User:Neeti m/B.Sc.(H)Chemistry-part-II/House Exam-2007

B.Sc. (Hons.)/II-(NS) CHEMISTRY-Paper-IX (Physical Chemistry-2) House Examination-2007 Acharya Narendra Dev College

Time: 3Hours'

Maximum Marks :50

Attempt Six questions in all. Question No. 1 is compulsory. Use of calculator is allowed.

1. Answer any five of the following:

1. The resistance of a decinormal solution of a salt occupying a volume between two platinum electrodes 1.8 cm apart and 5.4 cm² in area was found to be 32 ohm. Calculate the equivalent conductance of the solution.
2. An aqueous solution of sodium carbonate is alkaline, ferrous sulphate is acidic and potassium chloride is neutral. Explain.
3. How far is it justified to identify the chemical potential with the escaping tendency of a substance?
4. Molecular weight of acetic acid in water from osmotic pressure measurement is less than its normal molecular weight. Explain.
5. The conductance of a solution increases if high frequency AC is used while it increases if the electric field strength is increased from 25 volts cm^-1 to 10⁵ volts cm^-1. Explain.
6. The physical basis of lowering of chemical potential of a solute is not the energy of interaction of solute and solvent. Comment.
7. How can it be proved that the system showing only a halt in the cooling curve is a eutectic mixture and not a pure compound?
8. An equal number of polymer molecules, each of molecular weights 10,000 g/mol and 20,000 g/mol are mixed. Calculate the number average and weight average molecular weight of the polymer.

(10 marks)

2 (a)A solution containing 10.09g of CuSO₄ in 189.9 g of water was electrolyzed. After electrolysis, 275.4 g of solution around the anode was found to contain 15.4 g of CuSO₄ and 3.096 g of silver was deposited in a coulometer placed in series. Calculate the Hittorff’s number of Cu²⁺ and SO₄^2- ions.

(b)From the following molar conductivities at infinite dilution:

Λ_m⁰ for Ba(OH)₂ = 457.6 ohm^-1cm²mol^-1.

Λ_m⁰ for BaCl₂ = 240.6 ohm^-1cm²mol^-1.

Λ_m⁰ for NH₄Cl = 129.8 ohm^-1cm²mol^-1.

Calculate Λ_m⁰for NH₄OH.

(c) Describe the conductometric titration curve for the titration of acetic acid with sodium hydroxide.

(4,2,2)

3 (a) A monobasic acid HA in a 0.1 molal aqueous solution gave a depression in freezing point as 0.3˚ C. Calculate the degree of dissociation (α ) of the acid. Given K_f =1.86 K Kg mol^–1.

(b) The partial molar entropy of a gas i in the mixture is given by

S_i,m­ = S_i⁰ –Rlnx_i . Derive this relation. Write the expression for the entropy change when n₁ moles of nitrogen and n₂ moles of oxygen are isothermally mixed.

(c)Calculate ΔG_mix and ΔS_mix for a mixture containing 15% nitrogen, 55% hydrogen and 30% ammonia at STP. Assume that the gases behave ideally.

(2,3,3)

4 (a)The bicarbonate ion (HCO₃) can act as a base on hydrolyzing in an aqueous solution. Given only Ka(H₂CO₃) and Kw. What is the K_h for HCO₃.

(b)Dichloroacetic acid has an acid dissociation constant K_a = 3.32 x 10^-2. Calculate the degree of dissociation and pH of a 0.01 M solution.

(c) The pH of a solution is 8.8. Calculate its hydrogen and hydroxyl ion concentration.

(d)Lemon juice normally has a pH = 2.1. If all the acid in the lemon juice is citric acid undergoing dissociation as Hcit → H⁺ + cit ^– with K_a = 8.6x 10^-4 mole L^-1. What is the citric acid concentration?

(2,2,2,2)

5 (a)A membrane permeable only to water separates a 0.01M solution of sucrose from a 0.005M one. On what system and how much pressure must be applied to bring the system to equilibrium?

(b) What are macromolecules? Differentiate between additional and condensation polymers.

(c)A solution of Ni(NO₃)₂ is electrolyzed between Pt electrodes using a current of 5.0 amperes for 30 mins. What weight of Ni will be produced at the cathode?

(3,3,2)

6 (a)Suppose a one-component system exhibits a gas phase, a liquid phase and three solid modifications. How many different phase equilibria, namely, one, two, three, four and five are present in the system?

(b)Derive the Phase rule equation and show that the equation remains unchanged even if two components are missing in two phases.

(c)On the basis of phase rule equation, state the degree of freedom in the following systems. Give reason briefly.

1. a pure substance at its critical point
2. solid CaCO₃ heated in a sealed tube.
3. A binary eutectic mixture.

(2,3,3)

7 (a)Prove thermodynamically that in a chemical reaction an increase in temperature shifts the equilibrium to the high enthalpy side.

(b)Phosphorous pentachloride dissociates according to the equation

PCl₅(g) → PCl₃ (g) + Cl₂(g)

1. Show that K_P = α² (P/P^O)/(1– α²) where P is the total pressure and α is the degree of dissociation.
2. At 250˚C the equilibrium constant is 1.78. Calculate α for total pressure of 0.01 atm.

(c)Write the equilibrium constant for the following reactions:

1. 2HI(g) [math]\rightarrow[/math] H₂(g) + I₂(g)
2. 4HCl(g) + O₂(g) [math]\rightarrow[/math] 2Cl₂(g) + 2H₂O(g)

(3,4,1)

8 (a)How is the phase diagram of water helpful to explain flow of glaciers?

(b) Nitrosyl chloride(NOCl) when heated decomposes as: 2NOCl→2NO+Cl₂. The reaction is endothermic with ΔHº = 77.07 kJmol^-1. The standard entropies for NOCl, NO and Cl₂ are 261.6, 210.7 and 223.0 JK^-1mol^-1 respectively. Calculate the equilibrium constant for the reaction at 650K. Mention any assumption in the calculation. (c)Starting from the relation:

[math]\frac{\partial P}{\partial T}[/math] =[math]\frac{\partial S}{\partial V}[/math]

derive the integrated form of Clausius–Clayperon equation for a liquid-vapour equilibrium. Mention any assumptions made.

(1,4,3)

9 Write short notes on the following:

1. Kohlrausch’s law
2. Moving Boundary method for determination of Transport Number
3. Raoult’s Law
4. Eutectic system

(2,2,2,2)