Statement - I : NaCl in water and organic acids in benzene show abnormal molecular mass.
Statement - II : Abnormal molecular mass is obtained when the substance in the solution undergoes either dissociation or association.

(I) In water NaCl dissociates into Na⁺ and Cl^- ions, because of this 

(a) Observed molecular weight is less than actual molecular weight

(b) Observed colligative properties are more than expected

(II) acetic acid dimerizes in benzene due to intermolecular hydrogen bonding.

Due to this association ,

(a)  Observed molecular weight of CH3COOH is more than actual molecular weight

(b)  Observed colligative properties are lesser than expected

(I) & (II) are correct