The oxidation number of phosphorus in PO3/4 is +5.
To determine the oxidation number of phosphorus, we can use the fact that the sum of the oxidation numbers of all atoms in a neutral compound is zero. Since oxygen has an oxidation number of -2, we can set up the equation:
x + 4(-2) = 0
where x is the oxidation number of phosphorus. Solving for x gives:
x = +5
Therefore, the oxidation number of phosphorus in PO3/4 is +5.