Question

Sodium sulfate is slowly added to a solution containing 0.0500 M Ca2 (aq) and 0.0370 M Ag (aq). What will be the concentration of Ca2 (aq) when Ag2SO4(s) begins to precipitate?

Answer 1

I looked up the Ksp values. It would have helped if you had since my values in the book I used for Ksp values probably will not agreed with the values in your text. My book is old.
Ag2SO4 ==> 2Ag^+ + SO4^2-
Ksp = (Ag^+)^2(SO4^2-)
Plug in Ag^+ and solve for SO4. I obtained approximately 0.009M

CaSO4 ==> Ca^2+ + SO4^2-
Do the same. SO4^2- = approx 0.001M

You can see that if you have a solution containing those two ion in those concns that the CaSO4 will ppt first since Ksp for CaSO4 will be exceeded first.
CaSO4 will continue to ppt until the (Ca^2+) has been diminished to the point that Ag2SO4 can ppt. When that happens the (SO4^2-) will be 0.009.
Plug 0.009 in the Ksp expression for CaSO4 and calculate (Ca^2+).