Solution:
An antibiotic is contained in a solid inner core and is surrounded by and outer coating that makes it palatable. The outer coating and the drug are dissolved at different rates in the stomach, owing to their differences in equilibrium solubilities. If $D_2 = 4\ mm$ and $D_1 = 3\ mm$, please calculate the time necessary for the pill to dissolve completely.\\
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$Additional\ information$ :\\
Assume density of outer later $=$ inner layer\\
Amount of drug in inner core $= 500\ mg$\\
Solubility of outer layer at stomach conditions $= 1.0\ kg/cm^3$\\
Solubility of inner layer at stomach conditions $= 0.4\ kg/cm^3$\\
Volume of fluid in stomach $= 1.2\ L$\\
$Sh = 2$, $D_{AB} = 6 \times 10^{-4}\ cm^2/min$
