Question

Consider a 1.50 L aqueous solution of 3.75 M NH3, where 17.5 g of NH4Cl are dissolved. To this solution, 5.00 g of MgCl2 are added.

a. What is [OH-] before MgCl2 is added?
b. Will a precipitate form?
c. What is [Mg2+] after equilibrium is established?

I don't need the answers, I just need an explanation of how to do these problems. Thanks.

Answer 1

That's good because I usually don't give answers anyway.
.........NH3 + H2O ==> NH4^+ + OH^-
I........3.75...........0.......0
C........-x.............x.......x
E......3.75-x...........x.......x

Kb = (NH4^+)(OH^-)/(NH3)
You want to solve for OH^-.
You know Kb.(NH4^+) = x from NH3 + ? from NH4Cl.
mols NH4Cl = grams/molar mass
M NH4Cl = mols/L solution. Total (NH4^+) = x + ?M NH4Cl

That's part a.
part b. Will a ppt form? That is will Mg(OH)2 ppt? If so Qsp must exceed Ksp.
Qsp = (Mg^2+)(OH^-)^2.
(Mg^2+) = M
mols MgCl2 = grams/molar mass
M MgCl2 = mols/L solution.
OH^- you know from above.
Solve for Qsp and compare with Ksp.

part c. I'm assuming Mg(OH)2 will ppt.
Ksp for Mg(OH)2 = (Mg^2+)(OH^-)^2
You know Ksp and (OH^-), plug those in and solve for (Mg^2+)