sheet 3 / 4 dilute solutions - welcome to csquare …. what mass of sodium chloride (molar mass =...
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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)
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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
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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?
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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.
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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.
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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.