Final answer:
When low concentrations of NH3 are added to CuSO4, it results in the formation of the complex ion [Cu(NH3)4]2+, turning the solution from a pale blue to an intensely blue-violet color and increasing the solubility of Cu2+ due to the complex formation.
Step-by-step explanation:
When low concentrations of NH3 (ammonia) are added to CuSO4 (copper sulfate), a chemical reaction occurs that results in the formation of a complex ion, specifically [Cu(NH3)4]2+ (aq). This occurs because ammonia acts as a ligand, coordinating with the copper(II) ion, Cu2+. The pale blue color of the hydrated Cu2+ ion solution turns into an intensely blue-violet color as ammonia replaces water molecules from the hydrated ion, forming the complex ion [Cu(NH3)4]2+ (aq), which is more soluble.
The addition of ammonia not only changes the color of the solution but also increases the solubility of Cu2+ in solution because the formation of the complex ion pulls the equilibrium towards the right, allowing more CuSO4 to dissolve. If we define x as the amount of Cu2+ produced by dissociation from the complex ion, the final concentrations of all species in solution are determined by the stoichiometry of the reaction and the change in concentrations due to the dissociation until equilibrium is reached.