Sheet 3 / 4

DILUTE SOLUTIONS

(Abnormal Molar Masses, Van't Hoff Factor)

1. What is the value of Van't Hoff factor for a completely dissociated dilute K2SO4 Solution?

2. Which of the following has higher boiling point & Why? 0.1M NaCl or 0.1M Glucose.

On Dissociation, n↑ Mw↓ C.P ↑ (like ∆Tb, ∆Tf, π, RLOVP)

Also, ∆Tb = T - To hence if ∆Tb ↑ then T↑ as (T = ∆Tb + To)

But, ∆Tf = To - T hence if ∆Tf ↑ then T↓ as (T = To - ∆Tf)

On Association: Vice-versa like in CH3COOH

* Also compare NaCl with MgCl2

3. Assuming complete dissociation, calculate the expected freezing point of solution prepared by

dissolving 6.0 g of Glauber’s salt (Na2SO4.10H2O) in 0.1 kg of water. (Given value of kf =1.86

K kg 1mol ).

α = 1, n = 3 .: i = 3 (See Q1 of this sheet)

4. The freezing point of a solution containing 0.3 of acetic acid in 30 g benzene is lowered by

0.45oC. Calculate Van’t Hoff factor .(Kf for benzene = 5.12 K kg 1mol )

(Here α not = 1 Beware)

5. An aqueous solution containing 1.248 g of barium chloride (molar mass = 208.34 g 1mol ) in

100 g of water boils at . Calculate degree of dissociation of barium chloride (Kb for

water = 0.52 K kg 1mol ).

6. 75.2 g of phenol (C6H5OH) is dissolved in solvent of Kf =14 K kg 1mol . If depression in

freezing point is 7 K , find the percentage of phenol that dimerises.

100.0832

7. 17% K2SO4 solution at 27 C is isotonic with 4% NaOH Solution at the same temperature. If

NaOH is 100% ionised, what is the degree of ionisation of K2SO4 in aqueous solution?

8. 2.0g of benzoic acid dissolved in 25.0 g of benzene shows a depression in freezing point equal to

1.62K. Molal depression constant (Kf ) for benzene is 4.9 K kg 1mol .What is the percentage

association of the acid ?

9. Benzoic acid completely dimerizes in benzene .What will be the vapour pressure of a solution

containing 61 g of benzoic acid per 500 g benzene when the vapor pressure of pure benzene at

this temperature of experiment is 66.6 torr ? What would be vapor pressure in the absence of

dimerisation?

10. What mass of sodium chloride (Molar mass = 58.5 g 1mol ) must be dissolved in 65 g of water

to lower the freezing point by 7.5 C ? The freezing point depression constant Kf for Water is

1. 86 K kg 1mol . Assume Van’t Hoff factor for NaCl is 1.87.

11. The degree of dissociation of Ca(NO3)2 in dilute solution aqueous solution containing 7.0 g of

the solute per 100 C is 70 percent .If the vapor pressure of water at 100 C is 760 mm. Calculate

the vapour pressure of the solution.

12. 19.5g of CH3FCOOH is dissolved in 500g of water. The depression in freezing point of water

observed is 1oC. Calculate Van't Hoff factor and Dissociation constant of this acid. Kf = 1.86

K kg mol-1

.

13. Determine the osmotic pressure of a solution prepared by dissolving 25mg of K2SO4 in 2L of

water at 25oC, assuming that it is completely dissociated.

Note: For Q14 Also see the hand-made corrections for ease.

14. 0.6 ml of Acetic Acid having density 1.06gml-1 is dissolved in 1L of water. The depression in

freezing point observed was 0. 0205oC. Calculate the Van't Hoff Factor & dissociation constant

of acid.

15. Determine the amount of CaCl2 (i = 2. 47) dissolved in 2. 5L of water such that its osmotic

pressure is 0. 75atm at 27oC.