Pure aniline is evaporating through a stagnant air film of 1 mm thickness at 300 K and a total pressure of 100 KPa. The vapor pressure of aniline at 300 K is 0.1 KPa. The total molar concentration under these conditions is 40.1 mole/m³. The diffusivity of aniline in air is 0.74 × 10^-5 m²/s. The numerical value of mass transfer co-efficient is 7.4 × 10^-3. Its units are

We are told that:

Mass transfer coefficient =
7.4
×
10
−
3
7.4×10
−3
(value given)

Need to determine units of this value.

🔍 Step 1: Match value with likely unit
A mass transfer coefficient
𝑘
𝑐
k
c
​
of:

7.4
×
10
−
3
7.4×10
−3

is quite common for gas phase diffusion, where:

Mass transfer coefficient
𝑘
𝑐
k
c
​
has units of m/s when based on molar flux and concentration difference.

This is consistent with:

𝑁
𝐴
=
𝑘
𝑐
(
𝐶
𝐴
1
−
𝐶
𝐴
2
)
N
A
​
=k
c
​
(C
A1
​
−C
A2
​
)
Where:

𝑁
𝐴
N
A
​
: molar flux [mol/m²·s]

𝐶
𝐴
1
−
𝐶
𝐴
2
C
A1
​
−C
A2
​
: concentration difference [mol/m³]

So
𝑘
𝑐
k
c
​
: [m/s]

🔍 Step 2: Consider other units
cm/s would give a value 100× larger if expressed in cm/s

mole/m²·s·Pa or kmole/m²·s·Pa are for pressure-based driving forces, but the value 7.4×10⁻³ is too high for those units in this case.

✅ Final Answer: A. m/s

Solution