The oxidation number of As in Na3AsO2 is

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.