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: RESULTS : A total of 49 patients ( median age : 60 ; range 41 - 81 ) entered this phase 2 trial .
### Output Text:
RESULTS : A total of 49 patients ( median age : 60 ; range 41 - 81 ) entered this phase 2 trial . 