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MCAT Chemistry Question — Solubility Product

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

The math works out to (4 x 10-10) x (27 x 64 x 10-30), and only the correct choice c) is close enough.

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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