Answer to Question #153007 in General Chemistry for Joy

Question

Consider the equilibrium reaction: YZ2(s) ⇌ Y2+(aq) + 2Z-(aq). What is the molar solubility of YZ₂ (Ksp= 4.0 x 10⁻¹¹) at 25⁰C in a 0.03 M AZ₂ solution? Assume that AZ₂ is a strong electrolyte.

Answer 1

Because the stoichiometry of the reaction is four NH₃ to one Cu²⁺, the amount of NH₃ required to react completely with the Cu²⁺ is 4(0.0846) = 0.338 M. The concentration of ammonia after complete reaction is 1.00 M − 0.338 M = 0.66 M. These results are summarized in the first two lines of the following table. Because the equilibrium constant for the reaction is large (2.1 × 10¹³), the equilibrium will lie far to the right. Thus we will assume that the formation of [Cu(NH₃)₄]²⁺ in the first step is complete and allow some of it to dissociate into Cu²⁺ and NH₃ until equilibrium has been reached. If we define x as the amount of Cu²⁺ produced by the dissociation reaction, then the stoichiometry of the reaction tells us that the change in the concentration of [Cu(NH₃)₄]²⁺ is −x, and the change in the concentration of ammonia is +4x, as indicated in the table. The final concentrations of all species (in the bottom row of the table) are the sums of the concentrations after complete reaction and the changes in concentrations.