Precipitation is the formation of an insoluble substance. For the equation AB(s)

Question

Precipitation is the formation of an insoluble substance. For the equation AB(s)?A+(aq)+B?(aq), precipitation represents a shift to the left and the production of a solid. From Le Châtelier's principle, we know that when the product of the concentrations of A+ and B? gets above a certain level, the reaction will respond by shifting left to decrease the concentrations of A+ and B?. This critical level, Ksp, is a constant at a certain temperature. In this case, Q=[A+][B?]. where Q is the ion product.When Q>Ksp, precipitation occurs until Q is equal to Ksp.When Q?Ksp, a precipitate does not form. Part C What concentration of the lead ion, Pb2+, must be exceeded to precipitate PbBr2 from a solution that is 1.00×10?2 M in the bromide ion, Br?? Ksp for lead(II) bromide is 4.67×10?6 . Express your answer with the appropriate units.

Explanation / Answer

PBr2. ----------->> . Pb2+ . +. 2  Br-

1 mole .    1 mole . 2mole

as explained above for any AB which dissociate in solution as A+ + B- .  the precipetation will start when product of conc. of A+ & .B-  is greater than

solubility product Ksp .

Ksp for PbBr2 is giving 4.67×10-6

[Br-] = 1×10-2

Ksp PbBr2 = [Pb+2] × [Br-]2

4.67×10^-6 = [Pb2+] ×1×10-4

[Pb2+] . = 4.67×10-6 ÷10-4

= 4.67 ×10-2 M

So Pb2+ concentration must exceed 4.67×10-2 M in order to precipitation of PbBr2.

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