I don't really get the gist of what you're saying, but here's what I think you might be trying to do: are you trying to find the mass of the Epsom salt and the water in the Epsom salt, and asking what happens when you heat up the compound? If so, here's my answer.
Epsom salt has the chemical formula MgSO4·7H2O. To find the mass of this, simply add up the molar mass of all the elements in the whole compound: 1 magnesium, 1 sulfur, 11 oxygen, and 14 hydrogen. 24.3 + 32.06 + 11(16) + 14(1.01) = 246.50 g. Water's the same, just a bit easier: 7 oxygen, 14 hydrogen. 7(16) + 14(1.01) = 126.14 g.
As for the actual heating of the compound, this kind of reaction is a decomposition reaction, meaning that one compound becomes two different things. In this reaction, the reactant would be the Epsom salt, since that is what we're heating and is thus undergoing a chemical reaction. The products would be the two things that come about from this: Magnesium sulfate (MgSO4) and water (H2O). Keep in mind that, in this situation, there are 7 water molecules for every 1 magnesium sulfate molecule.
That should be it for the most part, I hope this helped in any way :)
