Question

ASSERTION-Vant Hoff factor for solute showing dissociation is always greater than for solute showing association.
REASON-Dissociation leads to increase in number of particles whereas, association leads to a decrease in number of particles.
(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) Assertion is incorrect but Reason is correct

Answer 1

Hint: The Van't Hoff factor i is a measure of the effect of a solute on colligative properties such as osmotic pressure, relative lowering in vapor pressure, boiling-point elevation and freezing-point depression.


Complete step by step solution:
For most non-electrolytes dissolved in water, the van 't Hoff factor is essentially 1. For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only, as occasionally ion pairing occurs in solution.
The Van't Hoff factor, symbol i expresses how many ions and particles are formed (on an average) in a solution from one formula unit of solute. When solute particles dissociate in solution, i is greater than 1 and when solute particles associate in solution, i is less than 1.
So, from this we can conclude that both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Therefore, option (a) is correct.


Note: Van't Hoff factor for association or dissociation is expressed in terms of: i= normal molecular mass / observed molecular mass. But in case of dissociation, the Van't Hoff’s factor is more than 1 because the observed molecular mass has a lesser value than the normal molecular mass. In case there is no dissociation the value of ‘i’ becomes equal to one. Since colligative properties are inversely proportional to molecular masses, the Van't Hoff’s factor may also be written as: i= observed value of colligative property ÷calculated value of colligative property.