Write the assumptions, draw the control volume, write the conservation law $($do shell balance$)$ and write the boundaries $($not conditions$)$ for the following problems:

a

A drug molecule dissolves in solution in spherical coordinates, you want to obtain the concentration $C_{\text{d}\text{r}\text{u}\text{g}}(r)$ in the solution.

A gas mixture is flowing through a borosilicate pipe. The pipe allows only the diffusion of gas A through its wall so that it separates gas A from the mixture. You want to obtain the concentration $C_A(r)$ through the pipe wall