Colligative Properties: Formulas & Concepts
Properties of solutions that depend only on the number of solute particles, irrespective of their nature, are called Colligative Properties. These are essential for determining molar masses of solutes.
1. The Four Colligative Properties
These properties apply to dilute solutions containing non-volatile solutes.
$K_b$: Ebullioscopic Constant
$K_f$: Cryoscopic Constant
Best method for polymers/proteins
Figure: Summary of Colligative Properties and Molar Mass Determination
2. Abnormal Molar Mass & Van't Hoff Factor ($i$)
When solutes associate or dissociate in solution, the number of particles changes, affecting colligative properties. The Van't Hoff factor ($i$) corrects this.
Calculating '$i$'
- Dissociation ($i > 1$): For $\text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^-$, $i \approx 2$.
- Association ($i < 1$): For Acetic Acid dimerization in benzene, $i \approx 0.5$.
- No Change ($i = 1$): For Urea, Glucose, Sucrose.
Modified Equations for Ionic Solutes
Always multiply the standard formula by '$i$' for electrolytes:
- $\Delta T_b = i K_b m$
- $\Delta T_f = i K_f m$
- $\Pi = i CRT$
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