Final answer:
The crucible should be filled with a hydrate to about one-third to one-half full to allow space for water vapor to escape during the dehydration process and prevent loss of the sample.
Step-by-step explanation:
When conducting an experiment to determine the percent of water in a hydrate, it is important to not overfill the crucible with the hydrate sample.
Typically, the crucible should be filled to a level where the material does not touch the lid when it is placed on top. Considering the mentioned information where nearly half of the mass of the hydrate is composed of water molecules, this suggests the sample will significantly decrease in mass during heating (as the water is expelled).
Therefore, the crucible should be filled with a hydrate to a moderate amount, usually about one-third to one-half full, allowing space for any water vapor to escape and prevent spattering of the hydrate as it decomposes.
This practice ensures good thermal contact and efficient heating while also avoiding any loss of sample from overflowing or violent reactions during the dehydration process.