Step 1:
Write down Chemical formula, Overall compound is neutral, so overall charge on it is zero,
Na₃AsO₂ = 0
Step 2:
Write down the oxidation state of known elements, i.e O.S of Na is +1, and O.S of O is -2, Now put these values in above formula.
(+1)₃ + As + (-2)₂ = 0
Step 3:
Now solve for As,
(+1)₃ + As + (-2)₂ = 0
+3 + As - 4 = 0
As = 4 - 3
As = +1
Result:
Oxidation state of Arsenic in Na₃AsO₂ is +1.
