Here are 1000 MCQs on Mass Transfer (Chapterwise).
1. What is mass transfer?
a) Transfer of momentum
b) Movement of energy across phases
c) Movement of mass due to a concentration difference
d) None of the mentioned
View Answer
Explanation: Mass transfer refers to the movement of mass from one location to another, driven by concentration gradients.
2. Which among the following is always true for mass transfer to occur?
a) Difference in concentration
b) Difference in chemical potential
c) Difference in pressure
d) Difference in temperature
View Answer
Explanation: Mass transfer occurs to attain an equilibrium position or to minimize the energy of the system. It can happen even without differences in concentration, pressure, or temperature, e.g., a ball rolls down a slope to minimize its potential energy.
3. The mass transfer by convection can take place under how many conditions?
a) 3
b) 4
c) 2
d) 1
View Answer
Explanation: Free or forced conditions.
4. Air at 30°C flows at 45 m/s past a wet flat plate of length 0.5 m. Estimate the value of the mass transfer coefficient, assuming the water vapor content of air is initially negligible. The thermophysical properties of air are: diffusion coefficient D = 0.256 × 10-4 m2/s, dynamic viscosity µ = 1.86 × 10-5 kg/m·s, specific heat cp = 1.005 kJ/kg·°C, Prandtl number Pr = 0.701, and density ρ = 1.165 kg/m3.
a) 0.3076 m/s
b) 0.1076 m/s
c) 0.4076 m/s
d) 0.2076 m/s
View Answer
Explanation: h m = 0.0296 (Re) -0.2 (V)/ (Sc) 0.667.
5. What is the empirical correlation for the local mass transfer coefficient for laminar boundary layer flow over a flat plate?
a) Sh X = 0.332 (Re) 0.5 (Sc) 0.33
b) Sh X = 0.332 (Re) 0.5 (Sc) 0.23
c) Sh X = 0.332 (Re) 0.5 (Sc) 0.43
d) Sh X = 0.332 (Re) 0.5 (Sc) 0.53
View Answer
Explanation: Sh = h ml/D = .332 (Re) 0.5 (Sc) 0.33.
6. What is the unit of diffusivity?
a) m/s
b) m3/s
c) m2/s
d) m/s2
View Answer
Explanation: Diffusivity is measured in m2/s, representing the rate of diffusion per unit area.
7. What causes the concentration at the interphase to be unequal, according to Lewis and Whitman theory?
a) Moderate mass transfer rate
b) Low mass transfer rates
c) High mass transfer rates
d) None of the mentioned
View Answer
Explanation: Theoretically proved by Lewis and Whitman that if the mass transfer rates are higher, the concentration deviates from equilibrium.
8. What will be poor if the holding time is longer in a mixer?
a) Momentum transfer
b) Mass transfer
c) Heat transfer
d) Both mass and momentum transfer
View Answer
Explanation: If the holding of liquids inside the mixer is poor, the rate of mass transfer decreases.
9. Find the gas phase mass transfer coefficient for unbound moisture removal if the flux is 5 kg/sq.m·s and the difference in humidity of the liquid and the main stream is 0.5 units.
a) 10
b) 30
c) 5
d) 20
View Answer
Explanation: Flux= Mass transfer co-efficient * (Ys-Y)
Mass transfer co-efficient= 5/0.5 = 10.
10. In humidification, where must the gas be for mass transfer to occur?
a) Soluble
b) Insoluble
c) Partially soluble
d) Inert
View Answer
Explanation: Generally, the purpose is to add the moisture to the gas. So, the gas won’t soluble in the liquid. Due to the heat transfer, the solute gets transferred between the phases.
11. In liquid-liquid mass transfer, what is the effect of increased viscosity on diffusivity?
a) No change in diffusivity
b) Diffusivity increases
c) Unpredictable
d) Diffusivity decreases
View Answer
Explanation: Diffusivity decreases as viscosity increases, as per the Stokes-Einstein equation.
