ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

The solubility of a compound with phosphate ions varies based on the cation it is paired with. Potassium phosphate, which contains potassium as the cation, tends to be soluble in water. This is largely due to the general rule that salts of alkali metals, like potassium, are soluble in water.

In contrast, compounds like calcium phosphate and iron phosphate tend to form precipitates in aqueous solutions due to their lower solubility. Calcium phosphate is known to have low solubility because of the common ion effect and the relatively small solubility product constant (Ksp) associated with it. Similarly, iron phosphate is also insoluble in water under most conditions. Thus, when evaluating the solubility of various phosphates, potassium phosphate stands out as the soluble option in the presence of phosphate ions.