Going back to your original question, if you wanted a solution of 1.8 x 10-5 M [NH3], the best way would be to find a solution of concentrated NH3 (ammonium hydroxide) and use the dilution equation to solve for how much water you must dilute the solution with (it will be a lot).
M1V1 = M2V2
Analytically, and using your first number (1.8 x 10-5 M NH3)
NH3 + H2O --> NH4+ + OH-
(1.8 x 10-5 - x) x x x
Kb = 1.8 x 10-5 = X2 / (1.8 x 10-5 - X)
Solving for X yields OH-
determine H+ using the dilute solution relationship (H+)(OH-) = 1 10-14
determine pH - value should be in the 8-9 region
The lower concentrations will be closer to 7. The addition of something like ammonium chloride (the conjugate acid of ammonia) will cause the pH to drop.
|