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: At 687 days mice that had been on 0 . 75 mg had average platelet counts of 2 . 5 x 10 ( 6 ) .
### Output Text:
At 687 days mice that had been on 0 . 75 mg had average platelet counts of 2 . 5 x 10 ( 6 ) . 