Can anyone tell me how to calculate the amount of water that needs to evaporate for a solution to precipitate?

exact problem from AP practice test

(d) An open container holds 1.000 L of 0.00400 M PbCl2, which is unsaturated at 25C. Calculate the minimum water, in mL, that must evaporate from the container before solid PbCl2 can preciptate.

Ksp=1.6X 10^-5 at 25C.

PLEASE HELP! i have to have this done for monday! Thanks!