Question

1)Suppose a solution contains 0.26 M Pb2 and 0.45 M Al3 . Calculate the pH range that would allow Al(OH)3 to precipitate but not Pb(OH)2. The Ksp values for Al(OH)3 and Pb(OH)2 can be found here

2a)Calculate the concentration of IO3– in a 3.89 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.

2b)A different solution contains dissolved NaIO3. What is the concentration of NaIO3 if adding excess Pb(IO3)2(s) produces [Pb2 ] = 5.30 × 10-6 M?

Answer 1

[Pb2+] = 0.26 M

[Al3+] = 0.45 M

Ksp of Al(OH)3 = 4*10^-15

Ksp of Pb(OH)2 = 4*10^-15

Al(OH)3    Al3+ + 3OH-

Ksp = [Al3+]*[OH-]³

4*10^-15 = 0.45 * [OH-]³

[OH-] = 2.07*10^-5 M

pOH = - log (2.07*10^-5)

= 4.68

pH = 14 - 4.68

pH = 9.32

Pb(OH)2        Pb2+ + 2OH-

Ksp = [Pb2+][OH-]²

4*10^-15 = 0.26 * [OH-]²

[OH-]² = 15.38*10^-15

[OH-] = 1.24*10^-7

pOH = - log (1.24*10^-7)

= 6.9

pH = 14 - 6.9

pH = 7.1

pH range 9.32 to 14 would allow Al(OH)3 to precipitate but not Pb(OH)2.