$3.$ A drug is injected into the blood stream in which it is sparing ly soluble in the form of nanospheres. The drug undergoes a first$-$order chemical reaction with rate constant k$1.$ At steady$-$state the diffusion is exactly balanced by the chemical reaction, where Co is the molar solubility of the drug and I is the drug diffusivity constant. Determine the concentration profile radially away from The nanosphere of radius R$.$ Calculate the total mass rate of drug from the sphere at radius R in moles per time. Assuming this steady$-$state continues, derive an expression for the nanosphere radius, R$,$ as a function of time. This is called a quasi$-$steady$-$state model!