12. What does the Schmidt number (Sc) signify in mass transfer?
a) Ratio of thermal diffusivity to momentum diffusivity
b) Ratio of momentum diffusivity to mass diffusivity
c) None of the mentioned
d) Ratio of thermal diffusivity to mass diffusivity
View Answer
Explanation: The Schmidt number is defined as the ratio of momentum diffusivity to mass diffusivity (Sc = ν/DAB).
13. According to two-film theory, what determines the overall mass transfer coefficient?
a) Resistance in the liquid phase only
b) None of the mentioned
c) Resistance in the gas phase only
d) Sum of resistances in both phases
View Answer
Explanation: The overall mass transfer coefficient is influenced by the resistances in both gas and liquid phases.
14. What is the physical significance of Henry’s law in mass transfer?
a) Diffusion of gases in solids
b) Solubility of a gas in a liquid
c) Vapor pressure of a liquid
d) Rate of chemical reaction
View Answer
Explanation: Henry’s law states that the amount of gas dissolved in a liquid is proportional to its partial pressure above the liquid.
15. Which term represents the enhancement of mass transfer in turbulent flow?
a) Eddy diffusivity
b) Laminar flow
c) Molecular diffusion
d) Surface tension
View Answer
Explanation: In turbulent flow, eddy diffusivity accounts for the increased mixing and mass transfer.
16. Which process utilizes gas absorption as a mass transfer operation?
a) Ammonia production
b) Filtration
c) Sedimentation
d) Water desalination
View Answer
Explanation: Gas absorption is used in ammonia production for scrubbing gases like CO2.
17. Which type of mass transfer involves the transfer of mass across a phase boundary?
a) Molecular diffusion
b) Interphase mass transfer
c) Diffusion
d) None of the mentioned
View Answer
Explanation: Interphase mass transfer occurs when mass is transferred across the boundary of two phases, such as gas to liquid or liquid to solid.
18. What is the significance of the concentration gradient in mass transfer?
a) It acts as the driving force
b) None of the mentioned
c) It does not affect the process
d) It slows down the process
View Answer
Explanation: The concentration gradient acts as the driving force, causing mass to move from higher to lower concentrations.
19. Which of the following is NOT a mass transfer operation?
a) Gas absorption
b) Heat conduction
c) Drying
d) Distillation
View Answer
Explanation: Heat conduction involves energy transfer, not mass transfer.
20. What is molecular diffusion?
a) Movement of molecules due to a concentration difference
b) Bulk movement of fluid
c) Energy transfer between molecules
d) Movement of molecules due to pressure
View Answer
Explanation: Molecular diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
21. Which of these is an example of mass transfer?
a) Expansion of gas
b) Heat conduction in a rod
c) Freezing of water
d) Mixing of two liquids
View Answer
Explanation: The mixing of two liquids involves mass transfer as molecules move to equalize concentrations.
22. What is the primary purpose of mass transfer in distillation?
a) To add heat to the system
b) To separate components
c) To increase temperature
d) To increase pressure
View Answer
Explanation: In distillation, mass transfer separates components based on differences in volatility.
23. Which operation uses mass transfer principles in wastewater treatment?
a) Filtration
b) Centrifugation
c) Aeration
d) Sedimentation
View Answer
Explanation: Aeration involves gas-liquid mass transfer to dissolve oxygen in wastewater.
24. 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 (m2/sec) if the diffusion flux is 1.4×10-8kg/m2-sec.
a) 9.87*10-13
b) 9.87*10-12
c) 9.87*10-10
d) 9.87*10-11
View Answer
Explanation: Convert from wt% to kg/m3 .
Concentration in kg/m3=[C1/(C1/ρ1+C2/ρ2)]*1000
0.012% and 0.075% in kg/m3 are 0.270 and 1.688 respectively.
Putting all the values in the equation JA=(-)DABdC/dx.
25. Which one is having the lowest value of diffusivity?
a) Glucose
b) Ethanol
c) Oxygen
d) Urea
View Answer
Explanation: Thermal diffusivity of glucose is 0.60 while that of urea, oxygen and ethanol are 8.06, 1.80 and 1.00 respectively. Its unit is cm2 /s.
