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: Beta - 2 - adrenoceptor - mediated hypokalemia and its abolishment by oxprenolol .
### Output Text:
Beta - 2 - adrenoceptor - mediated hypokalemia and its abolishment by <span class="chemical">oxprenolol</span> . 