Mass Transfer Multiple Choice Questions on “Concentration and Velocity”.
1. In which of the following conditions mass transfer will occur spontaneously? C and z is concentration and distance respectively.
a) dC/dz>0
b) dC/dz<0
c) dC/dz=0
d) None of the Mentioned
Answer: b
Clarification: Mass transfer occurs in the direction of decreasing concentration.
2. Which among the following is always true for mass transfer to occur?
a) Difference in concentration
b) Difference in Pressure
c) Difference in temperature
d) Difference in chemical potential
Answer: d
Clarification: Mass transfer occurs to attain an equilibrium position or to minimize the energy of the system. Mass occur can occur even if there is no difference in concentration, pressure and temperature. Example: – A ball rolls down a slope to minimize its potential energy.
3. A solution contains 0.3 moles of solute A, 0.2 moles of B and 0.5 moles of C. What will be the mole fraction of A in the mixture?
a) 0.3
b) 0.2
c) 0.5
d) 1
Answer: a
Clarification: Mole Fraction of A= Moles of A/Total moles
Paragraph for question Numbers (4-6)
A gas mixture containing 21% O2 and 79% N2 flows through a pipe of circular cross-section of diameter 2cm at atmospheric pressure. The velocity of O2 is 0.04m/s and N2 is 0.035m/s.
4. What is the value of molar average velocity (m/s) of the mixture?
a) 0.036
b) 0.035
c) 0.034
d) 0.033
Answer: a
Clarification: Molar average velocity= (C1u1+C2u2+……….)/C
=y1u1+y2u2…..
= 0.21*0.04+0.79*0.035
=0.036.
5. What will be the value of mass average velocity (m/s)?
a) 0.981
b) 0.891
c) 0.985
d) 0.857
Answer: a
Clarification: Mass average velocity= (ρ1u1+ρ2u2)/ ρ
ρ1= (p1*M1)/RT
ρ= (p*M)/RT.
Dividing these two equations
ρ1/ ρ = (p1*M1)/pM =(y1*M1)/M
M=y1*M1+y2*M2=0.79*28+0.21*32=28.84
Mass average velocity= (y1*M1*u1+y2*M2*u2)/M
= (0.21*32*0.04+0.79*0.035)/28.84
= 0.981.
6. What is the value of Volume average velocity?
a) 0.036
b) 0.035
c) 0.034
d) 0.033
Answer: a
Clarification: Volume average velocity= Molar average velocity.
7. In a gas mixture if molecular weights of all the species are equal than which one will be more, Mass average velocity or Molar average velocity?
a) Mass average velocity
b) Molar average velocity
c) Both will have the same value
d) Nothing definite can be said
Answer: c
Clarification: Average molar mass of the mixture=M(x1+x2+………+ xn)=M
(x1+x2+………+ xn=1)
Mass average velocity=Σuiρi/ρ=uiPui/P
Molar average velocity=Σciui/c=ρuiPi/P (P=cRT).
8. The velocity of various components of mixture(21% A, 78% B and 1% C) are:
Velocity is in the form: v=xi+yj+zk
Velocity of component A= (0.1, 0, 0)
Velocity of component B= (0, 0.1, 0)
Velocity of component B= (0, 0, 0.1)
Calculate the value of average molar velocity?
a) (0.021, 0.078, 0.01)
b) (0.1, 0.1, 0.1)
c) (0, 0 ,0)
d) (0.079, 0.021, 0.01)
Answer: a
Clarification: Molar average velocity = (c1u1+c2u2+c3u3)/C.
9. Calculate the value of mass average velocity in the previous question?
a) (0.0235, 0.0764, 0.00007)
b) (0.021, 0.078, 0.01)
c) (0.1, 0.1 0.1)
d) (0.079, 0.021, 0.01)