Thermodynamic theory of solutions

Abstract

A solution is a homogeneous mixture consisting of two or more components in which the composition can be continuously varied (within some range) with no change of phase. Solutions can be gases, liquids, or solids. We have already considered the properties of gaseous solutions (when we considered a mixture of ideal gases). In this chapter, we focus on condensed phases (i.e. liquids and solids). The composition of a solution can be described in several ways. Here are the most common: 1. The molar fraction of the ith component, xi, is the ratio of the number of moles of component i, ni, to the total number of moles of all species within the solution, n: 2. The weight fraction of the ith component, [i], is the ratio of the mass of component i, wi, to the total mass of all species within the solution, w: the weight fraction is often written as a weight percent [wt%] = 100[i]. 3. The molarity of the ith component, ci, is the number of moles of component i, ni per liter of solution, V: 4. The molality of ith component (used only for dilute solutions), mi, is the number of moles of component i, in 1 kg of solvent. There are several other definitions used to describe the composition of a solution, but we shall only refer to those described above in this text. The reason that there are so many definitions of the composition is related to how the term ‘‘concentration’’ is applied. For example, from the physical–chemical point of view, the molar fraction is the most convenient definition of the concentration since it is on an atomic basis. However, from the point of view of someone who has to prepare solutions from separate solutes and solvents, the mass fraction is the most convenient definition since it is directly related to the mass of the components, rather than the number of moles of the component. The former is easily measured, while the latter requires the additional knowledge of the molecular weight (and a trivial calculation).