# Mass Transfer Questions and Answers – Concentration and Velocity

This set of Mass Transfer Multiple Choice Questions & Answers (MCQs) focuses 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

Explanation: 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

Explanation: 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

Explanation: 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.

Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!

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

Explanation: 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

Explanation: Mass average velocity= (ρ1u12u2)/ 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

Explanation: 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

Explanation: 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)

Explanation: 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)

Explanation: (y1*M1*u1+y2*M2*u2)/M.

10. For the calculation of Mass average velocity, which velocity of the molecule is used?
a) Instantaneous Velocity
b) Mean Velocity
c) Instantaneous Velocity as well as Mean Velocity
d) Bulk velocity of the mixture

Explanation: None.

11. Which among the following can be the unit of Flux?
a) Kg*m-2*s-1
b) Kg*m2*s-1
c) Kg*m2*s1
d) Kg*m*s2

Explanation: Dimension analysis.
Flux= rate at which a species passes through a unit area, in unit time.

12. A molecule in a mixture moves with a velocity of 0.03m/s in (+x) direction. The concentration of the particle is 0.5 mol/liter. A observer is running in (-x) direction with a velocity of 2m/s. calculate the value of molar flux (mol/m2.s) relative to the observer.
a) 0.015
b) 15
c) 1.015
d) 1015

Explanation: Molar flux with respect to observer= C(u-U)
(C=500mol/m3, u=0.03m/s, U= (-) 2m/s).

13. In the previous question what will be the value of flux (mol/m2.s) with respect to a stationary observer?
a) 0.015
b) 15
c) 1.015
d) 1015

Explanation: Here U=0.

14. Under which of the following conditions mass average velocity and molar average velocity are equal?
a) In a very dilute solution
b) If molecular weights of all the species are equal
c) Always different
d) In dilute solutions as well as in solutions in which all components have same molecular weights

Explanation: None.

15. The mass flow rate of a component in a mixture in X-direction is 2kg/s. The area vector at a particular cross-section is (1.1, 3.2, 5.4) cm2. Calculate the value of flux (kg/cm2.s) through the cross-section.
a) 0.31
b) 2.2
c) 1.82
d) 6.45

Explanation: For flux calculation area normal to the flow direction should be taken into account.

Sanfoundry Global Education & Learning Series – Mass Transfer.

To practice all areas of Mass Transfer, here is complete set of 1000+ Multiple Choice Questions and Answers.