0600-1-mt-1mark

The units of diffusivity are:

• m/s

• m²/s

• kmol/(m².s)

• none of these

GATE-CH-1994-1-o-mt-1mark

Diffusion coefficient in a binary gas mixture at low pressures varies with pressure as

• \(P\)

• \(P^2\)

• \(1/P\)

• independent of \(P\)

GATE-CH-1997-1-15-mt-1mark

Molecular diffusivity of a liquid

• increases with temperature

• decreases with temperature

• may increase or decrease with temperature

• is independent of temperature

GATE-CH-2003-16-mt-1mark

The diffusion coefficient, in m²/s, of acetic acid in benzene (liquid in liquid) is

• \(2.09 \times 10^{-4}\)

• \(2.09 \times 10^{-5}\)

• \(2.09 \times 10^{-9}\)

• \(2.09 \times 10^{-12}\)

GATE-CH-2003-17-mt-1mark

Component \(A\) is diffusing in a medium \(B\). The flux \(N_A\) relative to a stationary point is equal to the flux due to molecular diffusion if

• mass transfer is accompanied by reaction

• diffusion of \(A\) is in stagnant medium \(B\)

• molecular mean free path is high

• there is equimolar counter-diffusion

GATE-CH-2005-20-mt-1mark

The ratio of the diffusion coefficient in a gas to that in a liquid is of the order of

• \(10^5\)

• \(10^{-5}\)

• \(10^{-2}\)

• \(10^2\)

GATE-CH-2006-13-mt-1mark

The reaction \(2A + B \rightarrow 2C\) occurs on a catalyst surface. The reactants \(A\) and \(B\) diffuse to the catalyst surface and get converted completely to the product \(C\), which diffuses back. The steady state molar fluxes of \(A\), \(B\) and \(C\) are related by

• \(N_A = 2N_B = N_C\)

• \(N_A = - (1/2) N_B = -N_C\)

• \(N_A = 2N_B = - N_C\)

• \(N_A = (1/2) N_B = N_C\)

GATE-CH-2011-19-mt-1mark

Ammonia (component 1) is evaporating from a partially filled bottle into surrounding air (component 2). The liquid level in the bottle and the concentration of ammonia at the top of the bottle are maintained constant. \(N_i\) is the molar flux relative to a fixed location in space and \(J_i\) is the molar flux with respect to the average molar velocity of the constituent species in the gas phase. Assume that air in the bottle is stagnant. Which ONE of the following is CORRECT ?

• \(N_1=\text {constant}, N_2=0, J_1+J_2=0\)

• \(N_1+N_2=0, J_1+J_2=0\)

• \(N_1+N_2=0, J_1=\text {constant}, J_2=0\)

• \(N_1=\text {constant}, N_2=0, J_1=\text {constant}, J_2=0\)

GATE-CH-2015-14-mt-1mark

For a binary mixture of components \(A\) and \(B\), \(N_A\) and \(N_B\) denote the total molar fluxes of components \(A\) and \(B\), respectively. \(J_A\) and \(J_B\) are the corresponding molar diffusive fluxes. Which of the following is true for equimolar counter-diffusion in the binary mixture?

• \(N_A + N_B = 0 \text { and } J_A + J_B \ne 0\)

• \(N_A + N_B \ne 0 \text { and } J_A + J_B = 0\)

• \(N_A + N_B \ne 0 \text { and } J_A + J_B \ne 0\)

• \(N_A + N_B = 0 \text { and } J_A + J_B = 0\)

GATE-CH-1988-15-i-mt-4mark

For equimolar counter diffusion from a sphere to a surrounding stationary infinite medium, the mass flux \(N_{Ai}\) of the diffusing component \(A\) at the interface is given by \(N_{Ai} = D_A(C_{Ai} - C_{Ab})/R\) where \(D_A\) is the diffusivity, \(R\) the radius of the sphere and \(C_{Ai}\) and \(C_{Ab}\) the molar concentrations of \(A\) at the interface and at a point far away from the sphere. Show that the Sherwood number, based on the diameter of the sphere, is equal to 2.

• Answer

GATE-CH-1988-5-a-i-mt-1mark

Gas \(A\) diffuses to the surface of a catalyst where it is converted into \(B\) according to the chemical reaction \(3A\rightarrow B\). The product diffuses back. The ratio of molar flux of \(A\) to \(B\) at steady state is ––––- .

• Answer

GATE-CH-1991-6-ii-mt-2mark

The Fick’s second law of diffusion in one dimension is ––––- .

• Answer

GATE-CH-1993-22-a-mt-5mark

Oxygen is diffusing in a mixture of oxygen-nitrogen at 1 std atm, 25\(^\circ \)C. Concentration of oxygen at planes 2 mm apart are 10 and 20 volume % respectively. Nitrogen is non-diffusing.

1. Derive the appropriate expression to calculate the flux of oxygen. Define units of each term clearly.
2. Calculate the flux of oxygen. Diffusivity of oxygen in nitrogen = \(1.89 \times 10^{-5}\) m\(^2\)/s.

• Answer

GATE-CH-1994-19-mt-5mark

In a gas mixture of hydrogen and oxygen, steady state equimolar counter diffusion is occurring at a total pressure of 100 kPa and temperature of 20\(^\circ \)C. If the partial pressures of oxygen at two planes 0.01 m apart, and perpendicular to the direction of diffusion are 15 kPa and 5 kPa, respectively and the mass diffusion flux of oxygen in the mixture is \(1.6 \times 10^{-5}\) kmol/m\(^2\).s, calculate the molecular diffusivity (in m\(^2\)/s) for the system.
____________\(\times 10^{-5}\)

• Answer