Question

Assertion: Molecular mass of $KCl$ calculated on the basis of colligative properties will be lower than the normal molecular mass.
Reason: Experimentally determined molar mass is always lower than the true value.
Which of the given statements is true?
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

Hint: As we know that colligative properties like freezing point and boiling point data can be used to calculate the molecular mass of an unknown solute. If we dissolve a known mass of any unknown substance into a known amount of solvent then we can calculate the molecular mass of that substance.

Complete step by step answer:
As we know that colligative properties like osmotic pressure, freezing point and boiling point data can be used to calculate the molecular mass of an unknown solute as they are related to molecular mass by the formula of $\Delta {T_f} = {K_f} \times m$ where m is the molality of the substance which can be defined as the ratio of moles of solute to the mass of solute and moles as we know are given as the ratio of given mass to the molecular mass.

-So, if we dissolve a known mass of any unknown substance into a known amount of solvent then we can calculate the molecular mass of that substance. But this calculated molecular mass will be lower than the normal molecular mass because if a solute associates, the number of particles in the solution will be less than the actual number dissolved.
-Therefore we can say Molecular mass of $KCl$ calculated on the basis of colligative properties will be lower than the normal molecular mass is correct.
-But, the experimentally determined molar mass is always lower than the true value is incorrect because it depends on the behaviour of solute in aqueous solution. It may undergo association, dissociation or may remain the same in aqueous solution.

Therefore the correct answer is (C).

Note: Colligative properties can be defined as the number of solute particles divided by the number of solvent particles present in a solution. So, molarity, molality and normality of a solute can be calculated easily. But only those properties are considered which results from the dissolution of non-volatile solutes in a volatile solvent.