The equilibrium constant for the reaction Cu3(PO4)2(s) 3 Cu2+(aq) + 2 PO43–(aq) is K = 1.40 x 10–37. Suppose that a beaker is filled to 1 L with a solution containing

Question

[Cu2+] = [PO43–] = 10.0 nM, and 300.0 g Cu3(PO4)2 . Which of the following observations is true?

a. 
Q < K, and products must be converted to reactants for the reaction to reach
equilibrium.
b. 
Q < K, and reactants must be converted to products for the reaction to reach
equilibrium.
c. 
Q > K, and products must be converted to reactants for the reaction to reach
equilibrium.
d. 
Q > K, and reactants must be converted to products for the reaction to reach
equilibrium.
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Ok, So my first thought is to write the expression for Q:

Q = [Cu+2]^3[PO4-3]^2

Substitute 10.0nM --> 10 x 10^-9

Q= [10 x 10^-9]^5
= 1 x 10^-40

I don't know what to do with the 300.0g Cu3(PO4)2. Help please.

Answer 1

So far you are good. So Q = 1E-40 < K = 1.4E-37 which lets you know c and d can't be right. The answer must be a or b. Now write the Keq expression.
Keq (which is really Ksp) =
Cu3(PO4)2 = (Cu^2+)^3(PO4^3-)^2 = 1.4E-37
Q must be larger. How can it get larger? No way except for Cu^2+ and PO4^3- to increase so some of the solid MUST dissolve in order to increase the ion concentrations. What did I do with the 300 g Cu3(PO4)2. I said, aha, so it has SOME solid (in order for Ksp to be a Ksp there must be at least a speck of solid in the beaker and 300 g qualifies as a speck). Another way of saying this is that the ions concentrations are not high enough so some of the solid must dissolve in order to reach the equilibrium state. So the answer is ........?

Answer 2

thank you :)