Hello candidate,

The formula for calculating out the formal charge of a compound is given by the formula- Formal Charge = [Number of valence electrons on atom] – [number of non-bonded electrons + number of bonds].

Hence, using this formula we get the formal charge on phosphate ion equal to -1, but on experimental basis it comes out to be -.75 because of the involvement of valence electrons in resonance effect other than just chemical bonding .

Hope you found it helpful!