Question about ammonia and nitrogen for a small experiment

Question

Hello! I am trying to set up a small experiment for my class but I am stuck on a problem. We have three saltwater tanks, with the following measurements:

The first tank will have a pH of 7.5, an NH3 level of 0.06 mg/L, and a TAN (total ammonia nitrogen) level of 20. Just NH3 is added.
The second will have a pH of 7.2, an NH3 level of ???, and a TAN of 40. In this tank, CO2 + NH3 are added.
The third tank will have a pH of 7.2, an NH3 level of ???, and a TAN of 40. In this tank, only NH3 is added.

Basically, I am trying to figure out what NH3 levels I need to get the TAN level of 40 in the last two tanks. I'm sure I am overthinking it but I don't want to overlook anything. Any tips are welcome. Thank you! :-)

Answer 1

An aqueous solution of ammonia, NH₃, is in equilibrium with NH4OH, a weak base. You can look up the Kb for NH₃ or the Ka for NH₄⁺ and then using the appropriate equilibrium equation and the given pH, you can solve for [NH₃].  TAN is the total nitrogen from NH₃ and NH₄⁺ combined, usually reported in mg/L.  In your case, values of 40 are no likely to be mg/L, so you need to tell us what the units are so that we might be able to better assist you.