Question

What minimum number of moles of NH4Cl must be added to 1.0 liter of solution that is 0.10 M in Mg(NO3)2 and 1.0 M in NH3 to prevent precipitation of Mg(No3)2?

Answer 1

Mg(OH)2 ==> Mg^+2 + 2OH^-

Ksp = (Mg^+2)(OH^-)^2

Kb for NH3 = (NH4^+)(OH^-)/(NH3)

Solve Ksp for OH^-. You now Mg so there is only one unknown. Substitute OH from that calculation into Kb for NH3. You know NH3 and Mg^+2, solve for NH4^+, then convert that molarity to grams NH4Cl. Post your work if you get stuck.

Answer 2

how do you know Mg?