NEET Solutions Colligative Properties

NEET Study Guide: Solutions & Colligative Properties

1. Opening Framing

Students often feel confident about colligative properties because the formulas (?T?,-= iCRT) seem straightforward. The real gap appears when questions combine multiple concepts—like van’t Hoff factor with molality and degree of dissociation—in a single problem. Under exam pressure, students misapply the formulas by overlooking whether the solute is ionic or non-ionic, or by confusing molarity with molality, leading to calculation errors that cost marks.

2. Core Concepts

Concept 1: Colligative Property A property of a solution that depends only on the number of solute particles, not their identity. Note: The key is "number," not "mass" or "volume"—students often plug in mass instead of moles, forgetting that colligative effects scale with particle count.

Concept 2: Molality (m) The number of moles of solute per kilogram of solvent. Note: Unlike molarity, molality is temperature-independent because it’s based on mass, not volume. Students confuse it with molarity, especially in freezing point depression problems where temperature changes.

Concept 3: van’t Hoff Factor (i) The ratio of the actual number of particles in solution to the number of formula units dissolved. Note: For non-electrolytes, i = 1; for electrolytes, i > 1 but never equals the theoretical maximum (e.g., NaCl’s i < 2 due to ion pairing). Students assume i = 2 for NaCl in all cases, ignoring real-world deviations.

Concept 4: Osmotic Pressure (?) The pressure required to stop the flow of solvent into a solution through a semipermeable membrane. Note: Osmotic pressure is the only colligative property that can be measured at room temperature for high-molecular-weight solutes (e.g., proteins). Students forget that-= iCRT applies to all solutes, not just electrolytes.

Concept 5: Raoult’s Law (for Non-Volatile Solutes) The vapor pressure of a solution is proportional to the mole fraction of the solvent. Note: Students misapply this to volatile solutes, forgetting Raoult’s Law only holds when the solute is non-volatile. For volatile solutes, both components contribute to vapor pressure.

3. Phase/Process Breakdown Table

4. Where Students Go Wrong (Mistake Taxonomy)

Mistake 1: Confusing Molality with Molarity Question (NEET 2020): A solution contains 18 g of glucose (C?HO?) in 100 g of water. Calculate the freezing point depression. (K? = 1.86 K kg mol?¹) Common Wrong Answer: ?T? = 1.86 K (using molarity instead of molality). Reasoning Error: Students calculate moles of glucose (0.1 mol) and divide by volume of water (?0.1 L), getting 1 M. They then use ?T? = K? * M, ignoring that molality requires mass of solvent (0.1 kg), not volume. Correct Answer: ?T? = 1.86 K kg mol?¹ * (0.1 mol / 0.1 kg) = 1.86 K.

Mistake 2: Ignoring van’t Hoff Factor for Electrolytes Question (NEET 2019): The osmotic pressure of a 0.1 M NaCl solution at 27°C is (R = 0.082 L atm K?¹ mol?¹). Common Wrong Answer:-= 0.1 * 0.082 * 300 = 2.46 atm (assuming i = 1). Reasoning Error: Students treat NaCl as a non-electrolyte, forgetting it dissociates into Na? and Cl? (i-1.8 for 0.1 M NaCl). They plug in i = 1, underestimating ?. Correct Answer:-= 1.8 * 0.1 * 0.082 * 300 = 4.43 atm.

Mistake 3: Misapplying Raoult’s Law to Volatile Solutes Question (NEET 2018): A solution contains 1 mole of ethanol and 3 moles of water. The vapor pressure of pure ethanol is 44.5 mmHg and pure water is 23.8 mmHg. What is the total vapor pressure of the solution? Common Wrong Answer: P_total = 0.25 * 44.5 + 0.75 * 23.8 = 29.0 mmHg (correct calculation, but wrong reasoning). Reasoning Error: Students assume Raoult’s Law applies only to non-volatile solutes and forget that for volatile solutes, both components contribute to vapor pressure. The trap is in the question’s phrasing—students might exclude ethanol’s contribution. Correct Answer: P_total = (1/4)44.5 + (3/4)23.8 = 29.0 mmHg.

5. Cross-Topic Connections

1. Colligative Properties-Electrochemistry (Nernst Equation): The van’t Hoff factor (i) appears in the Nernst equation for cell potentials, where ion dissociation affects the effective concentration of electrolytes in solution.

2. Osmotic Pressure-Physiology (Kidney Function): Osmotic pressure gradients drive water reabsorption in the kidneys—colligative properties explain why hypertonic solutions cause cellular dehydration.

3. Freezing Point Depression-Thermodynamics (Phase Diagrams): The ?T? formula is derived from the Clausius-Clapeyron equation, linking colligative properties to phase equilibrium and vapor pressure changes.

4. Molality-Chemical Kinetics (Rate Laws): Reaction rates often depend on molality (not molarity) for solvent-dependent reactions, as molality remains constant with temperature changes.

6. Past Year Questions — Pattern Recognition

PYQ 1 (NEET 2021): Question: The freezing point of a 0.1 m solution of acetic acid in benzene is 278.4 K. The freezing point of pure benzene is 278.7 K, and K? for benzene is 5.12 K kg mol?¹. What is the van’t Hoff factor for acetic acid in benzene? Hint: The question tests whether you recognize that acetic acid dimerizes in benzene (i < 1). Students who assume i = 1 or i = 2 (for dissociation) miss the non-ideal behavior. The correct approach is to calculate i from ?T? = i * K? * m.

PYQ 2 (NEET 2017): Question: A 5% solution of cane sugar (molar mass = 342 g mol?¹) is isotonic with a 1% solution of a substance X. What is the molar mass of X? Hint: The trap is in the units—students often miscalculate molality from percentage concentration. The key is recognizing that isotonic solutions have equal osmotic pressures (? = iCRT), so molalities must be equal. Convert % to molality first.

PYQ 3 (NEET 2016): Question: The boiling point of a 0.2 m solution of a non-volatile solute in water is 100.104°C. The ebullioscopic constant of water is 0.52 K kg mol?¹. What is the van’t Hoff factor of the solute? Hint: Students rush to plug numbers into ?T_b = i * K_b * m and forget that the question gives ?T_b (0.104°C) relative to pure water’s boiling point. The correct i is derived from 0.104 = i * 0.52 * 0.2.