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: Dissociated learning of rats in the normal state and the state of amnesia produced by pentobarbital ( 15 mg / kg , ip ) was carried out .
### Output Text:
Dissociated learning of rats in the normal state and the state of amnesia produced by <span class="chemical">pentobarbital</span> ( 15 mg / kg , ip ) was carried out . 