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: Confusion mostly occurred during the two first weeks following valproic acid exposure ( 39 . 7 % ) .
### Output Text:
Confusion mostly occurred during the two first weeks following <span class="chemical">valproic acid</span> exposure ( 39 . 7 % ) . 