Final answer:
Additional information such as mass or solution concentration and volume is needed to calculate the number of moles of phosphorus as phosphate (PO4^3-); chemical equations involving phosphorus compounds like H3PO4, Ca3(PO4)2, and PCl5 involve balancing phosphate ions as a unit.
Step-by-step explanation:
The question, "If phosphorus is present as PO43-, how many moles are there?" cannot be answered without additional information. To determine the number of moles of a substance, one must know either the mass of the substance or the concentration and volume of its solution.
In reference to the given information, when balancing chemical equations, the polyatomic ion phosphate (PO43-) is often treated as a unit. For example, when phosphoric acid (H3PO4) reacts with calcium hydroxide (Ca(OH)2) to form calcium phosphate (Ca3(PO4)2), the phosphate ions are balanced as a unit. The molar ratio is used to determine how many moles of reactants are needed to produce a certain amount of product, such as in the preparation of phosphorus from calcium phosphate or in the reaction of PCl5 with water to produce POCl3.