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: Omeprazole is the first drug designed to block the final step in the acid secretory process within the parietal cell .
### Output Text:
<span class="chemical">Omeprazole</span> is the first drug designed to block the final step in the acid secretory process within the parietal cell . 