# Class 12 Chemistry MCQ – Abnormal Molar Masses

This set of Class 12 Chemistry Chapter 2 Multiple Choice Questions & Answers (MCQs) focuses on “Abnormal Molar Masses”.

1. Only if the calculated molar mass is higher than the actual molar mass of the solute, the calculated molar mass is considered to be abnormal molar mass.
a) True
b) False

Explanation: When we correlate molar mass with osmotic pressure as a colligative property, we sometimes encounter a situation where the calculated molar mass of the solute is either higher or lower than the actual molar mass. This is called the abnormal molar mass. Abnormal molar mass occurs as a result of dissociation or association of molecules.

2. Which of the following statements is correct?
a) Solutes that dissociate in water have molar mass higher than the molar mass of the solute calculated theoretically
b) Solutes that associate in water have molar mass higher than the molar mass of the solute calculated theoretically
c) Solutes that dissociate in water experience a decrease in colligative properties
d) Colligative properties are independent of the number of particles of the solute in the solution

Explanation: On association of solute molecules, the number of particles in a solution is reduced. Since, colligative properties are directly proportional to the number of particles in a solution, the colligative properties reduce. The molecular mass of the solute is inversely proportional to its colligative properties. Therefore, solutes that associate in water have a higher molar mass than its value calculated theoretically.

3. What is the value of the Van’t Hoff factor (i) for solutes that dissociate in water?
a) > 1
b) < 1
c) = 0
d) Not defined

Explanation: Van’t Hoff’s factor (i) is defined as the ratio of the observed colligative property to the calculated colligative property. Since, the abnormal mass for solutes that dissociate is lesser than its normal molar mass, the value of Van’t Hoff’s factor will always be > 1.

4. In which of the following solutions will the Van’t Hoff Factor for the solute be lesser than 1?
a) Sodium chloride in water
b) Benzoic acid in benzene
c) Acetic acid in benzene
d) Phenol in benzene

Explanation: The Van’t Hoff factor is less than 1 for solutes that dissociate in solution. In the given list, only sodium chloride dissociates in water whereas the remaining carboxylic acids associate in benzene. Hence, the value of the Van’t Hoff factor is lesser than 1 for a solution of sodium chloride in water.

5. The Van’t Hoff Factor for a solution of glucose in water is equal to 1.
a) True
b) False

Explanation: The Van’t Hoff Factor (i) is equal to the ratio between the normal molar mass of the solute and its abnormal mass. The glucose molecules neither associate nor dissociate. Hence, the abnormal mass is equal to the normal molar mass of the solute and Van’t Hoff factor is equal to 1.
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6. The depression of freezing point of a solution of acetic acid in benzene is – 0.2°C. If the molality of acetic acid is 0.1 m, then find the ratio of the normal mass to the abnormal mass. (Assume Kf of acetic acid = 4.0°C m-1)
a) 1.5
b) 0.8
c) 0.5
d) 0.2

Explanation: Given,
Depression in freezing point(ΔT) = -0.2 °C
Molality of the solution(m) = 0.1 m
Freezing point depression constant (Kf) = 4.0 °C m-1
Let the Van’t Hoff factor = i
We know that, ΔT = i x Kf x m
0.2 = i x 4 x 0.1
i = 0.2/0.4 = 0.5
Therefore, the ratio between the normal mass and abnormal mass, which is equal to the Van’t Hoff factor is equal to 0.5.

7. What is the Van’t Hoff Factor for 1 mole of BaCl2, assuming 100% dissociation?
a) 0.33
b) 1
c) 3
d) 2

Explanation: Van’t Hoff Factor can also be written as the ratio between the total number of moles of particles after association / dissociation and the total number of moles of particles before association / dissociation. Since BaCl2 completely dissociates(into one Ba2+ and two Cl ions), the total number of moles after dissociation is equal to 3. Therefore, the Van’t Hoff Factor for BaCl2 is 3.

8. Which of the following aqueous solutions should have the least boiling point?
a) 1.0 M KOH
b) 1.0 M (NH4)2SO4
c) 1.0 M K2CO3
d) 1.0 M K2SO4

Explanation: KOH→K+ + OH
(NH4)2SO4→2NH4+ + SO42-
K2CO3→2K+ + CO32-
K2SO4→2K+ + SO42-
Concentration of particles in 1.0 M KOH solution is minimum (= 2 M). Hence, it will minimum elevation in boiling point.

9. The pH of a 2 M solution of a weak monobasic acid (HA) is 4. What is the value of the Van’t Hoff factor?
a) 0.00005
b) 1.005
c) 1.0005
d) 1.00005

Explanation: pH = 4 means [H+] = 10-4 M

 HA ⇌ H+ + A– Initial C mol L-1 0 0 After dissociation C – Cα Cα Cα

Total = C (1+α)
Thus, [H+] = Cα, i.e., 10-4 = 2 x α or α = 5 x 10-5 = 0.00005
i = 1 + α = 1 + 0.00005 = 1.00005.

10. Acetic acid associates as dimers in benzene. What is the Van’t Hoff factor (i) if the degree of association of acetic acid is 50%?
a) 0.25
b) 0.50
c) 0.75
d) 0.40

Explanation: Given,
Degree of association (α) = 50% = 0.5
Number of molecules associated (n) = 2
Let the Van’t Hoff factor = i
We know that for solutes that associate in solution, i = 1 + ((1/n) -1) x α
= 1 + ((1/2) – 1) x 0.5
= 1 – (0.5 x 0.5)
= 0.75
Therefore, the Van’t Hoff factor is equal to 0.75.

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