The chemical formula for an alkane with 125 carbon atoms is C₁₂₅H₂₅₂.

Here's how we get that:

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

* In this case, we have 125 carbon atoms (n = 125).

* Substituting into the formula: C₁₂₅H_(2*125)+2 = C₁₂₅H₂₅₂