Abnormal Molar Mass

 

Here are detailed notes based on the concept of Abnormal Molar Mass, typically covered in Class 12 Chemistry – Solutions and Colligative Properties:

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Abnormal Molar Mass

1. What is Molar Mass?

Molar mass is the mass of one mole of a substance, usually expressed in g/mol.
For a compound, it's calculated by summing the atomic masses of all the atoms in its formula.

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2. What is Abnormal Molar Mass?

Sometimes, the molar mass calculated from colligative properties (like depression in freezing point, elevation in boiling point, etc.) does not match the expected theoretical value.
This difference is referred to as Abnormal Molar Mass.

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3. Cause of Abnormal Molar Mass

Abnormal molar mass usually arises due to:

a. Association

• Molecules combine (associate) to form larger units.
• Common in non-polar solvents.
• Observed molar mass is higher than expected.
• Example: Acetic acid in benzene forms dimers.

b. Dissociation

• Molecules split (dissociate) into ions.
• Common in polar solvents like water.
• Observed molar mass is lower than expected.
• Example: NaCl, KCl, AlCl₃ in water.

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4. Role of van’t Hoff Factor (i)

To account for association/dissociation, van’t Hoff factor (i) is introduced:

i = \frac{\text{Observed colligative property}}{\text{Calculated colligative property assuming no dissociation/association}}

Or,

i = \frac{\text{Normal molar mass}}{\text{Abnormal molar mass}}

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5. Values of van’t Hoff Factor (i)

• i > 1: Dissociation
• i < 1: Association
• i = 1: No dissociation or association (ideal case)

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6. Modified Colligative Property Formulas

All colligative property formulas are multiplied by i:

• Relative lowering of vapour pressure:

  \frac{p^0 - p}{p^0} = i \cdot \frac{n_2}{n_1}

• Elevation of boiling point:

  \Delta T_b = i \cdot K_b \cdot m

• Depression of freezing point:

  \Delta T_f = i \cdot K_f \cdot m

• Osmotic Pressure:

  \pi = i \cdot C \cdot R \cdot T

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7. Numerical Example

Suppose, expected molar mass = 60 g/mol
Observed molar mass = 120 g/mol

i = \frac{60}{120} = 0.5 \quad \text{(Association)}

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8. Significance

Understanding abnormal molar mass is essential in calculating the actual molecular behavior of substances in solution — whether they associate, dissociate, or remain unchanged.

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Would you like some practice questions or numericals based on this concept?