If we add PbNO3 liquid to water, can we observe the precipitate of Pb(OH)2?

ajan_antonian@yahoo.com · science forum beginner Joined: 16 Apr 2006 Posts: 6

The solubility product of Pb(OH)2 is 2.5x10-16.In water [OH-] is
10-7moldm-3.At a point [Pb2+] will be 10-1.
So,=[Pb2+][OH-]
=0.1x(10-7)2
=0.1x10-14
=10-15mol3dm-9
So, there should be a precipitate of Pb(OH)2. But in our theory I
haven't met any point like this.

tressure@gmail.com · science forum Guru Wannabe Joined: 28 Oct 2005 Posts: 102

To add to what Bob said, Ksp values aren't exact methods of determining
solubility, because they ignore the formation of other species such as
PbOH+ or [Pb(OH)3]-, the formation of which will both increase
solubility of lead salts and decrease the pH.

Don't have any figures on lead nitrate or hydroxide, but I seem to
recall that the solubility of lead iodide (not pH dependant, or at
least much less so than the hydroxide) is about ten times as much as
would be predicted from the Ksp.

Oscar Lanzi III · science forum Guru Wannabe Joined: 30 Apr 2005 Posts: 176

I would add more to this about using ion products to assess solubility.
Say you are given a solubility product Ksp = 10^(-52) for HgS. So
[Hg(2+)] = [S(2-)] and [Hg(2+)]*[S(2-)] = 10^(-52), meaning each ion
separartely has a concentratikon of 10^(-26) mol/L.

OK, HgS has a low solubility in water, but 10^(-26) mol/L is lower than
low. One mole per liter is Avogadro's number of units per liter, and
Avogadro's number is only 6.02*10^(23). So 10^(-26) mol/L = 0.00602
unit/L is not possible unless you have almost 200 liters in your sample
and then happen to spot one Hg(2+) ion and one S(2-) ion in that vast
expanse.

Since 200-L beakers are rather hard to come by in most chemistry labs,
this can't be right. We have to assume a different solution model,
involving complex Hg-S species that have some reasonable presence in
typical lab samples. Such species behave thermodynamically like Hg(2+)
and S(2-) at super-low concentrations, but in material balance terms
they contain a much greater amount of Hg and S than would be possible
with the separated ions -- great enough to actually measure
thermodynamic properties in 200 mL rather than 200 L.

The moral of the story is not to read too much into the Ksp values. You
have to look at what's actually in solution, which for a sparingly
soluble salt (or even a highly soluble one, for that matter) is usually
not just separate ions. Only for some solutes with single-charged ions
can you tranmslate the Ksp equation directly into a an estimate of
material solubility.

--OL

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