Question

12. If a solution is 0.25 Min Ba2+ and 0.50 MPb+ upon addition of Na2CrO4(s) which solid will precipitate first and at what concentration of Cr042-will the second solid begin to form? Ksp(BaCrO.) 2.1 × 10-10 Ksp(PbCrO.) 2.3x 10-13 A. BaCrO4 will precipitate first, a [Cro,--] = 8.4 × 10-10 M will cause PbCrO4(s) to form. B. BaCrO4 will precipitate first, a [Cr042-] = 4.6 x 10-13 M will cause PbCrO4(s) to form. C. PbCrO4 will precipitate first, a [CrO41 8.4 x 1010 M will cause BaCr04(s) to form. D. PbCrO2 will precipitate first, a [Cr042-] = 4.6 10-13 M will cause BaCrO4(s) to form.

Answer 1

12. Selective precipitation of Ba2+ and Pb2+ with CrO4^2- addition,

(C) PbCrO4 will precipitate first and at [CrO4^2-] = 8.1 x 10^-10 M BaCrO4 will begin precipitation.

Among the two, PbCrO4 has a lower Ksp value, that means it will precipitate first from the solution (BaCrO4 remains soluble at this stage).

[CrO4^2-] concentration for BaCrO4 precipitation = Ksp/[Ba2+]

                                                                               = 2.1 x 10^-10/0.25 = 8.4 x 10^-10 M

11. The solubility of solids that increase in acidic medium are,

C. I and II

These solids in solution forms,

I. Ni(OH)2(s) <==> Ni2+(aq) + 2OH-(aq)

Now, OH- reacts with added acid and forms H2O(l), thereby decreasing concentration of OH- on the right handside. According to LeChatellier's principle, more solid Ni(OH)2 would dissolve to increase OH- concentration and reattain equilibrium state.

II. Mg3(PO4)2(s) <==> 3Mg2+(aq) + 2PO4^3-(aq)

Now, PO4^3- reacts with added acid and forms HPO4^2-(aq), thereby decreasing concentration of PO4^3- on the right handside. According to LeChatellier's principle, more solid Mg3(PO4)2 would dissolve to increase PO4^3- concentration and reattain equilibrium state.

14. For the given mixing of solutions, the true statement would be,

D. Qsp is greater than Ksp for CoCO3 so CoCO3 solid forms

Qsp = [Co2+][CO3^2-]

       = (4 x 10^-3 M x 10 ml/50 ml)(0.001 M x 40 ml/50 ml) = 6.4 x 10^-7

So, Qsp > Ksp, therefore, solution is saturated and precipitate would form.