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Note: Answer Copy for midterm exam
Q1) Determine the pressure difference on a droplet of mercury with a surface
tension of 480 dyn/cm if its radius is 1.0mm.
∆p = 2γ / r
r= 10-3 m
∆p = 2 × 480 / 10-3
= 9.6 105 dyn/cm2
Q2) Calculate the heat of vaporization of diethylether from the following vapor
pressures: 400 mmHg at 18oC and 760 mmHg at 35oC. (if R=8.31J/K mol)
T1= 18+283=291K
T2=35+273= 308K
Log P2 – log P1 = (-∆Hvap/2.303 R) (1/T1 -1/T2)
Log 760 – log 400 = (-∆Hvap /2.303 × 8.31) (1/291 – 1/308)
2.88-2.6= (∆Hvap) 3.51×10-4
∆Hvap= 2.82 × 104 J/mol
Q3) A liter of water at 25 oC dissolves 0.0404g O2 when the partial pressure of
oxygen is 1.0 atm. What is the solubility of oxygen from air, in which the partial
pressure of O2 is 159 mmHg.
S= K P
(0.0404 g/liter) = K (1 atm)
K= 0.0405 g /liter atm
S= K P
S= 0.0404 (g/liter atm) × (159/760) (atm)
1
S= 8.45 ×10-3 gO2/Liter
Q4) What is the amount heat required to convert 105g of ice at -15 oC to liquid
water at 0 oC. (Cice= 2.03 J/g oC and ∆Hfusion= 6010 J/mol)(20 Marks)
Q = qice + qfusion
qice = m Cice ∆T
= 105 × 2.03 × 15 = 3.2 × 104 J
qfusion= n ∆Hfusion
= (105/18) × 6010 = 3.5 × 105 J
Q= 3.2 × 104 + 3.5 × 105 = 3.82 × 105 J
Q5) Define the following terms:
1- Avogadro’s Law: It states that equal volumes of gases, measured at the same temperature
and pressure, contain equal numbers of molecules.
2- Newtonian Fluids: Fluids for which the shearing stress is linearly related to the rate of
shearing strain.
3- Viscosity index: It is an arbitrary measure for the change of viscosity with temperature. It
is used to characterize lubricating oil in the automotive industry.
4- Adhesion forces: It is the forces at the interface, between the molecules belonging to
different phases.
5- Critical point: It is the point where after that we cannot convert gas to liquid whatever the
pressure can be.
Q6) Describe five main differences between the physical adsorption and chemical
adsorption.
Comparison between physisorption and chemisorption
S. No. Physical adsorption
Chemical adsorption
1
Caused by intermolecular van der
Caused by chemical bond formation
Waals' forces
2
It is not specific
It is highly specific
3
It is reversible
It is irreversible
4
Heat of adsorption is low (20-40 kJ/mol) High heat of adsorption (80-240 kJ/mol)
5
6
Low temperature is favourable
Results multilayer adsorption
Increases with high temperature
Results unimolecular layer
2