How do we describe the solution behaviour of #PbCl_2(aq)#?

1 Answer
Mar 21, 2016

Answer:

Lead(II) chloride is very insoluble in aqueous solution. The filtrate is saturated with respect to #Pb^(2+)#.

Explanation:

A saturated solution is a solution in which the concentration of the solute is equal to THAT WHICH WOULD BE IN EQUILIBRIUM WITH UNDISSOLVED SOLUTE:

#PbCl_2(s) rightleftharpoons PbCl_2(aq)#, i.e.

#PbCl_2(s) rightleftharpoons Pb^(2+) + 2Cl^-#

The supernatant solution (the solution that lies atop undissolved solute) is said to be saturated with respect to lead chloride. Normally, a temperature is specified because a hot solution can usually hold more solute than a cold one.

AS for any equilibrium, we can express the reaction in terms of an equilibrium constant:

#K_(sp) = [Pb^(2+)][Cl^-]^2# #=# #"A small number"#. The subscript #"sp"# stands for solubility product. These are extensively tabulated for a range of sparingly soluble and insoluble salts; normally, laboratory temperature, #298# #K# is specified. So if #[Cl^-]# is artificially increased (by adding #Cl^-#), the ion product, #[Pb^(2+)][Cl^-]^2# will be momentarily GREATER than #K_(sp)#. In order to obey the #K_(sp)# expression, lead chloride will precipitate in order to reduce #[Pb^(2+)]# and #[Cl^-]#.

I acknowledge that I have gone on a long time about nothing to the power of less. The important definition is the following:

A saturated solution is a solution in which the concentration of the solute is equal to THAT WHICH WOULD BE IN EQUILIBRIUM WITH UNDISSOLVED SOLUTE.

An examiner would be quite justified in rejecting a definition of saturated solution as a solution in which the solvent holds all the solute that it can .