A asked in Science & MathematicsChemistry · 1 month ago

 Precipitates questions (2 parts). ?

Full question: You have a solution containing Cl- and F- ions, both at 0.010 M. You wish to separate the two ions from each other by precipitating one but not the other using Pb(NO3)2 as the precipitating agent. Ksp(PbF2) = 3.7*10^-8, 

Ksp(PbCl2) = 1.7*10^-5. 

a) Which precipitates first as lead(II) nitrate is added, PbF2 or PbCl2?

b) What is the concentration of the first ion that precipitates (Cl- or F-) when the second, more soluble salt begins to precipitate? 

Please describe your answers clearly and please include steps.

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  • 1 month ago
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    a) PbF2 will precipitate first because the two compounds have similar formulas and Ksp for PbF2 is smaller.

    b) Use the Ksp for PbCl2 to calculate the [Pb2+] at which the chloride will precipitate:

    Ksp = [Pb2+][Cl-]^2 = 1.7X10^-5

    [Pb2+](0.010)^2 = 1.7X10^-5

    [Pb2+] = 0.17 M

    Now, use this to calculate the remaining [F-] in the solution:

    Ksp = [Pb2+][F-]^2 = 3.7X10^-8

    (0.17) [F-]^2 = 3.7X10^-8

    [F-] = 4.7X10^-4 M

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