Anhydrous sodium sulfate is a drying agent*, which absorbs water to form hydrates.
How much drying agent should you add?
Well, usually one or two scoopfuls is enough, if the solution was pre-dried.
Make sure that there are freely moving particles of drying agent upon swirling (i.e., the drying agent is not all clumped).
Wait for 10-20 minutes before you remove the drying agent.
Organic Web Chem University of Alberta