Epsom salt is mgso4·7 h2o. what does the chemical formula become when this hydrate is gently heated?

Epsom salt, magnesium sulphate, or MgSO4.7H2O comes in different hydrates.

The heptahydrate (.7H2O) comes in as Epsom salt.

It readily loses one water of crystallization, or by gentle heating, to become the hexahydrate, (.6H2O).

When heated to 150 ° C, it further lose water of crystallization to the monohydrate (. H2O)

Finally, when heated to 200 ° C, it becomes anhydrous, i. e. a powder that has no more water of crystallization.

(ref. Wiki)