$(6)($i$)$ A $30m^3$ storage tank is half filled with water and half filled with air at $21C.$ After $76kg(1000$mol of liquid dipropylene glycol $(C_3{H}_8{O}_2)$ is added, the storage tank is sealed. Determine the equilibrium concentration of dipropylene glycol in water and the equilibrium partial pressure of dipropylene glycol in the air space. Henry's Law Constant $(K_H)$ for dipropylene glycol at $21C$ is ${10}^{6}mo\frac{l}{L*atm}.$

$(7)($ii$)$ A nitrogen analysis of a contaminated water sample gave the following results for ammonium, nitrite, nitrate and organic$-$nitrogen: $40m\frac{g}{L}$ as $N{H}_3,2m\frac{g}{L}$ as $N{O}_2^{-},10m\frac{g}{L}$ as $N{O}_3^{-}$and $20m\frac{g}{L}$ as organic$-N,$ respectively. Calculate the total nitrogen concentration in the water sample. Assume that all the ammonia is in the form of $N{H}_3$ and the following chemical atomic weights: $H=1,N=14$ and $O=16$ apply.

$(7)($iii$)$ Two different methods, the most probable number $($MPN$)$ and colony$-$forming unit $($CFU$),$ are commonly used to enumerate fecal indicator bacteria in water samples for environmental compliance. Briefly explain each method and two $(2)$ important differences to be considered when interpreting reported data from the microbiology lab. The image below shows Enterococci colonies growing on a selective agar after membrane filtration.