A sample of natural rubber (cis-1,4-polyisoprene) is vulcanized such that one of every 240 chain carbon atoms is cross-linked. The formula unit of the isoprene monomer is \( C_5H_8 \) (molecular weight = 68 g/mol). The average molecular weight (rounded off to nearest integer) between cross-links is \(\underline{\hspace{2cm}}\) g/mol.