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 charge left on the atom when all other atoms of the compound have been removed in their usual oxidation states that are assigned according to set of rules.
Step 1 : `As_2S_3+HNO_3+H_2Oto H_3AsO_4+H_2SO_4+NO`
Step 2 : Assign the oxidation number and identify the redox couples.

Step 3: Multiply `As_2S_3` by `3 and HNO_3` by 2.
`3As_2S_3+2HNO_3+H_2O to H_3As_O_4+H_2SO_4+NO`
Step 4 : Balance all the elements in the equation (As,S and N) except H and O.
`3As_2S_3+2HNO_3+H_2O to 6H_3AsO_4+9H_2SO_4+2NO`
Step 5: Balance the complete equation including O & H.
`3As_2S_3+28HNO_3+H_2O to 6H_2AsO_4+9H_2SO_4+28NO`.