Solve a mass diffusion problem
 

Hello, I'm at the beginning to use fluent therefore sorry if i ask maybe stupid question... I have the following problem: there is a capillary one open ended filled with a solution of initial mass fraction =1% with a note diffusion coefficient at fixed temperature. The capillary diffuses in pure water through only the open end of capillary. D is of order 10^-10 m^2/s and i want know the evolution of mean mass fraction in the capillary until a time of 10^6 seconds for PURE DIFFUSION.. How can I solve this problem at better..?I have a theoretic solution of this problem but in my simulation the evolution of concentration is completely different from the theoric curve!