Show work to receive credit 1 What is the solubility of CuOH

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1. What is the solubility of Cu(OH)₂ at a pH of 10.58? K_sp(Cu(OH)₂) = 4.800e-20.

2. A laboratory manager wants to prepare 250.00 mL of a buffer with a pH of 8.916 from equimolar solutions of HCN (K_a = 4.900e-10) and LiCN. What volume of the LiCN solution would be required?

Solution

1) pH = 10.58, pOH = 14-10.58 = 3.42, [OH} = 10^ -3.42 = 0.00038 ,

now Ksp = [Cu2+][OH-]^2 = 4.8 x10^ -20

S ( 2S +0.00038)^2 = 4.8 x10^ -20 ,

ignoring 2 S in bracket , S = 3.33 x10^ -13 is solubility ,

2) pH =pka + log[LiCN]/[HCN] , pka = -log(4.9x10^ -10) = 9.3

8.916=9.3 + log[LiCN]/[HCN]

[LiCN]/[HCN] = 0.413,

[LiCN] = 0.413[HCN] , since we have both equi molar solutions only vol change can change conc

0.413 x Vol of LiCN = vol of HCN

hence 0.413V + V = 250 ,

V = 176.9 ml of HCN and (250-176.9) = 73.1 ml of LiCN is required to make buffer solution