Define oxidation number. Balance the following equation using oxidation number method.
`As_(2)S_(3)+HNO_(3)+H_2O to H_3AsO_4+H_2SO_4+NO`

Explanation

Oxidation number : It is defined as the imaginary the compound have been removed in their usual oxidation states that are assigned according to set of rules.
Step 1 : `As_(2)S_(3)+HNO_(3)+H_(2)OrarrH_(3)AsO_(4)+H_(2)SO_(4)+NO`
Step 2 : Assign the oxidation numbers and identify the redox couples.

Step 3 : Multiply `As_(2)S_(3)"by "3 and HNO_(3)" by "2.`
3 `As_(2)S_(3)+2HNO_(3)+H_(2)OrarrH_(3)AsO_(4)+H_(2)SO_(4)+NO`
Step 4 : Balance all the elements in the equation (As,S and N) except H and O. `3As_(2)S_(3)+2HNO_(3)+H_(2)Orarr6H_(3)AsO_(4)+9H_(2)SO_(4)+2NO`
Step 5 : Balance the complete equation including O & H.
`3As_(2)S_(3)+28HNO_(3)+4H_(2)Orarr6H_(3)AsO_(4)+9H_(2)SO_(4)+28NO`