Why do we have different definitions of concentration?
It is worthy of comment that chemist's use a number of different ways to quantify concentration. Mass fraction, mole fraction, molarity and molality are the main ones, but density is also a measure of concentration and there are vaiours types of density as well. Why are there so many ways to talk about one aspect? First, there are many sides to the elephant. Mass fraction tells us something different than mole fraction. The relative mass may affect certain aspects of physical change or separation methods, but reactivity is governed by molar relationships that we call stoichiometry. In the laboratory we often use solutions and it is most convneient to talk about concentration in terms of molarity, which has units of moles per liter. However, when we consider colligative properties such as a the freezing point depression the magnetiude depends on the solute per amount of solvent and therefore molality is more appropriate as a unit of concentration. Molality is defined per kilogram of solvent irrespective of changes in volume, density etc. We can begin to see that the choice of concentration unit depends on the application. What is particularly vexing sometimes is converting from one to another. We will leave that exciting aspect of the problem until Chapter 3. However, be assured that we have worked out some handy methods to do this and it will not be that bad once you have mastered these.
