Question

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^–            K_f = 8 x 10²⁹

Answer 1

pOH = 14.0 – 10.0 = 4.0 

[OH^– ] = antilog pOH = 1.0 x 10^–4

Due to the large K_f 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.