i = observed colligative property / calculated colligative property $\frac{assuming no dissociation}{association}$. 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: $M_{observed}$ = $\frac{M_actual}{i}$. If i > 1 (dissociation): $M_{observed}$ < $M_{actual}$. If i < 1 (association): $M_{observed}$ > $M_{actual}$.