Final answer:
The molar solubility of a slightly soluble salt decreases when a common ion from another source is present. This change occurs due to the common ion effect where the added ion shifts the equilibrium towards the solid state. However, making the solution more acidic can increase solubility for salts with anions that are conjugate bases of weak acids.
Step-by-step explanation:
The molar solubility of a slightly soluble salt is affected by the presence of a common ion from another source due to the common ion effect. When an ionic compound is dissolved in water, it reaches a dynamic equilibrium represented by its solubility product constant (Ksp). Adding another salt that releases an ion already present in the equilibrium will shift the equilibrium towards the solid state, reducing the solubility of the original compound. This can be seen in the example of adding CaCl2 to a solution of calcium phosphate Ca3(PO4)2, which will increase the concentration of Ca2+ ions, causing more Ca3(PO4)2 to precipitate and thus reducing its solubility.
On the other hand, when the pH of the solution is made more acidic, it can increase the solubility of sparingly soluble salts whose anion is the conjugate base of a weak acid. The added H+ ions would react with the anion, lowering its concentration in the solution, and leading to the dissolution of more solid salt to restore equilibrium. This is an application of Le Chatelier's principle.