What is the oxidation number for all elements of AsO4^3-?
Oxygen is -2 (by assignment) and As is +5
The convention is to assign -2 to oxygen when it appears in any compound (except for
Then, we require that the total of all oxidation numbers in any molecule or ion add up to the real electric charge on that particle.
Now, you solve like an algebra problem:
As + 4(-2) = -3
As = -3 +8 = +5