If a solution is made 0.080 M in Mg(NO3)2, 0.075 M in aqueous ammonia, and 3.5 M in NH4NO3, will Mg(OH)2 precipitate? What is the pH of this solution?

Answer 1

Use the Henderson-Hasselbalch equation to solve for pH of the solution. Use NH4Cl as the acid and NH3 as the base. Convert the pH to (OH^-) and set up Ksp with Mg(OH)2 and see if Qsp is greater than Ksp for Mg(OH)2. In the regular qual scheme of things, Mg ions are in group V and the solution in group IV is treated with NH3 and NH4Cl to keep Mg(OH)2 from pptng in group IV instead of group V.