How to Find the Mass of Anhydrous Salt?

To find the mass of an anhydrous salt (salt without water), subtract the mass of the empty container (crucible, evaporating dish, etc.) from the mass of the container and the anhydrous salt after all water has been removed by heating.

Here's a breakdown of the process:

Steps Involved:

1. Weigh the Empty Container: Determine and record the mass of the clean, dry container (e.g., crucible) that you will use to heat the hydrated salt. Let's call this Mass of Container.

2. Weigh the Container with Hydrated Salt: Add the hydrated salt (the salt containing water molecules) to the container. Weigh and record the combined mass. This is Mass of Container + Hydrated Salt.

3. Heat the Hydrated Salt: Carefully heat the container and hydrated salt. The heat will cause the water molecules to evaporate, leaving behind the anhydrous salt. The heating process may require multiple heating cycles until a constant mass is achieved. This ensures all the water has been removed.

4. Cool and Weigh the Container with Anhydrous Salt: After heating, allow the container and its contents to cool to room temperature in a desiccator (to prevent reabsorption of moisture from the air). Weigh and record the mass. This is Mass of Container + Anhydrous Salt.

5. Calculate the Mass of Anhydrous Salt: Subtract the mass of the empty container (from step 1) from the mass of the container plus the anhydrous salt (from step 4).

   Mass of Anhydrous Salt = (Mass of Container + Anhydrous Salt) - Mass of Container

Example:

Let's say you have the following data:

• Mass of empty crucible: 25.00 g
• Mass of crucible + hydrated salt: 27.46 g
• Mass of crucible + anhydrous salt (after heating): 26.50 g

Then, the mass of the anhydrous salt is:

Mass of Anhydrous Salt = 26.50 g - 25.00 g = 1.50 g

Important Considerations:

• Complete Dehydration: Ensure all the water is removed during heating. Continue heating until successive weighings (after cooling) are the same, indicating that no more water is being lost.
• Prevent Decomposition: Avoid overheating the salt, as this can lead to decomposition and inaccurate results. Use a moderate heat source.
• Desiccator Use: Cooling in a desiccator is crucial to prevent the anhydrous salt from reabsorbing moisture from the atmosphere before weighing.
• Constant Mass: A constant mass (within experimental error) between subsequent heating cycles indicates complete dehydration.

By following these steps carefully, you can accurately determine the mass of the anhydrous salt.