1)Why is it necessary to heat the alum until its mass does not change by more than 2mg?

2)Why its it necessary to store the alum in a disccator while it cools before measuring the mass?

( systhesis and analysis of alum crystals experiment)

1. By reaching constant weight you know you have driven off all of the water it contains.

2. If allowed to stand in air the alum will reabsorb, from the air (which contains some water), some of the water diven off by previous heating. Cooling it in a desiccator keeps it from picking up moisture.

1) It is necessary to heat the alum until its mass does not change by more than 2mg because heating helps to remove any remaining water molecules or impurities from the sample. Alum typically contains water molecules in its crystal structure, known as water of crystallization. By heating the alum, we can drive off these water molecules and achieve a more accurate measurement of the mass of the pure alum.

To determine when the heating is complete, we weigh the alum before heating and then continue heating until the mass remains constant. This means that all the water molecules have been removed, and any further loss in mass would indicate the loss of the desired compound (alum) rather than just water. Heating until the mass does not change by more than 2mg provides a balance between ensuring complete removal of water and preventing excessive loss of the alum compound.

2) It is necessary to store the alum in a desiccator while it cools before measuring the mass to prevent the re-absorption of moisture from the surrounding air. A desiccator is a sealed container that contains a drying agent, such as silica gel or anhydrous calcium chloride. This drying agent absorbs any moisture present in the air inside the desiccator, creating a dry environment.

After heating the alum, it becomes very hygroscopic, meaning it readily absorbs moisture from the air. If the alum were exposed to the atmosphere while cooling, moisture molecules could cling to the alum crystals, increasing their mass and altering the accuracy of the measurement.

By storing the alum in a desiccator, we ensure that its mass remains constant and accurately represents the pure and dry alum compound. After the alum has cooled and equilibrated in the desiccator, we can remove it and promptly measure its mass without the interference of any moisture absorption.