26. The vapour pressure of the nitrogen gas is 400mmHg and the partial pressure is 300mmHg. Find the relative saturation.
a) 100
b) 75
c) 50
d) 133
View Answer
Explanation: Relative saturation or relative humidity = (partial pressure / vapour pressure) x 100 = 75.
27. Find the rate of non-diffusing solute, if the mole fraction of the gas phase is 0.65 and the diffusing rate is 70 moles/hr.
a) 24.5 moles/hr
b) 37.7 moles/hr
c) 200 moles/hr
d) 18.18 moles/hr
View Answer
Explanation:
=
(
1
−
)
=
70
×
(
1
−
0.65
)
=
24.5
G
s
=G(1−y)=70×(1−0.65)=24.5 moles/hr.
28. Which of the following is not an example of ideal solution?
a) Solution of benzene in toluene
b) Solution of ethyl and propyl alcohol
c) Paraffin hydrocarbon gas in paraffin oil
d) Solution of isobutane and olefins
View Answer
Explanation: Only adjacent homologous series fall in the ideal solution category. Here, isobutane is an alkane and olefins are alkenes.
29. Find the ratio of distillate to residual flow rates, if a binary mixture containing 65% of more volatile component while the distillate and the residue compositions are 90% and 30%.
a) 0.4
b) 1.4
c) 2.4
d) 3.4
View Answer
Explanation: -W/D=(xd – xf)/(xw-xf)
Finally we ger –W/D= -0.416
D/W= 2.4.
30. What happens to the liquid after preheating when entering the cyclonic flash drum?
a) Starts to evaporate
b) Split with centrifugal force
c) Comes out in residue
d) None of the mentioned
View Answer
Explanation: Since the column is like a cyclone, it will split the liquid with centrifugal force for further separation.
31. Which of the following reboilers is considered as an additional tray?
a) Internal reboiler
b) Jacketed kettle
c) Kettle-reboiler
d) None of the mentioned
View Answer
Explanation: The kettle-reboiler is an external boiler considered an additional tray.
32. Find the separation factor if the ratio of the weight fraction of solute in the extract to raffinate is 0.75. Also, given the ratio of dilutant in raffinate to extract is 0.5.
a) 1
b) 1.25
c) 1.5
d) 1.75
View Answer
Explanation: Distribution coefficient = the ratio of the weight fraction of solute in the extract to raffinate * the ratio of dilutant in raffinate to extract
= 0.75 / 0.5 = 1.5.
33. What is the effect of increasing pressure on gas-phase diffusion?
a) No effect on diffusion
b) Increases diffusion
c) Decreases diffusion
d) Diffusion becomes independent of concentration
View Answer
Explanation: Gas-phase diffusion decreases with an increase in pressure due to a reduced mean free path of molecules.
34. Find the time taken to dry 50% to 40% of moisture (dry basis) if critical moisture content is 30%. If the weight of the wet solid is 5 kg and the product of area and the constant rate is 0.5 kg/min.
a) 40 sec
b) 80 sec
c) 60 sec
d) 20 sec
View Answer
Explanation: Time for drying = (wet solid weight * change in moisture) / (Area * constant rate)
= (5 * 0.1) / 0.5 = 1 min = 60 sec.
35. Which gas will diffuse faster according to Graham’s law?
a) Oxygen
b) Carbon dioxide
c) Hydrogen
d) Nitrogen
View Answer
Explanation: Graham’s law states that lighter gases like hydrogen diffuse faster than heavier ones.
36. Find the moisture content in dry basis if the weight of dry solid is 5 kg and the moisture is 2 kg.
a) 0.4
b) 0.5
c) 0.3
d) 0.2
View Answer
Explanation: As it is a dry basis, the moisture content = 2 / 5 = 0.4.
37. What does Fick’s first law describe?
a) Pressure transfer
b) Mass diffusion rate
c) Heat transfer
d) Energy transfer
View Answer
Explanation: Fick’s first law describes how mass diffuses at a rate proportional to the concentration gradient.
38. What does the term “steady-state diffusion” imply?
a) No diffusion occurs
b) Diffusion rate is zero
c) Concentration remains constant with time
d) Concentration changes over time
View Answer
Explanation: In steady-state diffusion, the concentration profile does not change over time.
