This set of Mass Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Fick’s Law-2”.

1. For what kind of mixtures D_{AB}=D_{BA} holds?

a) Ideal

b) Real

c) For both real and ideal

d) This relation is never true.

View Answer

Explanation: None.

2. In the expression for molar flux N_{A}=(N_{A}+N_{B})C_{A}/C – D_{AB}dC_{A}/dz, the terms representing bulk flow and molecular diffusion are, respectively

a) (N_{A}+N_{B})C_{A}/C, D_{AB}dC_{A}/dz

b) (N_{A}+N_{B})C_{A}/C, N_{A}dC_{A}/B

c) (N_{A}+N_{B})C_{A}/C, (N_{A}+N_{B})C_{A}/C

d) (D_{AB}dC_{A}/dz, D_{AB}dC_{A}/dz)

View Answer

Explanation: None.

3. Consider loss of ethanol vapor by diffusion from a half-filled open test tube. At what point in the diffusion path will the contribution of the bulk flow term to the molar flux be maximum?

a) At the liquid-gas interface

b) In the bulk liquid

c) In the bulk gas

d) None of the mentioned

View Answer

Explanation: At the liquid-gas interface concentration will be maximum.

Bulk liquid is not a part of the diffusion path.

4. A reaction 3A+2B+C=1.5D+E occurs in a reactor at steady state. What will be the value of the flux ratio N_{A}/N_{D}

a)-2

b)-0.5

c) 2

d)-0.5

View Answer

Explanation: N

_{A}/N

_{D}= (rate of disappearance of A)/(rate of appearance of D)

= -3/1.5

= -2.

5. For a component A of a mixture, concentration as a function of x is given:

C_{A}=5e^{-10x} (x is in cm and C_{A} in mol/liter)

Calculate the value of diffusion velocity (m/s)of the component A at the point x=0, if diffusivity of A in the mixture is 2.567*10^{-5}m^{2}/s.

a) 0.2567

b) 2.567

c) 0.0025

d) 3.541

View Answer

Explanation: By Fick’s law, J

_{A}=(-)D

_{AB}*dC/dx

C

_{A}(u

_{A}-U)= (-)D

_{AB}dC/dx…………1

Diffusion velocity= (u

_{A}-U)=(-D

_{AB}dC/dx)/C

_{A}

dC/dx = -50e

^{-10x}

C

_{A}= 5 mol/lit

Put all the values in equation 1.

6. A sheet of Fe 1.0 mm thick is exposed to an oxidizing gas on one side and a deoxidizing gas on the other at 725°C. After reaching steady state, the Fe membrane is exposed to room temperature, and the C concentrations at each side of the membrane are 0.012 and 0.075 wt%. Calculate the diffusion coefficient (m^{2}/sec) if the diffusion flux is 1.4×10^{-8}kg/m^{2}-sec.

a) 9.87*10^{-12}

b) 9.87*10^{-13}

c) 9.87*10^{-11}

d) 9.87*10^{-10}

View Answer

Explanation: Convert from wt% to kg/m

^{3}.

Concentration in kg/m

^{3}=[C

_{1}/(C

_{1}/ρ

_{1}+C2/ρ

_{2})]*1000

0.012% and 0.075% in kg/m

^{3}are 0.270 and 1.688 respectively.

Putting all the values in the equation J

_{A}=(-)D

_{AB}dC/dx.

7. At which condition molar flux with respect to a stationary observer and with respect to an observer moving with molar average velocity?

a) In a very dilute solution

b) In a highly concentrated solution

c) At moderate concentration

d) Never

View Answer

Explanation: In a very dilute solution, the contribution of the bulk flow term becomes negligible.

N

_{A}= (N

_{A}+N

_{B})C

_{A}/C – D

_{AB}dC

_{A}/dz (general equation)

(N

_{A}+N

_{B})C

_{A}/C =0 (in dilute solution)

N

_{A}= -D

_{AB}dC

_{A}/dz = J

_{A}.

8. Which among the following is the statement of the ‘Fick’s Law’?

a) The molar flux of species relative to an observer moving with the molar average velocity is proportional to the concentration gradient of the species.

b) The mass flux of species relative to an observer moving with the molar average velocity is proportional to the concentration gradient of the species.

c) The molar flux of species relative to an observer moving with the mass average velocity is proportional to the concentration gradient of the species.

d) The molar flux of species relative to a stationary observer is proportional to the concentration gradient of the species.

View Answer

Explanation: None.

9. The gas A diffuses through non-diffusing B from point 1 to point 2. The total pressure is 2atm and y_{A1}=0.1 and y_{A2}=0.Then the ratio (dP_{A}/dz)1/(dP_{A}/dz)2 is

a) 10

b) 1.11

c) 0.9

d) 2.3

View Answer

Explanation: None.

10. In industries titanium is hardened through diffusion of carbon. The concentration of carbon at 1mm into the surface of the titanium slab is 0.25kg/m^{3} and at 3mm the concentration is 0.68kg/m^{3}. The rate at which the carbon is entering into its surface is 1.27*10-9kg/m^{2}.s. calculate the value of diffusion coefficient of carbon.

a) 5.91*10^{-12}

b) 5.91*10^{-10}

c) 5.91*10^{-11}

d) 5.91*10^{-13}

View Answer

Explanation: J=-D(∂C/∂x)

∂C/∂x= Δc/Δx=(0.25-0.68)/(0.003-0.001)=-215kg/m

^{4}

1.27*10

^{-19}kg/m

^{2}.s= -D*(-215kg/m

^{4})

D=5.91*10

^{-12}m

^{2}/s.

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