Final answer:
To precipitate calcium ions (Ca2+) and not ammonium ions (NH4+), one can add a sulfate ion solution, causing calcium sulfate to form and precipitate due to its low solubility.
Step-by-step explanation:
To precipitate one cation while keeping the other cation in solution from a mixture of NH4+ (aq) and Ca2+ (aq), one could add a solution containing sulfate ions, such as Na2SO4. According to the solubility rules, calcium sulfate (CaSO4) has low solubility and will likely precipitate out of the solution. This is because the solvated ions reach a concentration at which their product exceeds the Ksp (solubility product) for calcium sulfate, leading to precipitation. On the other hand, ammonium sulfate ((NH4)2SO4) formed from the combination of NH4+ and SO42- ions is highly soluble and will remain in solution.