[OH^-][OH−] =10^-510−5
From KspKsp of Cu(OH)_2Cu(OH)2, it starts precipitating at:
2.2*10^-202.2⋅10−20 = [Cu^(2+)][Cu2+]*[OH^-]^2[OH−]2
[OH^-][OH−]=sqrt (0.22*10^-20)√0.22⋅10−20=0.46*10^(-10)0.46⋅10−10
and from KspKsp of Mg(OH)_2Mg(OH)2, it starts precipitating at
6.3*10^-106.3⋅10−10 =[Mg^(2+)][Mg2+]*[OH^-]^2[OH−]2
[OH^-][OH−]=sqrt (1.26*10^-10)√1.26⋅10−10 =1.12*10^(-5)1.12⋅10−5
therefore at [OH^-][OH−]=10^(-5)10−5 all Cu(OH)_2Cu(OH)2 is precipitated and all Mg(OH)_2Mg(OH)2 is in solution