Diffusivity is a measure of how quickly a substance diffuses or spreads out in a given medium. It is often denoted by the symbol D and has units of square meters per second (sq.m/sec).

The diffusivity of AB can be calculated using Fick's Law of Diffusion, which states that the flux (N) of a substance is equal to the product of the diffusivity (D), the concentration difference (∆C), and the area perpendicular to the concentration gradient (A), divided by the distance (L) over which diffusion occurs.

The formula for Fick's Law of Diffusion is:

N = -D * ∆C * A / L

where N is the flux, D is the diffusivity, ∆C is the concentration difference, A is the area, and L is the distance.

In this case, the flux of A is given as 5 mol/sq.m sec, the concentration difference is 2 mol/cu.m, and the distance is 3 m. We need to find the diffusivity of AB.

Substituting the given values into the formula, we have:

5 mol/sq.m sec = -D * 2 mol/cu.m * A / 3 m

Rearranging the equation to solve for D, we get:

D = -5 mol/sq.m sec * 3 m / (2 mol/cu.m * A)
D = -7.5 sq.m/sec

Since diffusivity cannot be negative, we take the absolute value of the calculated diffusivity, which is 7.5 sq.m/sec.

Therefore, the diffusivity of AB is 7.5 sq.m/sec.

In summary,

- The flux of A is given as 5 mol/sq.m sec.
- The concentration difference is 2 mol/cu.m.
- The distance is 3 m.
- We can use Fick's Law of Diffusion to calculate the diffusivity.
- Substituting the given values into the formula, we find that the diffusivity of AB is 7.5 sq.m/sec.