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: Habitual use of acetaminophen as a risk factor for chronic renal failure : a comparison with phenacetin .
### Output Text:
Habitual use of <span class="chemical">acetaminophen</span> as a risk factor for chronic renal failure : a comparison with <span class="chemical">phenacetin</span> . 