Epsom salts is magnesium sulfate heptahydrate. If a student takes 2.50 g of Epsom salts and heats it to drive off the water, how much mass should the student expect to lose? ( I know that hepta is 7 but that's still not making sense to me)

Answer 1

There is a long way and a short way. Here's the long way (the modern way).
MgSO4.7H2O ==> MgSO4 + 7H2O

mols MgSO4.7H2O = grams/molar mass = approx 0.0101
Convert that to mols H2O. That will be 7x that or 0.0707 which you get from the coefficients in the balanced equation.
Now convert to g H2O. mols x molar mass = approx 1.3 g H2O.

The short way which no one teaches anymore. It's (the 7*molar mass H2O/molar mass MgSO4.7H2O) is called a chemical factor but it disappeared from text books years ago.
2.50 g x (7*molar mass H2O/molar mass MgSO4.7H2O) = 2.5 x (18/246.5) = about 1.3g

You can clean up the math since I estimated the numbers.