Question

in part iii, determine the water of hydration, why is the crucible heated until it reaches a constant mass? select one: to cause the sulfate in the alum to decompose. to ensure all of the water of hydration has been driven off as vapor. to make sure the crucible itself is completely dry before adding the alum. to melt the alum into a liquid for easier measurement.

Explanation:

In the experiment to determine the water of hydration, heating the crucible until constant mass ensures that all water of hydration (which is loosely bound water in the hydrate compound) is removed. If not heated to constant mass, there might still be some water left. Sulfate decomposition (first option) is not the goal here. The crucible is pre - dried before adding alum (third option is about pre - drying, not related to the heating during the hydration determination). Alum melting (fourth option) is not relevant as the aim is to remove water of hydration.

Answer:

To ensure all of the water of hydration has been driven off as vapor.