A cup of water is carefully degassed so there is no dissolved oxygen in the water. From time zero onward, the water surface is exposed to fresh air, so that the concentration of oxygen in the water, at the surface, jumps from $0$ to $9\frac{g}{m^3}.$ The water is kept perfectly stagnant the whole time.

I

What is the concentration of oxygen in the water, $1mm$ below the surface, at time $120$ seconds?

Data: The diffusivity of oxygen in water is $2{10}^{-9}\frac{m^2}{s}.$

State your assumptions for full marks.