Why does adding sodium chloride to a saturated solution of lead chloride reduce
the solubility of PbCl2?
Respuestas
Respuesta:
Explicación:
This is a standard problem in solubility equilibria; the solubility of PbCl2 depends on the concentration of ions FROM ALL SOURCES.
Explanation:
We can write the solubility expression in this way:
PbCl2(s)⇌Pb2++2Cl−, is the reaction, AND
Ksp=[Pb2+][Cl−]2 is the solubility expression; sp stands for solubility product, and Ksp has been measured for a variety of salts at various temperatures.
The solubility expression depends on just the concentration of the lead and chloride ions; from where the ions come makes no difference. So if [Cl−] is artificially raised (by adding sodium chloride), it logically follows that [Pb2+] must be correspondingly reduced in order for the solubility expression to be obeyed. The only way for [Pb2+] to be reduced is for more PbCl2 to precipitate. The same effect would pertain if I added lead nitrate, a soluble lead salt. [Pb2+] would increase, and how would [Cl−] evolve?
Such a phenomenon is known as salting out. It pushes the solubility equilibrium to the left hand side as WRITTEN. IF you were trying to isolate a precious metal, i.e. gold or platinum or rhodium, you want to salt out the precious metal salt and leave little of its ions in solution.