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 dose calculation on the basis of body surface area results in higher doses in young children than in older patients ( 16 to 28 mg / kg ) .
### Output Text:
The dose calculation on the basis of body surface area results in higher doses in young children than in older patients ( 16 to 28 mg / kg ) . 