The chemical formula for an alkane with 42 carbons is C₄₂H₈₆.

Here's how to determine the formula:

* Alkanes have the general formula C_nH_2n+2, where 'n' is the number of carbon atoms.

* Substitute 'n' with 42: C₄₂H_(2*42)+2

* Simplify: C₄₂H₈₆