$(30/100)$ A solid sphere $($B$)$ of radius $r_1$ is suspended from a wire in gas A$.$ Gas $A$ is transferred from the medium to the surface of the sphere and enters the following reaction with $B.$

$A($ gas$)+B(solid)$ longrightarrow$2D(gas)$

Product $D$ evaporates immediately and diffuses into the environment. Subtract and integrate the equation for the $N_{}$ flux. You can use the following equation as the starting point.

$N_A=-\frac{D}{RT}\frac{d{P}_A}{dr}+\frac{P_A}{P}(N_A+N_D)$