Determining the Chloride amount in an unknown salt with explanations of the techniques used in the process of gravimetric analysis.
Theory:.
The Chloride in the unknown salt is precipitated by the reaction of Chloride ions with Silver ions producing Silver Chloride which is insoluble in water making it possible to collect it as a precipitate later on through filtration. The above process is shown in the equation below:.
Ag+(aq) + Clï€(aq) ï‚® AgCl(s).
Though the produced salt is insoluble, some small amount of the precipitate does dissolve in water as shown in the equation below:.
AgCl(s) ï‚® Ag+(aq) + Cl-(aq) .
Looking at the above two equations it is clear that the precipitation reaction is reversible. As a result some of the precipitate returns back to separate Silver and Chloride ions. The greatest chance of solubility of the precipitate back to ions occurs when the exact amount of Silver ions are added to precipitate the chloride ions making the system in equilibrium hence it is preferred that some of excess Silver salt is to be added to make the reversible reaction favor the right hand side of the equation that is shown below to obtain as much of the chloride ions in the unknown salt as possible for accurate results.
AgCl(s) ⇌ Ag+(aq) + Cl-(aq) .
Nevertheless, the amount or concentration of salt that does dissolve back or stay in the solution is expressed using the Solubility product; Ksp which indicates how many moles per liter of the salt dissolves back in the solution .
For Silver Chloride salt the Ksp = 1.6 x 10-10 moles per liter which is a very small value that it is considered negligible. If we consider worst case scenario where the maximum amount of chloride ions dissolve and remain in the solution the following concept helps to calculate the solubility of the Chloride ions: For this reaction, each mole of AgCl that dissolves produces 1 mole of both Ag+ ions and Cl- ions.