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The van't Hoff factor (i) quantifies the deviation of colligative properties from non-electrolyte predictions. Definition: i = observed property / calculated property (assuming i=1). For dissociation: electrolyte produces n ions per formula unit. i = 1 + (n-1)alpha. Complete dissociation (alpha=1): i = n. Examples: NaCl (n=2, i_max=2), CaCl2 (n=3, i_max=3), Al2(SO4)3 (n=5, i_max=5), K4[Fe(CN)6] (n=5, i_max=5). For association: n molecules combine into one aggregate. i = 1 - alpha(1-1/n). Complete dimerisation (n=2, alpha=1): i = 0.5. Examples: acetic acid in benzene (dimerises, i ≈ 0.5), benzoic acid in benzene. From i, find alpha: dissociation alpha = (i-1)/(n-1). Association alpha = (1-i)/(1-1/n). From i, find Ka of weak acid HA: Ka = c*alpha^2/(1-alpha) where alpha = i-1 and c = molality. Abnormal molar mass: M_observed = M_actual/i. If you measure M_obs from colligative data (assuming i=1), the true M_actual = M_obs * i. For electrolytes: M_obs < M_actual. For associating solutes: M_obs > M_actual. Common JEE trap: calculating colligative properties without using i for electrolytes, or confusing dissociation (i>1) with association (i<1).