Which pair of compounds are isomers?
1) ch3ch2ch3 and ch2chch3
2) ch2ch2 and ch3coch3
3) ch3och3 and ch3coch3
4) ch3coch3 and ch3ch2cho

i'd like to know the steps to answering this question.

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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 containing 3 carbon atoms, 6 hydrogen atoms and one oxygen atom each but different functional groups attached.

Rest compounds do not have similar molecular formulas.

Thus and are isomers.

the answer to this question is 3.