Final answer:
There are 18 constitutional isomers for the compound with the molecular formula C8H18, which include various branched and unbranched structures.
Step-by-step explanation:
The question concerns the number of constitutional isomers for a compound with the molecular formula C8H18. Constitutional isomers are compounds with the same molecular formula but different structural connectivities. To determine the number of constitutional isomers for C8H18, we need to consider all the possible different arrangements of eight carbon atoms and eighteen hydrogen atoms that result in distinct structures. These arrangements take the form of linear or branched chains of carbon atoms.
For C8H18, there are 18 constitutional isomers. These include the straight chain n-octane and various branched forms such as isooctane (2,2,4-trimethylpentane), which is used as the standard for the gasoline octane rating, as well as other less branched isomers.