NEET · Chemistry
Own official Unit 5: concentration terms, vapour pressure/Raoult law, ideal and non-ideal solutions at current depth, colligative properties, molecular-mass determination and van't Hoff factor with applicability conditions.
In short
Solution calculations are reliable only when the concentration basis and physical model are stated. Molarity depends on solution volume and therefore temperature; molality depends on solvent mass. Raoult-law and colligative-property equations apply under their stated ideal/dilute assumptions, while the van't Hoff factor accounts for effective particle-number changes from association or dissociation.
Molarity M is moles of solute per litre of solution and changes if solution volume changes with temperature. Molality m is moles of solute per kilogram of solvent and is independent of volume. Mole fraction is dimensionless and the mole fractions in a mixture sum to one. Mass percentage uses a mass/mass basis unless another basis is explicitly stated.
For an ideal solution component i, p_i = x_i p_i°, and total vapour pressure is the sum of component partial pressures. For a nonvolatile solute in an ideal dilute model, the relative lowering of solvent vapour pressure is related to solute mole fraction. Non-ideal solutions show positive or negative deviations from Raoult-law behaviour depending on intermolecular-interaction changes; do not assume ideality if the question explicitly signals association or strong interaction differences.
Colligative properties depend on the number of solute particles rather than their chemical identity in the ideal dilute treatment. The current-scope relations include:
i is the van't Hoff factor. For a non-electrolyte with no association/dissociation, i is approximately 1. Electrolyte dissociation can raise the effective particle count; association can lower it. The actual model must match the species and concentration regime.
A colligative measurement can be combined with sample mass and concentration definitions to infer molar mass. The workflow must show which amount basis is measured and whether an i correction is justified; do not automatically insert a textbook integer dissociation count when the problem gives an observed van't Hoff factor.
M = n_solute / V_solution: mol L^-1 if V in L. Solution volume at stated T
M = n_solute / V_solution
Use when — Solution volume at stated T
m = n_solute / mass_solvent(kg): mol kg^-1. Solvent mass basis
m = n_solute / mass_solvent(kg)
Use when — Solvent mass basis
x_i = n_i / sum(n): dimensionless. Mole basis
x_i = n_i / sum(n)
Use when — Mole basis
p_i = x_i p_i°: pressure. Ideal-solution component model
p_i = x_i p_i°
Use when — Ideal-solution component model
Delta Tb = i Kb m: K; Kb in K kg mol^-1. Dilute colligative model
Delta Tb = i Kb m
Use when — Dilute colligative model
Delta Tf = i Kf m: K. Dilute colligative model
Delta Tf = i Kf m
Use when — Dilute colligative model
pi = i C R T: pressure; C mol volume^-1; T K. Dilute solution/osmotic ideal model
pi = i C R T
Use when — Dilute solution/osmotic ideal model
i = observed colligative effect / ideal effect for same formula amount: dimensionless. Defined comparison
i = observed colligative effect / ideal effect for same formula amount
Use when — Defined comparison
Original NEET-style item: A 0.100 mol kg^-1 aqueous solution of a non-electrolyte has Kf = 1.86 K kg mol^-1. Assuming i=1, estimate the freezing-point depression.
Reasoning: Delta Tf = i Kf m = 1 x 1.86 x 0.100 = 0.186 K. This is the magnitude of the depression; the new freezing point would be lower than the pure solvent value by that amount.
Cannot distinguish molarity from molality denominator
Knowledge gapWhy it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Identify solution volume vs solvent mass before formula selection.
Forgets colligative equations
Recall gapWhy it happens
Preparation Intelligence classification: Recall Gap.
How it is corrected
Retrieve property + concentration basis + i together.
Uses Celsius in pi=iCRT
Execution errorWhy it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Convert to kelvin.
Assumes i=2 for every 1:1 electrolyte regardless of data
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Use stated/observed model and concentration context.
Ideality or association state is ambiguous
Needs reviewWhy it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
State the assumption or hold the exact numerical inference.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
Molality uses solvent mass, while molarity uses solution volume, which can change with temperature.
It compares the effective particle-number effect in a colligative measurement with the ideal effect expected from the formula amount.
The standard relation Delta Tf=iKf m uses molality.
Official scope is taken from the NEET (UG) 2026 syllabus and official NTA documents; concept structure follows current NCERT contents. No performance, weightage or question-frequency data is asserted.
