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Discussion in 'DAT Discussions' started by Dencology, May 28, 2008.

  1. Dencology

    Dencology 2+ Year Member

    Mar 7, 2008
    given the reaction CaF29(s)____>Ca2+(aq) + 2F-(aq), determine whether a precipitate will form. initial concentration of Ca2+ is 0.5M and of F- is 0.25M. Ksp is 0.025.

    a. precipitate will form
    b. precipitate will not form
    a precipitate will from and then disappear

    i guess one way of solving this is to realize that the concentration of Ksp is less than the concentration of both Ca and F-. And we know that for a solution to form precipitation Ksp < 1 and for a solution not to from a precipitation Ksp > 1. Am i correct?
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  3. Zerconia2921

    Zerconia2921 Bring your A-game! 2+ Year Member

    Mar 24, 2008
    The initial concentration is defined as I.P. so the initial concentration of Ca and F are 0.5M and 0.25M respectivly. These concentrations do not represent equilb or a saturated solution, while Ksp does.

    This salt has its own distinct Ksp at a given temp. If the I.P. is equal to its Ksp then the solution is saturated and the rate at which the salt dissolves equals the rate at which it ppts out of solution.

    If a salts IP exceeds its Ksp the solution is supersaturated and unstable.

    Having this background information I believe the answer is C (edit mail)

    Ksp = 2.5x10^-2

    I.P = Ca2+ + 2F-
    IP = [0.5M] [2[.25M]^2
    IP = 1.25x10^-1

    IP > Ksp therefore supersaturated adding more salt will cause precipitate and then over time the IP will equal Ksp.


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