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What is the Cr3+ concentration when 0.010 mol of Cr(NO3)3 is dissolved in a liter of solution buffered at pH of 10.0. Cr3+ forms a complex ion with hydroxide shown below:

Cr3+ (aq) + 4 OH (a)⇌ Cr(OH)4–            Kf = 8 x 1029

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pOH = 14.0 – 10.0 = 4.0 

[OH ] = antilog pOH = 1.0 x 10–4

Due to the large Kf all of Cr3+ is converted to the complex ion, and some subsequently dissociates back to Cr3+. Then at equilibrium,

Note: Since the solution is buffered, [OH] will remain constant during the reaction.

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