van't Hoff Factor — Prelims Strategy

Prelims Strategy

To excel in NEET questions involving the van't Hoff factor, adopt a systematic approach. First, always identify the nature of the solute: is it a non-electrolyte, a dissociating electrolyte, or an associating solute?

This determines whether 'i' is 1, >1, or <1. For electrolytes, correctly determine 'n', the number of ions produced per formula unit (e.g., NaCl, $n=2$; $CaCl_2$, $n=3$; $Al_2(SO_4)_3$, $n=5$). If the degree of dissociation ($alpha$) or association ($\beta$) is given, use the respective formulas: $i = 1 + alpha(n-1)$ for dissociation, and $i = 1 - \frac{\beta(n-1)}{n}$ for association.

If 'i' needs to be calculated from observed colligative properties, rearrange the modified colligative property formulas. For numerical problems, pay close attention to units and significant figures. For conceptual questions, focus on the relationship between 'i' and the number of particles, and how it affects the magnitude of colligative properties.

Practice comparing solutions with different 'i' values to determine which will have the highest/lowest boiling point, freezing point, or osmotic pressure. Be wary of trap options that assume ideal behavior (i.

e., $i=1$) for electrolytes.