Question $2$:

A spherical balloon is pumped up with pure $He$ gas to a pressure of $\frac{2}{b}ar$ so that it is $25cm$ in

diameter. In $5$ hours of drifting through the air at $1$atm and $20C$ the pressure in the balloon

drops to $1\mathrm{.}9$ bar. The balloon volume and surface area remain approximately constant.

a$)$ Determine the effective diffusivity of $He$ through the balloon material, which is $80$

$m$ thick. You can assume that the ideal gas law applies and that the average pressure

in the balloon can be used for the calculation. There is no counter diffusion of air

from the outside

b$)$ Explain in a sentence or two why the balloon will take more than $2$ days to completely

deflate.