Here we say that the average charge is the charge an element possesses from the overall charge of all its resident structures. Here it says determine the average charge of the oxygen atom within the phosphate ion. So phosphate ion basically has phosphate double bonded to one oxygen but then single bonded to all the others. It gives us 4 resonant structures.
Now, if only given the molecular formula, then draw one of the resonance structures, and if given multiple resonance structures, again just choose one. Here we're going to calculate the formal charge for the elements and add them to determine their overall charge. So we're going to focus on one of them. Let's focus on this one. Remember formal charge equals group number minus the bonds the element is making plus each nonbonding electron calculated or added individually.
So let's say that the double bonded oxygen is oxygen A and all these single bonded ones are oxygen B. So oxygen A is in Group 6A. It's making two bonds and it has four electrons that are not bonding, so its formal charge is 0. Then oxygen B, they are in Group 6A as well. One of them is making one bond and it has six non-bonding electrons, so this one is 0 and these are all -1.
So now we're going to add up all the formal charges together, so 0+-1+-1+-1=-3 for the overall charge. Step 4 says divide the overall charge by the total number of those elements. So we have a -3 overall charge, and we're dealing with four oxygen. So each of those oxygens would have a charge of -34, so we'd say their average charges is -34.
It's as simple as that. Draw the molecule correctly. Then you're going to figure out the total overall charge of all the elements in question and divide by the total number that exists. That gives us each of their individual or average charges.