Quest (1) determine the ksp for magnesium hydroxide Mg(OH)2 where the molar solubility of Mg(OH)2 is 1.4×10−4M?
We will use ICE table to solve this question.
The dissolution of Mg(OH)2 can be written as follows:
Mg(OH)2(s)→Mg2+(aq)+2OH−(aq)
Initial: 0M 0M
Change: −sM +sM +2sM
Equilibrium: sM 2sM
The expression of Ksp can be written as follows:
Ksp=[Mg2+(aq)]⋅[OH−(aq)]2
⇒Ksp=s⋅(2s)2=4s3=4×(1.4×10−4)3
⇒Ksp=1.1×10−11
Quest (2) The Ksp for Zn(OH)2 is 5.0×10−17 . Determine the molar solubility of Zn(OH)2 in a buffer solution with a pH of 11.5?
We will use ICE table to solve this question.
pH=11.5⇒[H+]=10−11.5
⇒[OH−]=10−2.5=3.2×10−3M
The dissolution of Zn(OH)2 can be written as follows:
Zn(OH)2(s)→Zn2+(aq)+2OH−(aq)
Initial: 0M 3.2×10−3M
Change: −sM +sM +2sM
Equilibrium: sM (3.2×10−3+2s)M
The expression of Ksp can be written as follows:
Ksp=[Zn2+(aq)]⋅[OH−(aq)]2
5.0×10−17=s⋅(3.2×10−3+2s)2
Solve for s=4.9×10−12M
Here is a video that explains the solving of Q2 (start at minute 5:14):