Molarity vs molality confusion: $\Delta Tb$ and $\Delta Tf$ require molality (mol/kg solvent); osmotic pressure requires molarity (mol/L solution). Using molarity in boiling point calculations — or molality in osmotic pressure — is a direct NEET mark loss.

Forgetting van't Hoff factor for electrolytes: Calculating $\Delta Tf$ for NaCl without multiplying by i ≈ 2 gives an answer that is half the correct value. Every electrolyte question in colligative properties requires i.

Temperature in Kelvin for osmotic pressure: π = iCRT requires T in Kelvin. If 27°C is given, T = 300 K. Failing to convert is a consistent exam trap.

Association → apparent molar mass is LARGER, not smaller: Students frequently invert this. Association reduces particles (i < 1) → smaller $\Delta Tf$ → $M_{2}$ formula gives a larger value ($\Delta Tf$ in denominator is small). Apparent M > true M for dimers.

Minimum/maximum boiling azeotrope confusion: Positive deviation → MINIMUM boiling (higher vapour pressure = easier to boil = lower boiling point). Negative deviation → MAXIMUM boiling. This is inverted in student memory more often than any other single point in this chapter.

Units of solvent mass in molar mass formula: $M_{2}$ = Kb × w_{2} × 1000 / ($\Delta Tb$ × w_{1}). Here w_{1} must be in grams (the 1000 factor converts g to kg). Using kg directly and also writing 1000 doubles the error.

Gas solubility with temperature: Gas solubility decreases with temperature (K_H increases). Most solids become more soluble with T. Gas behaviour is the opposite — frequently confused in MCQs.

i is not always an integer: Partial dissociation or association gives non-integer i. For 50% dissociation of NaCl: i = 1 + (1)(0.5) = 1.5, not 2. Using i = 2 for partial dissociation overstates the colligative effect.