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.

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