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: A 3 mg / kg ( i . p . ) dose induced this response in all of the mice tested and the peak response of 73 jerks per min was observed between 27 and 45 min .
### Output Text:
A 3 mg / kg ( i . p . ) dose induced this response in all of the mice tested and the peak response of 73 jerks per min was observed between 27 and 45 min . 