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Chem 221: Physical Chemistry Ii Question Paper
Chem 221: Physical Chemistry Ii
Course:Bachelor Of Science & Bachelor Of Education Science
Institution: Chuka University question papers
Exam Year:2013
CHUKA
UNIVERSITY
UNIVERSITY EXAMINATIONS
SECOND YEAR EXAMINATIONS FOR THE AWARD OF DEGREE IN BACHELOR OF SCIENCE & BACHELOR OF EDUCATION SCIENCE
CHEM 221: PHYSICAL CHEMISTRY II
STREAMS: BSC& BED SCI Y2S2 TIME: 2 HOURS
DAY/DATE: FRIDAY 19/4/2013 8.30 AM – 10.30AM
INSTRUCTIONS:
Answer question ONE and any other TWO questions.
USEFUL DATA:
Kacetic acid = 1.8 ×?10?^(-5) mol l^(-1)
Kammonium hydroxide =1.8×?10?^(-5 ) mol l^(-1)
K formic acid =1.77×?10?^(-4) mol l^(-1)
Ksp for ?Ag?_2 ?cro?_4=9.0×?10?^(-12)
At STP v_1=22.4 litres,t=273k and p=1 am.
Density of mercury =13.6 g/?cm?^3
Acceleration due to gravity=9.81 ?ms?^(-2)
Gas constant (R) =0.0821 litre atm k^(-1 ) ?mol?^(-1)
=8.314 JK ?mol?^(-1)
QUESTION ONE (30 MARKS)
1. (a) (i) Calculate the solubility of silver chromate Ag2Cr04 in a 0.1 M solution of
AgNo3. [2 Marks]
(ii) Explain briefly how the following factors influence the degree of
dissociation of a substance.
(a) Nature of solvent. [2 Marks]
(b) Temperature. [1 Mark]
(iii) Explain the following observations:
(a) Presence of H2s is essential in AS2S3 sol, though H2s ionizes and
should be precipitated as AS2S3”. [2 Marks]
(b) “Both emulsifier and peptizing agents stabilize colloids but their
actions are different” [3 Marks]
(b) (i) 0.900 g of a solute was dissolved in 100 ml of benzene at 25oC when its
density is 0.879 g/ml. this solution boiled 0.250oC higher than the boiling point of benzene. Molar elevation constant for benzene is 2.52oC – Kg mol-1 calculate the molecular weight of the solute. [2 Marks]
(ii) When can there be freezing point elevation? [3 Marks]
(c) (i) State Henry’s law. [1/2 Marks]
(ii) At 20oC the solubility of nitrogen gas in water is 0.0150g/litre when the
partial pressure of N2 is 580 torr. Calculate solubility of N2 in H2o at 20oC when its partial pressure is 800 ton. [1 Mark]
(iii) Briefly discuss critical solution temperature. [4 Marks]
(d) Two components A (m.pt 0oC) and B (m.pt -25oC) form two compounds and Y melting congruently. X melts at 49.6oC and contains 40 mole per cent B while Y melts at -40oC and contain 60 mole percent B. There are three eutectic points at
-51.20C, 28 mole B; 63.5?. 48 mole % B and -49.4? , 66 mole % B. deduce the formulae of the compounds and draw a tentative phase diagram using the above given data. [5 Marks]
(e) (i) Calculate the RMS velocity of chlorine molecules at 12? and 78 cm
pressure. [3 Marks]
(ii) Calculate the pressure exerted by 1.00 mole of methane (CH4) in a 250 ml container at 300 k using van der Waals equation. What pressure will be predicted by ideal gas {a=2.253L^2 atm ?mol?^(-2) b-0.0428 L ?mol?^(-1),R=0.0821 atm ?mol?^(-1) K Y. [2 Marks]
QUESTION TWO (20 MARKS)
2. (a) (i) What is meant by the terms “Degree of Hydrolysis” and “Hydrolysis
constant”? [1 Mark]
(ii) Derive an expression for the relationship between PH, hydrolysis constant and dissociation constant of the base and acid in the case of the hydrolysis of a salt of a weak acid and a weak base. [8 Marks]
(iii) Calculate the hydrolysis constant, degree of hydrolysis and PH of
(i) 0.1M ammonium acetate. [2 Marks]
(ii) 0.1M ammonium formate [2 Marks]
(b) Comment on the following statements:
(i) “Deltas are formed at a place where rivers pour water into sea”
(ii) “Colloid solutions are thermodynamically unstable”
(iii) “Lyophobic colloids flocculate only when the charges on the colloidal
particles are completely neutralized by addition of electrolysis”
QUESTION THREE (20 MARKS)
3. (a) (i) “Lowering of vapour pressure of a liquid by a non volatile solute is due to
the attraction of solvent molecule through salvation” justify or criticize. [5 Marks]
(ii) Derive an expression relating the relative lowering of the vapour pressure
of dilute solution with the molecular mass of the solute dissolved.
[4 Marks]
(iii) 18.2 g of cambamide (urea), CO (NH2)2 was dissolved in 100g of water at
50?. The lowering of vapour pressure produced is 5 mm Hg. Calculate the molecular mass of urea. The vapour pressure of water at 50? is
92mmHg.{H=1, C=12.0, 0=16, N=14 } [1 Mark]
(b) The vapour pressure composition curve for a water ethanol mixture deviates
positively from Raoults law. The corresponding boiling point composition diagram has a minimum at 95.6% ethanol.
(I) Sketch clearly labeled diagrams for:
(i) Vapour pressure=-composition diagram. [1 Mark]
(ii) The corresponding boiling point composition diagram [1 Mark
(iii) The expected boiling point composition diagram if the deviation
had been negative (B.P, water = 100?, B.pt ethanol 78.5?. [1 Mark]
(II) Explain why some liquids have a negative deviation from Raoults law
while others have a positive deviation. [5 Marks]
(III) A liquid X, with a vapour pressure of 20 KNM-2 is mixed on equimolar
portion of liquid Y. The vapour pressure of the mixture is found to be 101.325 KNM-2. Determine the vapour pressure of pure Y. [2 Marks]
QUESTION FOUR (20 MARKS)
4. (a) (i) How many components are present in a system in which ammonium
chloride undergoes thermal decomposition? Explain your answer.
[3 Marks]
(ii) The system Pb (m.pt 327?) and Sb (m.p 631? ) exhibits a simple eutectic
at 86% Pb (by mass) and 246?. Breaks in the cooling curves in thermal analysis were found for the following compositions.
T (?) 550 500 400 300 296
Mass % Pb 30 44 66 80 96
(I) Sketch a phase diagram for the system and clearly label it.
(II) Calculate the amount of antimony that crystallizes out from 20 Kg
of a melt on attaining 35% Pb by mass after cooling it to a temperature of 400?.
(III) How much is the maximum amount of Sb that can be recovered
from this melt?
(b) (i) What is meant by critical phenomenon?
(ii) Derive the expression for the critical constants of a gas using Van der
Wall’s equation of state.
(iii) How do you determine the Van der Waal’s constant from the critical
valves of pressure, temperature and volume?
(iv) Vander Waal’s constants for hydrogen chloride gas are a=3.67 atm lit2 and
b=40.8ml mol-1. Find the critical temperature and critical pressure of the gas. [2 Marks]
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