Anhydrous is a state of a chemical indicating that it is free of water. The exact drying procedures vary from chemical to chemical, though most powdered salts can be produced using this procedure.
Place the salt (aqueous or solid) into the round bottom flask.
Place the round bottom flask in the heating mantel.
If the salt is aqueous, gently heat the solution until steam occurs and then keep at a constant temperature until nearly all of the water has evaporated.
Connect the 3-way valved adapter to the top of the round bottom flask using a Keck clip. Connect the vacuum line to one hose port and the nitrogen line to the other.
Open the valve to the vacuum line, reducing the pressure inside of the round bottom flask.
Heat for another 20 minutes with the vacuum line open.
Turn the valve so that the round bottom flask is filled with nitrogen.
Repeat Steps 5-7 three times then remove the flask from heat.
Close off the valve. This will trap the dry salt inside and prevent it from absorbing moisture from the air.