Q. What is the oxidation number of phosphorous in PO4 3-?
Let us consider the oxidation of phosphorous to be y
Also, The oxidation number of oxygen is -2
here the ion has three negative charges.
y + (-2) × 4 = -3
y – 8 = -3
y=+5
Hence, the oxidation number of P in PO4 3- is +5.