Abnormal Molar Masses
Molar masses that are
lower or higher than expected values when calculated (generally using
colligative properties) are called abnormal molar masses. Abnormal molar masses
depends upon the total number of moles particles either after dissociation or
association of solute molecules in solvent or solution.
Dissociation of Solutes:
There are many
solutes, such as organic acids, salts and bases, when dissolved in water
dissociate into ions.
Example
–
When one mole of NaCl is dissolved into water, it
dissociates into Na + and Cl -
. After dissociation, there would be one mole of sodium ions (Na+)
and one mole of chloride ions (Cl-
) released in solution. Thus, we get two moles of particles in the
solution.
After the dissociation of NaCl or KCl or any other solutes
which breaks into ions into water, the boiling point of water increases. And
when molar mass is determined experimentally, it always comes lower than the
expected value.
Association
of Solutes:
There are many
solutes, when dissolved, get associated into the solution, this decreases the
number of moles into the solution. This increases the molar mass of the
solution than expected value, when determined experimentally, using colligative
properties methods.
Example – When acetic acid (ethanoic acid) is dissolved into benzene, molecules of
acetic acid gets associated i.e. dimerize due to
hydrogen bondings and number of particles reduced.
This happens generally in the solvents having low dielectric constants.
This increases the molar mass than
expected value when determined experimentally.
Van’t Hoff
Factor:
Van’t Hoff, a Dutch scientist, after many
experiments, introduced a factor known as Van’t Hoff
factor after keeping in account of the increase or decrease in molar masses
than expected because of dissociation or association of solutes.
The Van’t
Hoff Factor is denoted by letter ‘i’.
Van’t Hoff defined this factor as follows:
Where abnormal molar mass is determined
experimentally and colligative properties is obtained by keeping in mind that
solute (non-volatile) neither associated nor dissociated.
In the case of dissociation i < 1
And in the case of association i< 1
Thus, equations for colligative
properties can be modified as follows after inclusion of Van’t
Hoff factors.
Relative lowering of vapour pressure of
solvent
Elevation of boiling point,
= i
m
Depression of Freezing point,
= i
m
Osmotic pressure of solutions,
= 
RT/V
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