The degree of unsaturation or double bond equivalents(DBE) can be determined by using a formula. This formula gives us the number of rings and / pi bonds.
As mentioned in the problem the number of pi bond is one so if we calculate the DBE this will give us the number of rings by subtracting one from DBE.
There is a rule in naming polyatomic anions containing oxygen.
Those names ending in <em>–ate</em> are given to the most common oxyanion of an element.
While those names ending in <em>–ite</em> are for those that contains fewer O atom for
the same charge. Since P has fewer O atoms, its name ends in <em>–ite</em>.
Thus, P is phosphite while P is phosphate.