Page 107 - Instant notes

Non-electrolyte solutions 93

charge (are not ions) and cannot interact electrostatically, termed non-electrolyte
solutions. Electrostatic interaction is the added complication of the relatively long-range
attractive and repulsive forces between ions of opposite and like charges found in ionic or
electrolyte solutions; these are considered in Section E.
Concentration is the normal variable used to define the composition, or the relative
amounts of solvent and solute in a solution. The concentration of a species A, c A, is
defined as:

where n A is the number of moles of A in solution and V is the volume of the solution.
−3
This normally has units of mol dm (also written M for molar). The concentration of A
is often also written as [A]. A solution is typically prepared by dispersing (dissolving) n A
moles of solute in solvent to produce a final total volume of V. This is generally not the
same as mixing n A moles of solute with a volume, V, of water, as the volume after mixing
is usually not V in this case.
An alternative measure of the composition of a mixture is the mole fraction, x A. The
mole fraction is given by:

where n is the total number of moles of species present in the mixture. For example, for a
solution containing two solutes, A and B, and a solvent, C:
n=n A+n B+n C

where n i is the number of moles of a species, i. By dividing both sides by n, this means
that x A+x B+x C=1 or more generally, Σ ix i=1.
Concentration and mole fraction are clearly closely related, as V is related to n.
Concentration is the more frequently used composition variable, but mole fraction is
more general and is often preferred to concentration in mixtures where there is no
obvious solvent. These systems are commonly found in phase diagrams, which is why
mole fraction is the composition variable of choice in the-phase diagrams of mixtures.
A third composition variable used occasionally for solutions is the molality of species
A, m A, defined by:

where M is the total mass of the solute and solvent. When water is used as a solvent, 1
3
3
dm (1000 cm ) of water has a mass of 1.000 kg at room temperature (298 K). This
−1
means that the molality of a solute (in mol kg ) is approximately equal to its
−3
concentration (in mol dm ) for dilute aqueous solutions, where the contribution of the
mass of the solute to the overall mass is negligible.
Chemical potential