Relative lowering of vapour pressure of solvent, o o – . i n Elevation of Boiling point, D T b = i K b m Depression of Freezing point, D T f = i K f m Osmotic pressure of solution, P = i n R T / V g of benzoic acid (C H COOH) dissolved in g of benzene shows a depression in freezing point equal to . K. Molal depression constant for benzene is .
K kg mol – . What is the percentage association of acid if it forms dimer in solution? The given quantities are: w = g; K f = . K kg mol – ; w = g, D T f = .
K Substituting these values in equation ( . ) we get: M = – – . K kg mol × g × g kg g × . K = .
g mol – Thus, experimental molar mass of benzoic acid in benzene is = . g mol – Now consider the following equilibrium for the acid: C H COOH ⇌ (C H COOH) If x represents the degree of association of the solute then we would have ( – x ) mol of benzoic acid left in unassociated form and correspondingly x as associated moles of benzoic acid at equilibrium. Therefore, total number of moles of particles at equilibrium is: Thus, total number of moles of particles at equilibrium equals van’t Hoff factor i . But Normal molar mass Abnormal molar mass i