C
CraftExam
medium difficulty9 min

Colligative Properties and Molar Mass Calculations

Colligative properties are solution properties that depend on the number of solute particles present, not on their chemical nature, for dilute solutions.

In one line: Colligative means count of particles controls the effect.

Practice This Concept

Understand it

The four main colligative properties are relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure. They are used to calculate molar mass of solutes and to compare particle numbers in solution. For electrolytes or associating solutes, the observed effect may differ because the number of particles changes.

Example

Adding salt to water increases the boiling point and lowers the freezing point because the number of solute particles affects phase-change conditions.

Rule to remember

Elevation of boiling point: Delta Tb = Kb m.

Depression of freezing point: Delta Tf = Kf m.

Osmotic pressure: π = C R T = nRT/V.

Relative lowering of vapour pressure: (p0 - p)/p0 = xsolute for ideal dilute solutions.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A solution has molality 0.20 mol kg−1 and Kf = 1.86 K kg mol−1.

  1. 1Delta Tf = Kf m = 1.86 × 0.20 = 0.372 K.
  2. 2Interpretation: the freezing point is lowered by 0.372 K from the pure solvent value.

Method you can reuse

  1. 1Identify which property is mentioned: boiling point, freezing point, vapour pressure, or osmosis.
  2. 2Choose the matching formula.
  3. 3Convert temperature to kelvin difference where needed, volume to litres or cubic metres consistently, and solvent mass to kilograms for molality.
  4. 4Substitute values, then state whether the result is property change, molar mass, or osmotic pressure.

Visual tip

A phase-change comparison sketch may help show elevation in boiling point and depression in freezing point, but numerical formula selection is the central skill.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Name the colligative property most suitable for determining molar mass of biomolecules at room temperature.

Think first, then show answer
Osmotic pressure is suitable because it can be measured at room temperature and gives appreciable values for dilute solutions.

Common wrong answer

Using molarity in Delta Tb = Kb m or Delta Tf = Kf m is a common error because these formulas require molality.

Don't mix this up

A common error is assuming colligative properties depend on mass of solute rather than number of solute particles. Equal masses of different solutes can produce different effects if their molar masses differ.

Exam tip

For numerical answers, always write the selected formula, convert units, substitute values, and interpret whether molar mass or property change is being calculated.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions usually ask calculation of Delta Tb, Delta Tf, osmotic pressure, molar mass, or comparison of colligative effect for different solutes. Long-answer questions may combine formula selection with abnormal molar mass.
  1. 1

    Enough for 1 mark

    Colligative properties are solution properties that depend on the number of solute particles present, not on their chemical nature, for dilute solutions.

  2. 2

    Add for 2 marks

    Example: Adding salt to water increases the boiling point and lowers the freezing point because the number of solute particles affects phase-change conditions.

  3. 3

    Add for 3 marks

    The four main colligative properties are relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.

MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results
Practice This Concept

Help improve this page

Found something confusing, incorrect, or missing?

Practice This Concept