The chemical formula for water is H₂O.

Sodium thiosulfate and water can exist in solution together, but they don't form a new compound with a unique chemical formula.

If you're asking about a specific hydration state of sodium thiosulfate, such as sodium thiosulfate pentahydrate, then the formula would be Na₂S₂O₃·5H₂O. This indicates five water molecules are associated with each sodium thiosulfate molecule.