molality = Moles of solute 

The numerators in molarity and molality calculations are identical, but their denominators differ greatly. Molarity deals with liters of solution, while molality deals with kilograms of solvent. A solution is a mixture of solvent and solute a solvent is the medium into which the solute is mixed. 

A further complication to the molarity/molality confusion is how to distinguish between their variables and units. The letter m turns out to be overused in chemistry. Instead of picking another Vciriable (or perhaps a less confusing ncime that stcirted with a nice uncommon letter like z, chemists decided to give molality the lowercase script m as its variable. To help you avoid uttering cmy four-letter words when confronted with this plethora of m-words and their abbreviations, we ve provided Table 13-1. 

Occasionally, you may be asked to calculate the mole fraction of a solution, which is the ratio of the number of moles of either solute or solvent in a solution to the total number of moles of solute and solvent in the solution. By the time chemists defined this quantity, however, they had finally acknowledged that they had too many m variables, and they gave it the variable X. Of course, chemists still need to distinguish between the mole fractions of the solute and the solvent, which unfortunately both start with the letter s. To avoid further confusion, they decided to abbreviate solute and solvent as A and B, respectively, in the general formula, although in practice, the chemical formulas of the solute and solvent eire usually written as subscripts in place of A and B. For example, the mole fraction of sodium chloride in a solution would be written as 

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