Solutions — NEET Chemistry Notes

Solutions are homogeneous mixtures of two or more substances. This chapter covers concentration terms, colligative properties, Raoult’s Law, and the role of solutes in changing physical properties of solvents — a scoring topic in NEET Chemistry.

Quick Revision

• Solute + Solvent = Solution
• Concentration terms: Molarity, Molality, Mole Fraction, Normality, ppm
• Raoult’s Law: P_solution = X_solvent × P°_solvent
• Ideal Solution: Obeys Raoult’s Law at all concentrations
• Non-ideal Solution: Deviates from Raoult’s Law (positive or negative deviation)
• Colligative Properties: Boiling point elevation, freezing point depression, osmotic pressure
• Van’t Hoff Factor (i): Number of particles per formula unit

Standard Study

Concentration Terms

Molarity (M): Moles of solute per litre of solution

• M = (mass solute / M_w) / (volume solution in L)
• Unit: mol/L or M

Molality (m): Moles of solute per kg of solvent

• m = (mass solute / M_w) / (mass solvent in kg)
• Unit: mol/kg — independent of temperature (unlike molarity)

Mole Fraction (X): Ratio of moles of component to total moles

• X_A = n_A / (n_A + n_B)
• X_A + X_B = 1
• Dimensionless

Normality (N): Number of gram equivalents per litre of solution

• N = (mass solute / Eq_w) / (volume solution in L)
• Related to molarity: N = M × n_factor (where n = number of replaceable H⁺ or OH⁻)

Parts Per Million (ppm): Parts of solute per million parts of solution

• ppm = (mass solute / mass solution) × 10⁶

Raoult’s Law

• For a solution of volatile liquids: P_total = P_A + P_B
• P_A = X_A × P°_A (vapour pressure of pure A × mole fraction)
• P_B = X_B × P°_B
• Positive Deviation: P_solution > P_ideal (A-B attraction < A-A or B-B) — e.g., ethanol-hexane
• Negative Deviation: P_solution < P_ideal (A-B attraction > A-A or B-B) — e.g., acetone-chloroform

Azeotropes

• Constant boiling mixtures — composition of liquid and vapour is same
• Minimum boiling azeotrope: positive deviation (e.g., ethanol-water 95.6%)
• Maximum boiling azeotrope: negative deviation (e.g., nitric acid-water 68%)

Colligative Properties

1. Lowering of Vapour Pressure:

• ΔP = P°_solvent − P_solution = X_solute × P°_solvent
• Solute molecules reduce the fraction of solvent molecules at the surface

2. Boiling Point Elevation:

• ΔT_b = K_b × m (molality)
• K_b = (R × M_solvent) / (ΔH_vap × 1000)
• Non-volatile solute elevates boiling point

3. Freezing Point Depression:

• ΔT_f = K_f × m
• K_f = (R × M_solvent × T_f) / (ΔH_fus × 1000)
• Freezing point is lowered when solute is added

4. Osmotic Pressure (π):

• π = i × M × R × T (in molarity terms)
• πV = nRT
• Osmotic pressure is the pressure needed to stop osmosis

Abnormal Molecular Mass

• When solute dissociates or associates, observed molecular mass differs from true value
• Van’t Hoff Factor: i = (Normal molecular mass) / (Observed molecular mass) × n
• i > 1 for dissociating solutes (e.g., NaCl in water → i ≈ 2)
• i < 1 for associating solutes (e.g., acetic acid in benzene → i ≈ 0.5)
• Modified Colligative Properties: ΔT_b = i × K_b × m; π = i × M × R × T

Deep Study

Energetics of Solution Formation

• ΔH_solution = ΔH_mixing + (ΔH_solute−solute + ΔH_solvent−solvent)
• Solution forms if ΔH_mixing is negative (exothermic) or small and TΔS is positive

Solubility and Temperature

• For solid in liquid: solubility increases with temperature if dissolution is endothermic
• For gas in liquid: solubility decreases with increasing temperature (Henry’s Law)

Van’t Hoff Factor Details

• For strong electrolytes: i = number of ions (complete dissociation assumed)
• For weak electrolytes: i depends on degree of dissociation α
• i = 1 + (v − 1)α where v = number of ions from one formula unit

Osmosis and Reverse Osmosis

• Isotonic: equal osmotic pressure
• Hypotonic: lower concentration (cell swells)
• Hypertonic: higher concentration (cell shrinks)
• Reverse osmosis: apply pressure > π to force solvent from concentrated to dilute side

Exam Tips

1. Molality (m) is temperature-independent — better than molarity for colligative property calculations
2. Boiling point elevation and freezing point depression are both proportional to molality
3. Van’t Hoff factor i accounts for dissociation/association in colligative property formulas
4. For ionic compounds like NaCl: i ≈ 2 (two ions), for MgSO₄: i ≈ 2, for Ca₃(PO₄)₂: i ≈ 5
5. Raoult’s Law applies to both components of a volatile solution
6. Remember to use mole fraction (not mass fraction) in Raoult’s Law
7. Osmotic pressure formula π = iMRT is similar to ideal gas equation PV = nRT

Common Pitfalls

• Confusing molarity and molality — watch for L vs kg in denominator
• Forgetting to include Van’t Hoff factor i in colligative property formulas
• Confusing lowering of vapour pressure with boiling point elevation
• Misidentifying positive vs negative deviation in Raoult’s Law
• Forgetting that the solute must be non-volatile for boiling point elevation

Suggested Study Order

1. Concentration terms: molarity, molality, mole fraction, normality
2. Raoult’s Law and ideal/non-ideal solutions
3. Colligative properties: vapour pressure lowering
4. Boiling point elevation and freezing point depression
5. Osmotic pressure and van’t Hoff factor
6. Abnormal molecular mass
7. Azeotropes and solubility