39. Which of the following processes can accelerate leaching?
a) Heating
b) Drying
c) Crushing or grinding
d) Crushing
View Answer
Explanation: Crushing or grinding can result in the removal of more solute from the solids, thereby accelerating leaching.
40. A mixture of 60% hydrogen and 40% oxygen is blowing over a pan of water at 1 atm pressure. The bulk gas temperature is 30°C and the water temperature is 20°C. The vapor pressure of water at this temperature is 17.53 mm Hg. The concentration of moisture in the bulk gas is negligible, and the diffusivity values are: DO2-H2O = 0.357 cm2/s, DH2-H2O = 0.972 cm2/s at 34°C, DH2-O2 = 0.891 cm2/s at 43°C. Which of the following statements are correct?
a) Hydrogen and oxygen are diffusing through water vapor
b) Water vapor diffuses through non-diffusing hydrogen and oxygen
c) Hydrogen is diffusing through water vapor and oxygen
d) None of hydrogen, oxygen, and water vapor is diffusing
View Answer
Explanation: Water vapor has a source (pan) and a sink (bulk gas), while hydrogen and oxygen have no sink.
41. In a tapered tube connecting two vessels with a gaseous mixture at different concentrations but the same total pressure, where will the magnitude of the flux be maximum along the length of the tube, considering the tube has a variable cross-sectional area?
a) At the smaller diameter end
b) Midway in the tube
c) At the larger diameter end
d) Somewhere between the midway and the larger diameter end
View Answer
Explanation: The flux is maximum at the smaller diameter end, as the concentration gradient is steeper there due to the variation in cross-sectional area.
42. For a gas mixture (A+B) equimolar counterdiffusion takes place. Which among the following statements are correct for molar flux?
a) If temperature is halved, the flux will get halved
b) If temperature is doubled and length is halved, the flux will become four times
c) If diffusion length is halved, the flux will get halved
d) If temperature is doubled, the flux will get halved
View Answer
Explanation: For equimolar counterdiffusion, the molar flux is inversely proportional to the temperature, meaning that doubling the temperature will halve the flux.
Chapterwise Multiple Choice Questions on Mass Transfer
- Modes and Diffusion
- Interphase Mass Transfer
- Equipment for Gas-Liquid Operations
- Humidification Operations
- Gas Absorption
- Distillation
- Liquid Extraction
- Drying
- Leaching and Mass Transfer Co-efficients
- Diffusion in Solids and Molecular Diffusion in Fluids
1. Modes and Diffusion
The section contains multiple choice questions and answers on concepts of velocity and concentration, convective mass transfer, fick’s law, schimdt number, density concepts, emissivity and various questions on melting point.
2. Interphase Mass Transfer
The section contains questions and answers on interphase mass transfer, various material balance concepts and cascade stages.
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3. Equipment for Gas-Liquid Operations
The section contains MCQs on concepts of gas liquid equipments.
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4. Humidification Operations
The section contains multiple choice questions and answers on humidification operations and vapour gas mixtures.
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5. Gas Absorption
The section contains questions and answers on absorption concepts for operating line, gas absorptio principle and absorption tower design.
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6. Distillation
The section contains MCQs on vapour liquid equilibria, idealities concepts, flash vapourization methods, differential distillation methods, distillation column operation, azeotropic mixture rectification, reflux ratio and its cost variance, various distillation types and flash vaporization laws.
7. Liquid Extraction
The section contains multiple choice questions and answers on various concepts which include extraction liquid equilibria, unit operation solvent selection, extraction equipments and ternary equilibrium diagram.
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8. Drying
The section contains questions and answers on Drying rate, drying time period calculation and moisture concept of drying.
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9. Leaching and Mass Transfer Co-efficients
The section contains MCQs on technique of leaching operation.
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10. Diffusion in Solids and Molecular Diffusion in Fluids
The section contains multiple choice questions and answers on molecular diffusion in solids, liquids and gases, multicomponent diffusion, diffusion through variable and binary gas mixture.
Wish you the best in your endeavor to learn and master Mass Transfer!
