Question

Assuming that no equilibria other than dissolution are involved, calculate the concentration of all solute species in each of the following solutions of salts in contact with a solution containing a common ion. Show that it is not appropriate to neglect the changes in the initial concentrations of the common ions.

a) MgSO4 in contact with a Mg2+ common ion solution
b) Al(OH)3 in contact with an OH− common ion solution
c) Cu(NO3)2 in contact with a NO3− common ion solution
d) NH4Cl in contact with a NH4+ common ion solution

Answer 1

The common ion effect significantly impacts the solubility of ionic compounds in solution and cannot be neglected. When a common ion is present, solubility generally decreases, and the equilibrium position shifts in favor of the undissolved solid.

Step-by-step explanation:

The student's question concerns the calculation of solute species concentrations in solutions containing common ions. Specifically, they are considering salts that dissolve to give ions that are already present in the solution. For instance, when MgSO4 is dissolved in a solution that already contains Mg2+, due to the common ion effect, the solubility of MgSO4 would decrease compared to when it is dissolved in pure water.

The addition of a common ion will generally shift the solubility equilibrium to favor the formation of the undissolved solid, a concept based on Le Chatelier's principle. This means that when calculating the concentrations of solute species in the presence of a common ion, one must consider the initial concentration of the common ion already present in the solution and cannot be neglected as it has a significant effect on the final equilibrium state.