Suppose that a 15-mL sample of a solution is to be tested for I- ion by addition of 1 drop (0.20 mL) of 0.30 M Pb(NO3)2. What is the minimum number of grams of I- that must be present for PbI2(s) to form? (Ksp = 7.9 10-9)

Question

Suppose that a 15-mL sample of a solution is to be tested for I- ion by addition of 1 drop (0.20 mL) of 0.30 M Pb(NO3)2. What is the minimum number of grams of I- that must be present for PbI2(s) to form? (Ksp = 7.9  10-9)

DrBob222 · Answer

PbI2 ==> Pb^2+ + 2I^-

Ksp = (Pb^2+)(I^-)^2
0.2mL is a big drop but the concn Pb^2+ in the 15 mL sample will be 0.3M x 0.2/15 = ?
Substitute Ksp, ? for Pb^2+, and solve for (I^-) in moles/L.
Convert to moles in 15 mL, then to grams. g = mols x molar mass.