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: Normoammonemic encephalopathy : solely valproate induced or multiple mechanisms ?
### Output Text:
Normoammonemic encephalopathy : solely <span class="chemical">valproate</span> induced or multiple mechanisms ? 