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: The serum magnesium concentration was 3 . 0 mEq / L , which is usually well tolerated .
### Output Text:
The serum <span class="chemical">magnesium</span> concentration was 3 . 0 mEq / L , which is usually well tolerated . 