Question

An acidic solution is 2 mM in each of the following metal ions: Zn²⁺, Cu²⁺, Co²⁺, Ca²⁺.

a) Which of the metals would precipitate as their hydroxides at pH = 6.00? (Select all that apply. See theappendix.)

-zinc

-copper

-cobalt

-calcium

b) Which of the metals would precipitate as their hydroxides at pH = 8.00?

-zinc

-copper

-cobalt

-calcium

c) At what pH do the metal hydroxides begin to participate?

Zn(OH)₂ ____

Cu(OH)₂ ____

Co(OH)₂ ____

Ca(OH)₂ ____

Substance Ksp at 25 oC Aluminum Al(OH)3 1.9 x 10-33 Barium Ba(OH)2 5.0 x 10-3 BaCO3 8.1 x 10-9 BaSO4 1.1 x 10-10 Ba3(PO4)2 3.4 x 10-23 Cadmium Cd(OH)2 2.5 x 10-14 CdCO3 5.2 x 10-12 CdS 8.0 x 10-27 Calcium Ca(OH)2 5.5 x 10-6 CaCO3 4.8 x 10-9 Ca3(PO4)2 1.0 x 10-26 CaF2 3.9 x 10-11 Chromium Cr(OH)3 6.3 x 10-31 Cobalt Co(OH)2 1.6 x 10-15 CoS 4.0 x 10-21 Copper Cu(OH)2 2.2 x 10-20 Cu2S 1.6 x 10-48 CuCO3 1.4 x 10-10 CuS 6.3 x 10-36 Iron Fe(OH)2 8.0 x 10-16 FeS 6.3 x 10-18 Fe(OH)3 2.5 x 10-39 FePO4 9.9 x 10-29 Lead Pb(OH)2 2.8 x 10-16 PbF2 3.7 x 10-8 PbCl2 1.7 x 10-5 PbBr2 6.3 x 10-6 PbI2 6.5 x 10-9 PbCrO4 1.8 x 10-14 PbSO4 1.7 x 10-8 PbS 8.4 x 10-28 Magnesium Mg(OH)2 1.8 x 10-11 Manganese Mn(OH)2 1.9 x 10-13 MnCO3 8.8 x 10-11 MnS 5.6 x 10-16 Nickel Ni(OH)2 1.6 x 10-14 Silver AgCl 1.8 x 10-10 AgBr 5.0 x 10-13 AgI 8.3 x 10-17 AgCN 1.2 x 10-16 Ag2S 6.3 x 10-50 Ag2CrO4 1.1 x 10-12 Ag3PO4 2.6 x 10-18 Tin Sn(OH)2 1.4 x 10-28 SnS 1.3 x 10-23 Zinc ZnCO3 1.0 x 10-10 Zn(OH)2 4.5 x 10-17 ZnS 1.1 x 10-21

Answer 1

1) We can determine that ppt will form or not by taking into account the ksp

-zinc

Zn(OH)2 -----> Zn2+ + 2OH-
Ksp = [Zn2+][OH-]^2

To form a precipitate, the ionic product must be greater than the Ksp.

4.5 x 10^-17 = [Zn+2][OH-]^2

Zn +2 = 1/2 OH-
1/2[OH-]^3 = = 4.5 x 10^-17
[OH] = 11.61 x 10^-6M
pOH = - log (6.7 x 10^-6) =4.93
pH = 14.00 - pOH = 14.00 - 4.93 = 9.06

So minimum pH required is 8.82 for Zn+2 so it will not ppt out

-copper

Cu(OH)2 -----> Cu2+ + 2OH-
Ksp = [Cu2+][OH-]^2

To form a precipitate, the ionic product must be greater than the Ksp.

2.2 x 10^-20= [Cu+2][OH-]^2

Cu +2 = 1/2 OH-
1/2[OH-]^3 = 2.2 x 10^-20

[OH] = 4.69 x 10^-7M
pOH = - log (4.69 x 10^-7) = 6.32
pH = 14.00 - pOH = 14.00 - 4.72 = 7.67

So minimum pH required is 7.67 for Cu+2 so it will not ppt out

-cobalt

Co(OH)2 -----> Co2+ + 2OH-
Ksp = [Co2+][OH-]^2

To form a precipitate, the ionic product must be greater than the Ksp.

1.6 x 10^-15= [Co+2][OH-]^2

Co +2 = 1/2 OH-
1/2[OH-]^3 = 1.6 x 10^-15

[OH] = 2.4 x 10^-5M
pOH = - log (2.4 x 10^-5) = 4.61

pH = 14.00 - pOH = 14.00 - 4.61= 9.38

So minimum pH required is 9.38 for Co+2 so it will not ppt out

-calcium

Ca(OH)2 -----> Ca2+ + 2OH-
Ksp = [Ca2+][OH-]^2

To form a precipitate, the ionic product must be greater than the Ksp.

5.5 x 10^-6 = [Ca+2][OH-]^2

Ca +2 = 1/2 OH-
1/2[OH-]^3 = 5.5 x 10^-6

[OH] = 3.31 x 10^-2M
pOH = - log (3.31 x 10^-2) = 1.48

pH = 14.00 - pOH = 14.00