Final answer:
The structural formula of an alkane with eight carbon atoms and only primary hydrogen atoms is C8H18.
Step-by-step explanation:
An alkane with eight carbon atoms that only has primary hydrogen atoms can be represented by the structural formula C8H18. In this alkane, each carbon atom is bonded to four other atoms, either carbon or hydrogen. The carbon chain of the alkane is a straight line, and the bond angles are close to 109.5°.