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.