Solid Na2SO4 is added to a solution which is 0.014 M in Pb(NO3)2 and 0.041 M in AgNO3. Assume the volume remains constant. Ksp = 2.0 10-8 for PbSO4 and Ksp = 1.2 10-5 for Ag2SO4.

Question

Solid Na2SO4 is added to a solution which is 0.014 M in Pb(NO3)2 and 0.041 M in AgNO3. Assume the volume remains constant. Ksp = 2.0  10-8 for PbSO4 and Ksp = 1.2  10-5 for Ag2SO4.
(a) Which compound precipitates first?
1 PbSO4
 (b) What is the concentration of the first ion precipitated when the second ion starts to precipitate?

I NEED HELP WITH PART B THANK YOU!

DrBob 222 · Answer

Divide Ksp of one by the other.
[KspPbSO4/KspAg2SO4]=[(Pb^+2)(SO4^=)/(Ag^+)^2(SO4^=)].
Note (SO4^=) cancels and you are left with
2*10^-8/1.2*10^-5 = (Pb^+2)(Ag^+)^2
Plug in (Ag^+)^2 and solve for (Pb^+2), Check my thinking.