MCAT Chemistry Question — Solubility Product

  • Reviewed By: Liz Flagge
  • Solid Fe(OH)3 is added to distilled water and allowed to reach equilibrium. At that time, [Fe3+]aq= 4 x 10-10 M. Calculate the Ksp of Fe(OH)3.

    A. 4 x 10-10

    B. 4.8 x 10-19

    C. 6.9 x 10-37

    D. cannot be determined without knowing the [OH] concentration

    Click for Explanation

    The solubility product, Ksp, is given by the product of the equilibrium concentrations of the dissociated ions [A]a + [B]b in the form Ksp= [A]a[B]b. In this case, FeOH3 dissociates to [Fe3+]aq and 3[OH]aq. Even though we are not given the concentration of OH, we know that it must be three times the concentration of Fe3+, since one Fe(OH)3 molecule dissociates into one iron cation and three OH anions.

    Therefore, the solubility product Ksp = [Fe3+]1[OH]= [Fe3+]1{3[Fe3+]}3

    A. 4 x 10-10, incorrect, This answer disregards the [OH] concentration.

    B. 4.8 x 10-19, incorrect, This answer disregards the power of 3 on the [OH] ion.

    C. 6.9 x 10-37, correct.

    D. cannot be determined without knowing the [OH] concentration, incorrect, The [OH] concentration is known by the multiplying the given [Fe3+] concentration by 3.

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