A solution contains 1.39 times 10^-2 M zinc nitrate and 1.02 times 10^-2 M nicke

Question

A solution contains 1.39 times 10^-2 M zinc nitrate and 1.02 times 10^-2 M nickel(II) acetate. Solid sodium hydroxide is added slowly to this mixture. What is the concentration of zinc ion when nickel ion begins to precipitate? A solution contains 5.76 times 10^-3 M calcium acetate and 6.46 times 10^-3 M copper(II) nitrate. Solid ammonium carbonate is added slowly to this mixture. A. What is the formula of the substance that precipitates first? B. What is the concentration of carbonate ion when this precipitation first begins?

Explanation / Answer

1. Ksp of Zn(OH)2= 1.2×10–17 < Ksp of Ni(OH)2 = 2.0×10–15

So, Zn(OH)2 will precipitate first.

When Ni ion begins to precipitate after some time, concentration of Ni2+ at that point = 0.0139 M

Ksp of Ni(OH)2 =[Ni2+][OH-]2 = 2.0×10–15

or, [OH-]2 = 2.0×10–15/[Ni 2+] = 2.0×10–15/0.0139 = 1.44*10-13 M

Ksp of Zn(OH)2=[Zn2+][OH-]2 = 1.2×10–17

[Zn 2+] = 1.2×10–17 / [OH-]2 = 1.2×10–17 /1.44*10-13

= 8.34*10-5 M

2. Ksp of CaCO3 = 2.8×10–9 > Ksp of CuCO3 = 1.4 x 10-10

CuCO3 will precipitate first.

Ksp of CuCO3 = [Cu2+][CO32-]= 1.4 x 10-10

[CO32-] = 1.4 x 10-10 / [Cu2+] = 1.4 x 10-10 / 0.00646 = 2.17*10-8 M

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