What are important question in number system and polynomials class 9.

state of any element:(1) The total charge of a stable compound is always equal to zero (meaning no charge).For example, the H2O molecule exists as a neutrally charged substance.(2) If the substance is an ion (either there is a positive or negative charge) the total oxidation state of the ion is the charge (i.e. oxidation state of NO-13 ion is -1).(3) All elements from Group 1A has an oxidation state of +1 (e.g. Na+1 , Li+1 ). All Group 2A and 3A elements have an oxidation state of +2 and +3, respectively. (e.g. Ca2+ , Mg2+ , Al3+ )(4) Oxygen always have a charge -2 except for peroxide ion ( O2-2 ) which has a charge of -1.(5) Hydrogen always have a charge of +1 if it is bonded with a non-metal (as in the case of HCl ) and always have a -1 charge if it is bonded with a metal (as in AlH3 ).So for your problem, according to rule 2, the overall oxidation state of NO−3 is -1.NO−3 = -1According to rule 4, the O atom should have a -2 charge. But since the oxygen has a subscript in the formula, we need to multiply the charge by the subscript.x + [(-2) (3)] = -1 where x is the oxidation state on N atomsolving for x ,x + (-6) = -1x = -1 + (+6)x = +5Therefore, answer is N5+