Van't Hoff Factor and Abnormal Molar Mass MCQ - Practice Questions & Answers

Quick Facts

van't Hoff factor(i) or Abnormal Colligative Property is considered one the most difficult concept.
Calculation of Extent of Dissociation in an Electrolytic Solution is considered one of the most asked concept.
41 Questions around this concept.

Concepts Covered - 3

van't Hoff factor(i) or Abnormal Colligative Property

If a solute gets associated or dissociated in a solution, the actual number of particles are different from expected or theoretical consideration.

We know, that:

$\mathrm{Colligative\: property \propto number \: of \: particles}$

Thus, we can say that:

$\\\mathrm{i\: =\: \frac{Observed\: number\: of\: solute\: particles}{Number\: \: of\: particles\: initially\: taken}}\\\\\mathrm{i\: =\: \frac{Observed\: value\: of\: colligative\: property}{Theoretical\: value\: of\: colligative\: property}}\\$

Again, we have:

$\mathrm{Colligative\: property \propto\: \frac{1}{molecular\: mass\: of\: solute}}$

Thus;

$\mathrm{i\: =\: \frac{Theoretical\: molecular\: mass\: of \: solute}{Observed\: molecular\: mass\: of \: solute}}$

Calculation of Extent of Dissociation in an Electrolytic Solution

van't Hoff Factor for dissociation of solute

Suppose we have the solute A which dissociates into n moles of A. Then the dissociation occurs as follows:

$\mathrm{A_{n}\rightarrow nA}$

At time t = 0 1 0

At time t = t 1 - $\alpha$ n $\alpha$

At time t = 0, initial number of solute particles = 1

And, at time t = t, observed number of solute particles = 1 - $\alpha$ + n $\alpha$

= 1 + (n-1) $\alpha$

Thus, we know that:

$\mathrm{i\: =\: \frac{observed\: number\: of\: solute\: particles}{initial\: number\: of\: solute\: particles}}$

$\mathrm{i\: =\: \frac{1+(n-1)\alpha }{1}}$

where n = number of solute particles

$\alpha$ = Degree of dissociation

For strong electrolytes, the degree of dissociation is taken to be unity.

Using the above equation, the van’t Hoff factor and the degree of dissociation can be related which can be further related to the theoretical and observed colligative properties.

Calculation of Extent of Association in an Electrolytic Solution

Suppose we have a solute A and it associates into (A)_n. Then the association occurs as follows:

$nA\rightarrow \left ( A \right )_{n}$

At time t = 0 1 0

At time t = t 1 - $\beta$ $\beta$ /n

Now, the initial number of solute particles = 1

And, the observed number of solute particles = $1-\beta+\frac{\beta }{n}$

$=1+\beta\left [ \frac{1}{n}-1 \right ]$

Thus, van't Hoff factor is given as:

$i=1+\beta\left [ \frac{1}{n}-1 \right ]$

where, $\beta$ is the degree of association

Using the above equation, the van’t Hoff factor and the degree of association can be related which can be further related to the theoretical and observed colligative properties.