Question

a) A solution contains 9.54×10^-3 Mlead acetate and1.38×10^-2 M calcium nitrate.

Solid potassium sulfide is added slowly to this mixture.

What is the formula of the substance that precipitates first? Formula = ___
What is the concentration ofsulfide ion when this precipitation first begins? [S^2-] = ___ M

b) A solution contains 5.90×10^-3 Msodium phosphate and1.09×10^-2 M potassium hydroxide.
Solid lead acetate is added slowly to this mixture.

What is the formula of the substance that precipitates first? Formula = ___
What is the concentration of leadion when this precipitation first begins? [Pb²⁺] = ___ M

c) A solution contains 1.32×10^-2 Mzinc acetate and6.30×10^-3 M copper(II) nitrate.
Solid sodium hydroxide is added slowly to this mixture.

What is the formula of the substance that precipitates first? Formula = ____
What is the concentration ofhydroxide ion when this precipitation first begins? [OH^-] = ___ M

Answer 1

a)

Ksp of PbS = 3.2 x 10^-28

Ksp of CaS = 8 x 10^-6

here PbS will precipitates first . because Ksp value is low.

formula of the substance that precipitates first = PbS

PbS    ------------> Pb2+ + S2-

Ksp = [Pb2+ ][S2-]

3.2 x 10^-28 = 9.54 x 10^-3 x [S2-]

[S^2-] = 3.35 x 10^-26 M

b)

Ksp of Pb3(PO4)2 = 3.0 x 10^-44

Ksp of Pb(OH)2 = 2.8 x 10^-16

formula of the substance that precipitates first = Pb₃(PO₄)₂

Pb3(PO4)2 ----------> 3 Pb2+ + 2 PO43-

Ksp = [Pb2+]^3 [PO43-]^2

3.0 x 10^-44 = [Pb2+]^3 x ( 5.90×10^-3)^2

[Pb2+] = 9.52 x 10^-14 M

c)

formula of the substance that precipitates first = Cu(OH)2

[OH-] = 5.04 x 10^-9 M