Prepicting Precipitation calculating mixed concentrations
MX is an example of a solid that is slightly soluble in water. The equation for the equilibrium between dissolved substance and its ions as well as the reaction quotient expression Qs is given below.
Comparing the value of Qs for a solution containing the cation and anion of an sparingly soluble solid with the equilibrium constant Ks for the slightly soluble solid allows one to establish whether precipitate will form for a given solution or when two solutions are mixed.
If two solutions, each containing one of the ions of the sparingly soluble solid in known concentration, are mixed, the ion concentrations used to calculate Qs must be those in the mixture. These can be calculated by
MX(s)  M+(aq) + X–(aq) | Qs = [M+][X–] |
Comparing the value of Qs for a solution containing the cation and anion of an sparingly soluble solid with the equilibrium constant Ks for the slightly soluble solid allows one to establish whether precipitate will form for a given solution or when two solutions are mixed.
If two solutions, each containing one of the ions of the sparingly soluble solid in known concentration, are mixed, the ion concentrations used to calculate Qs must be those in the mixture. These can be calculated by
- calculating the amount in moles of the solute present in the original solution (n = cV where c and V are the concentration and volume of the solution containing that ion before mixing).
- multiplying that amount in moles by the number of that type of ion present in the solute
- dividing the result by the total volume in litres of the solution after mixing.