Final answer:
The oxidation state of phosphorus in the phosphate ion (PO43-) is +5, balancing out the -8 charge from the four oxygen atoms to give the ion's overall charge of 3-.
Step-by-step explanation:
The oxidation state of phosphorus in the phosphate ion (PO43-) can be determined by using the overall charge of the ion and the known oxidation states of oxygen. Since oxygen typically has an oxidation state of -2 and there are four oxygen atoms, the total oxidation contribution from oxygen is -8. Considering that the overall charge of the phosphate ion is 3-, phosphorus needs to have an oxidation state that balances the charge from the oxygens. Therefore, phosphorus will have an oxidation state of +5 to bring the total charge to 3- when combined with the -8 from oxygen.