✔ 最佳答案
First of all, I think you can weigh the sample (hydrated salt) before heating.
After heating the sample for long time, i.e. 5 -10 minutes, then re-weigh the sample and record the mass.
Heat and weigh the sample repeatingly, if the weigh of the sample has no change after a long time. Then, I think it has been converted to a anhydrous salt.