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Homework Help: Calculating Ksp of salt (answer check)

  1. Apr 25, 2006 #1
    69. Calculate the Ksp for each of the salts whose solubility is listed below.

    a) CaSO4 = 3.3 x 10-3 mol/L
    b) MgF2 = 2.7 x 10-3 mol/L


    Answers:

    a) Solution

    1. Write the dissociation reaction and the Ksp expression for the equilibrium reaction. Thus,

    CaSO4(s) <---> Ca2+ (aq) + SO42- (aq)

    Ksp = [Ca2+][SO42-]

    2. Calculate the concentrations of the ions at equilibrium from the stoichiometric information provided in the equilibrium equation. The equation shows that 1 mol of CaSO4 produces 1 mol of Ca2+ and 1 mol of SO42- at equilibrium. Therefore,

    [Ca2+] = 3.3 x 10-3 mol/L
    [ SO42-] = 3.3 x 10-3 mol/L

    3. Substitute the concentrations into the Ksp expression:

    Ksp = (3.3 x 10-3)( 3.3 x 10-3)

    = 1.2 x 10-5


    b) MgF2 <---> Mg2+ (aq) + F22+ (aq)

    Ksp = [Mg2+][F22+]

    [Mg2+] = 2.7 x 10-3
    [F22+] = 2.7 x 10-3
    Ksp = (2.7 x 10-3)(2.7 x 10-3)

    = 7.3 x 10-6
     
  2. jcsd
  3. Apr 25, 2006 #2
    your first one looks right, but the F2 in the second equation will actually dissociate into two F- ions
     
  4. Apr 25, 2006 #3
    Alright I fixed part b), is it right now?

    MgF2 <---> Mg2+ (aq) + 2F- (aq)

    Ksp = [Mg2+][F-]2

    [Mg2+] = 2.7 x 10-3
    [F-] = 2 x 2.7 x 10-3
    Ksp = (2.7 x 10-3)(5.4 x 10-3)

    = 1.5 x 10-5
     
    Last edited: Apr 26, 2006
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