Here's how to determine the molecular formula for an 18-carbon alkane:

* Alkanes are hydrocarbons with only single bonds between carbon atoms. Their general formula is C_nH_2n+2, where 'n' is the number of carbon atoms.

* For an 18-carbon alkane:

* n = 18

* Substitute into the general formula: C₁₈H_(2*18)+2

* Simplify: C₁₈H₃₈

Therefore, the molecular formula for an 18-carbon alkane is C₁₈H₃₈.