Masses Π Fig. . : Reverse osmosis occurs when a pressure larger than the osmotic pressure is applied to the solution. CH COOH ⇌ (CH COOH) dissociation, we could be led to conclude that the mass of mol particles is .
g and the mass of one mole of KCl would be . g. This brings into light the rule that, when there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value. Molecules of ethanoic acid (acetic acid) dimerise in benzene due to hydrogen bonding.
This normally happens in solvents of low dielectric constant. In this case the number of particles is reduced due to dimerisation. Association of molecules is depicted as follows: It can be undoubtedly stated here that if all the molecules of ethanoic acid associate in benzene, then D T b or D T f for ethanoic acid will be half of the normal value. The molar mass calculated on the basis of this D T b or D T f will, therefore, be twice the expected value.
Such a molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass. In van’t Hoff introduced a factor i , known as the van’t Hoff factor, to account for the extent of dissociation or association. This factor i is defined as: Normal molar mass Abnormal molar mass i Observed colligative property Calculated colligative property Total number of moles of particles after association/dissociation Number of moles of particles before association/dissociation i Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volatile solute is neither associated nor dissociated. In case of association, value of i is less than unity while for dissociation it is greater than unity.
For example, the value of i for aqueous KCl solution is close to , while the value for ethanoic acid in benzene is nearly . . Inclusion of van’t Hoff factor modifies the equations for colligative properties as follows: