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.

1 Answer

  • 1 month ago
    Favorite Answer

    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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