Anhydrous sodium sulfate is added to the organic solution remaining after the extraction with 6 M HCl and NaOH to remove any residual water present in the organic phase. This is important because the presence of water can interfere with subsequent reactions or analysis of the organic compounds.

Sodium sulfate is a hygroscopic salt, meaning that it readily absorbs water from its surroundings. When added to the organic solution, it will therefore attract and bind to any water molecules present, forming hydrated sodium sulfate. This process effectively removes the water from the organic phase, leaving behind a dry organic solution.

The use of anhydrous sodium sulfate as a drying agent is particularly advantageous because it is insoluble in most organic solvents. This means that it can be easily separated from the organic phase by filtration or centrifugation, without introducing any additional impurities into the solution.

In summary, anhydrous sodium sulfate is added to the organic solution remaining after the extraction with 6 M HCl and NaOH to remove residual water and ensure that the organic phase is dry. This is essential for preventing water interference in subsequent reactions or analysis.