In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Median follow - up was 11 . 8 months ( range of those surviving : 2 . 7 - 11 . 8 months ) .
### Output Text:
Median follow - up was 11 . 8 months ( range of those surviving : 2 . 7 - 11 . 8 months ) . 