Which pair of compounds are isomers?
(1) no2 and n2o4
(2) p2o5 and p4o10
(3) hcooh and ch3cooh
(4) ch3och3 and c2h5oh

and

Explanation: The compounds having similar molecular formula but different arrangement of atoms or groups in space are called isomers and the phenomenon is called as isomerism.

and are functional isomers which have same molecular formula but different functional groups attached.

(1) and are different compounds.

(2)   and are different compounds.

(3) and are different compounds.

50.23 i believe is correct but don't quote me on that.