Chemistry: Solutions - Lecture 6 | CHEMCA

Class 12 / JEE / NEET Chapter 1

Chemistry - Solutions | Lecture 6

By Abhishek Sengar

Lecture Chapters

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Theory & Quick Notes

1 Osmosis & Osmotic Pressure

Osmosis is the spontaneous flow of solvent molecules through a Semi-Permeable Membrane (SPM) from a region of lower solute concentration (pure solvent) to a region of higher solute concentration (solution).

Driving Force & Osmotic Pressure (π)

The solvent flows from a high vapor pressure side to a low vapor pressure side. The minimum excess pressure that must be applied to the solution side to just stop this flow is called Osmotic Pressure.

π = C R T

Where C = Concentration (Molarity), R = Gas constant (0.0821 L·atm/mol·K), T = Temperature (K).

Reverse Osmosis (RO)

If pressure applied on the solution side is greater than the osmotic pressure (π), the flow reverses. Pure solvent flows out of the solution through the SPM. Used in water purification.

2 Types of Solutions Based on Osmotic Pressure

When two solutions are separated by an SPM, their relative osmotic pressures dictate the flow.

Isotonic

π_A = π_B

No net flow of solvent.

Hypotonic

Lower π, lower concentration

Solvent flows OUT of this solution.

Hypertonic

Higher π, higher concentration

Solvent flows INTO this solution.

Example: A grape in salt water (hypertonic) shrinks. A raisin in pure water (hypotonic) swells.

3 Van 't Hoff Factor (i)

Colligative properties depend on the number of particles. If a solute dissociates (NaCl) or associates (dimerizes), the number of particles changes. The Van 't Hoff factor corrects this.

i = (Particles after Dissociation/Association) / (Particles before)

Non-Electrolytes (Glucose, Urea, Sucrose): i = 1
Strong Electrolytes (100% dissociation): i = n (e.g., NaCl i=2, MgCl₂ i=3)

For Weak Electrolytes (Dissociation)

α = Degree of dissociation

α = (i - 1) / (n - 1)

(i > 1)

For Association (e.g., Dimerization)

n = particles joining together (dimer n=2)

α = (1 - i) / (1 - 1/n)

(i < 1)

4 Modified Formulas & Abnormal Molar Mass

When applying colligative properties to electrolytes, we must multiply the theoretical formula by the Van 't Hoff factor (i).

ΔP / P°_A = i × x_B

ΔT_b = i × K_b × m

ΔT_f = i × K_f × m

π = i × C × R × T

Because colligative properties are inversely proportional to molar mass, an abnormal molar mass is observed if i ≠ 1.

Abnormal Molar Mass = Normal Molar Mass / i

Deep Dive

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Lecture 6 Checkpoint

Answer these quick questions to solidify your understanding of Osmosis and the Van 't Hoff Factor.

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