Calculate the concentration of IO3– in a 4.73 mM Pb(NO3)2 solution saturated with Pb(IO3)2. The Ksp of Pb(IO3)2 is 2.5 × 10-13. Assume that Pb(IO3)2 is a negligible source of Pb2 compared to Pb(NO3)2.

Answer 1

This is a Ksp problem with a common ion of Pb from the Pb(NO3)2.

..........Pb(IO3)2 ==> Pb^2+ + 2IO3^-
I.........solid.........0........0
C.........solid.........x........2x
E.........solid.........x........2x

Pb(NO3)2 is 100% dissociated.

........Pb(NO3)2 ==> Pb^2+ + 2NO3^-
I.......4.73 mM........0........0
C......-4.73 mM......4.73mM..2*4.73mM
E........0...........4.73mM..9.46 mM

Ksp Pb(IO3)2 = (Pb^2+)(IO3^-)^2
Ksp = 2.5E-13
(Pb^2+) = x from Pb(IO3)2 which the problem tells you to ignore + 4.73 mM from the Pb(NO3). Convert this to 0.00473M
(IO3^-) is 2x
Substitute into the Ksp expression as follows:
2.5E-13 = (0.00473)(2x)^2 and solve for x = (IO3). The unit will be M or mols/L.