$word_{count}$: 220

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 = $\frac{i-1}{(n-1)}$. Association alpha = $\frac{1-i}{(1-1/n)}$. From i, find Ka of weak acid HA: Ka = c*alpha^$\frac{2}{1-alpha}$ where alpha = i-1 and c = molality. Abnormal molar mass: $M_{observed}$ = $\frac{M_actual}{i}$. 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).