A molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass.
In order to account for all such abnormalities, introduced a factor
(i) known as van 't Hoff's factor, which represents the extent of association (or) dissociation of a solute.
Van't Hoff's factor is defined as the ratio of the observed colligative property to the calculated colligative property.
i = Observed colligative property / Calculated colligative property
Observed colligative property ∝ 1/Molar Mass
i = Mc/Mo
Van't Hoff's factor
(i) represents the extent of association (or) dissociation of a solute
i = Total number of moles of particles after association or dissociation / Number of moles of particles before association or disscussion
Experimentally determined molar mass is always lower than actual value for dissociation.
Molar Mass ∝ 1/Colligative Property
If the solute undergoes association in a solution, then the value of van 't Hoff's factor is less than one. If the solute undergoes dissociation then 'i' is greater than one.
KCl → K+ + Cl-
1 molecule of KCl furnishes 2 ions in solution
i = Total number of moles of particles after dissociation / Number of moles of particles before dissociation
i = 2/1 = 2
Ethanoic acid Dimer of Ethanoic acid
i = Total number of moles of particles after association / Number of moles of particles before association
i = 1/2 = 0.5
Molarity (M) is defined as number of moles of solute dissolved in one litre (or one cubic decimetre) of solution.
(a) Mol. mass of
Volume of solution = 4.3 L
(b) Number of moles present in 1000 ml of 0.5M H2SO4= 0.5 mol
therefore number of moles present in 30ml of 0.5M H2SO4=mol =0.015mol
therefore molarity =0.015/0.5L
thus molarity is 0.03M
Calculate (a) molality (b) molarity and (c) mole fraction of KI if the density of 20% (mass/mass) aqueous KI is 1.202 g mL-1.
Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent and is expressed as: