i = observed colligative property / calculated colligative property (assuming no dissociation/association). For dissociation into n ions with degree alpha: i = 1 + (n-1)*alpha. At complete dissociation: i = n. NaCl: n = 2, i_max = 2. K2SO4: n = 3, i_max = 3. AlCl3: n = 4, i_max = 4. For association (n molecules forming 1 aggregate): i = 1 + (1/n - 1)*alpha = 1 - alpha(1 - 1/n). At complete dimerisation (n=2, alpha=1): i = 0.5. Example: acetic acid in benzene dimerises via H-bonding. Abnormal molar mass: M_observed = M_actual/i. If i > 1 (dissociation): M_observed < M_actual. If i < 1 (association): M_observed > M_actual.