Abnormal Molecular Mass — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on Abnormal Molecular Mass, a systematic approach is essential. Here's a strategy:

1
Master the Basics of Colligative Properties: — Ensure you are comfortable with the standard formulas for relative lowering of vapor pressure, elevation in boiling point, depression in freezing point, and osmotic pressure *before* introducing the van't Hoff factor.
2
Understand the Van't Hoff Factor (i): — Clearly define 'i' as the ratio of observed to normal colligative property, or observed to initial number of particles. Remember its inverse relationship with observed molecular mass: $i = \frac{\text{Normal molecular mass}}{\text{Observed molecular mass}}$.
3
Categorize Solutes:

* Non-electrolytes (e.g., glucose, urea): $i = 1$. * **Strong electrolytes (e.g., NaCl, CaCl$_2$):** Assume complete dissociation, so $i = n$ (where 'n' is the number of ions produced per formula unit).

For NaCl, $i=2$; for CaCl$_2$, $i=3$; for K$_2$SO$_4$, $i=3$; for AlCl$_3$, $i=4$. * **Weak electrolytes (e.g., CH$_3$COOH, NH$_4$OH):** Dissociation is partial, so $1 < i < n$. Use $i = 1 + \alpha(n-1)$ to relate 'i' and degree of dissociation ($\alpha$).

* **Associating solutes (e.g., CH$_3$COOH in benzene):** Association is partial or complete, so $i < 1$. Use $i = 1 + \alpha(\frac{1}{n}-1)$ to relate 'i' and degree of association ($\alpha$), where 'n' is the number of molecules associating.

1
Apply Modified Formulas: — Always use the 'i'-modified colligative property formulas: $\Delta T_f = i K_f m$, $\Delta T_b = i K_b m$, $\pi = i CRT$, and $\frac{P^0 - P_s}{P^0} = i \frac{n_2}{n_1}$.
2
Comparison Questions: — For questions asking to compare colligative properties (e.g., highest boiling point, lowest freezing point), calculate the effective concentration, which is $i \times m$ (or $i \times C$). The solution with the highest effective concentration will have the greatest boiling point elevation, freezing point depression, and osmotic pressure.
3
Numerical Problems: — Pay close attention to units (especially temperature in Kelvin for osmotic pressure) and significant figures. Be careful with algebraic manipulations when solving for $\alpha$ or 'i'.
4
Trap Options: — Be aware that many incorrect options are derived by ignoring 'i' or using an incorrect 'i' value. Always double-check your 'i' calculation.