[kschapp]

Hello.  What is the ammonium concentration of a solution at 25 deg C that emits 5 ppm of ammonia?  If I assume a partial pressure, I can use Raoult's law to convert 5 ppmv to ppmw in the solution. I think my liquid concentration is both ammonia and ammonium ions. So where do I go from here?

[enahs]

Since it is a gas it will fill up the room, so the concentration depends on the volume size.

[kschapp]

it is not the weight emitted I am looking for. The fraction, xi, should be a function of yi and partial pressure.  If I can find xi, I should be able to find the mg/l of the ammonia/ammonium in solution.  My problem is, how much is ammonia and how much is ammonium, if I don't know the pH?  I've tried solving it using [NH3][H+]/[NH4+] = Ka, but my quadratic equation gives me an imaginary number.

[kschapp]

Okay.  I think I have worked it out.  Please tell me if this is correct.

p = yP = x Ha   (Henry's Law)

Ha = 1.6 x 10^-6 atm mol/m3

therefore, x = yP/Ha = (5 x 10^-6 mol/mol)(1 atm) / 1.6 x 10^-6 atm mol/m3 = 3.125 mol/m3 = 3.125 x 10^3 mol/liter.

This is the molarity of NH3, right? 

So..

[NH3][H+] / [NH4] = 5.7 x 10^-10

(3.125 x 10^-3)(3.125 x 10^-3) / x = 5.7 x 10^-10

x = 17132 mol NH4/l =

Also note that since [H+] = 3.125 x 10^-3,
pH = -log [H+] = 2.5. 

This pH does not make sense to me, since there is practically no dissociation of a NH3 until you reach a pH of 7.25 or less.  Can anyone make any sense of this?