Explore topic-wise InterviewSolutions in .

(b) Ethyl bromide and ethyl iodide 

a, c and d are non-ideal solutions whereas b is an ideal solution.

1. The egg which is placed in sodium chloride solution will shrink due to osmosis of water out of the egg and thus get hardened.

2. Osmotic pressure method.

Certain compounds when dissolved in suitable solvents either dissociate or associate.  

For example ethanoic acid dimerises in benzene due to hydrogen bonding,  while in water, it dissociates and forms ions. As a result the number of chemical species in solution increases or decreases as compared to the number of chemical species of solute added to form the solution. Since the magnitude of colligative property depends on the number of solute particles, it is expected that the molar mass determined on the basis of colligative properties will be either higher or lower than the expected value or the normal value and is called abnormal molar mass.  

In order to account for the extent of dissociation or association of molecules in solution, Van’t Hoff introduced a factor, i, known as the Van’t Hoff factor. It can be defined as follows.

i = Expected molar mass/Abnormalmolar mass

= Observed colligative property/Calculated colligative property

= Total number of moles of particles after association/dissociation/Number of moles of particles before association/dissociation

1. Reverse osmosis 

2. In fresh water container from salt water container. 

3. Cellulose acetate is semipermeable membrane (SPM) 

4. Purification of water

(a) 2B, C, 2D, E/3, F, G/5.

(b) 2B and 2 D are due to association

i = \(\frac{Normal \,molar \,mass}{Abnormal \,molar \,mass}\)

OR

i = \(\frac{Observed\,Colligative\, Property}{Calculated\,Colligative\, property}\)

Consider 1 dm³ of a solution containing m moles of an electrolyte AxBy. The electrolyte on dissociation gives number of A^y+ ions and y number of B^x- ions. Let α be the degree of dissociation.

At equilibrium,

AxBy \(\rightleftharpoons\) xA^y+ + yB^x-

For 1 mole of electrolyte : 1 – α, xα, yα and For ‘m’ moles of an electrolyte : m(1 – α), mxα, myα are the number of particles.

Total number of moles at equilibrium, will be, Total moles = m(1 – α) + mxα + myα

= m[(1 – α) + xα + yα] 

= m[1 – xα + yα – α] 

= m[1 + α(x + y – 1)]

The van’t Hoff factor i will be,

\(i = \frac{\text{Observed colligative property}}{\text{Theoretical colligative property}}\)

\(=\frac{m[1 + \alpha(x + y - 1)]}{m}\)

i = 1 + α (x + y - 1)

If total number of ions from one mole of electrolyte is denoted by n, then (x + y) = n

\(\therefore\) 1 + α (n - 1)

\(\therefore\) α (n - 1) = i - 1

\(\therefore\) α \(=\frac{i - 1}{n - 1}\,....(1)\)

This is a relation between van’t Hoff factor i and degree of dissociation of an electrolyte.

Correct answer is

(c) they are isotonic solutions

Given : m = 0.1 m, 

ΔT_f = 0 – (-0.43) = 0.43 °C 

K_f = 1.86 K kg mol^-1, i = ? 

ΔT_f = i × K_f × m

∴ i = \(\frac{ΔT_f}{K_f\times m}\) 

= \(\frac{0.43}{1.86\times 0.1}\) 

= 2.312

∴ van’t Hoff factor = i = 2.312

Definition of the van’t Hoff factor, i : It is defined as a ratio of the observed colligative property of the solution to the theoretically calculated colligative property of the solution without considering molecular change.

The van’t Hoff factor can be represented as,

\(i = \frac{\text{Observed value of colligative property}}{\text{Theoretical value of the colligative property}}\)

This colligative property may be the lowering of vapour pressure of a solution, the osmotic pressure, the elevation in the boiling point or the depression in the freezing point of the solution. Hence,

\(i = \frac{\text{Observed lowering of vapour pressure}}{\text{Theoretical lowering of vapour pressure}}\)

\(=\Delta\,P_\text{(ob)}/\Delta P_\text{(th)}\)

\(i = \frac{\text{Observed elevation in boiling point}}{\text{Theoretical elevation in boiling point}}\)

\(=\Delta\,T_\text{b(ob)}/\Delta T_\text{b(th)}\)

\(i = \frac{\text{Observed depression in freezing point}}{\text{Theoretical depression in freezing point}}\)

\(=\Delta\,T_\text{f(ob)}/\Delta T_\text{f(th)}\)

\(i = \frac{\text{Observed osmotic pressure}}{\text{Theoretical osmotic pressure}}\)

\(=\frac{\pi_\text{(ob)}}{\pi_\text{(th)}}\)

• When the solute neither undergoes dissociation or association in the solution, then, i = 1
• When the solute undergoes dissociation in the solution, then, i > 1
• When the solute undergoes association in the solution, then i < 1

From the value of the van’t Hoff factor, the degree of dissociation of electrolytes, degree of association of nonelectrolytes can be obtained.

van’t Hoff factor gives the important information about the solute molecules in the solution and chemical bonding in them.

Given : m = 0.1 m, ΔT_f = 0 – (-0.43) = 0.43°C

K_f = 1.86 K kg mol^-1, i = ?

ΔT_f = i × K_f × m

\(\therefore\) \(i = \frac{\Delta T_f}{K_f \times m} = \frac{0.43}{1.86 \times 0.1} = 2.312\)

van’t Hoff factor = i = 2.312.

1. While calculating osmotic pressure by equation, π = CRT, the concentration is expressed in molarity but not in molality.

2. This is because the measurements of osmotic pressure are made at a certain constant temperature. 

3. Molarity depends upon temperature but molality is independent of temperature. 

4. Hence in osmotic pressure measurements, concentration is expressed in molarity.

(ii), [Hint: If added substance dissolves, the solution is unsaturated. If it does not dissolve solution is saturated. If precipitation occurs solution is supersaturated.]

(b)  Ternary