Consider a desorption process in a spherical sample. Initially, the sphere has uniform species A mass

fraction $!"$ and radius $$; for $>0,$ the species A mass fraction at the surface of the sphere is

maintained at $!$#$ $.$ Assuming $$ is constant and that $$ is approximately constant, write in dimensionless

form the evolution equation for $!$ with the opportune initial and boundary conditions. Derive a

solution for the obtain partial differential equation using the change of variable $=!$