question_answer

  Why is the mass determined by measuring a colligative property in case of some solutes abnormal? Discuss it with the help of van't Hoff factor.                                                                                                                                      [2]

Answer:

  Colligative properties depend on the number of ions in solution. (a) If there is ionisation, number of ions increases hence colligative properties also increase. (b) If there is association, number of ions decreases and in this case also colligative properties decrease. van't Hoff factor i = [1 + (y -1) x] where, y = number of ions or molecules from 1 mole solute. x = degree of ionization or association. Also

Example		Equilibrium	i	Abnormal molecular weight
(i)	Glucose	No change	1	Normal value
(ii)	NaCl		( 1 + x )
(iii)			(1+2x)
(iv)	Benzoic acid	(dimer)

Molar mass of NaCI determined using depression in freezing point methodMolar mass of benzoic acid in benzene solvent using the above method as it forms dimer by H-bonding   [2]