The student placed 10 ml of pbcl2 (saturated solution) in the test tube and added a pinch of nacl. a white precipitate of pbcl2 formed. how will this affect the concentration of pb2+?

Adding NaCl to a saturated PbCl2 solution will decrease the Pb2+ concentration.

Explanation:

If a solution is saturated this means it cannot dissolve more material.  

PbCl2 ⇆ Pb2+ + 2Cl-

Adding more Cl- in the solution is the same thing as trying to dissolve more of the material. In the solution, the concentration of Cl- will increase.

In order for the Ksp to maintain a constant value, the concentration of Pb2+ must decrease if the concentration of Cl- is increased.

Adding NaCl to a saturated PbCl2 solution will decrease the Pb2+ concentration.

they are all solids.