Question

Epsom salts are hydrates of magnesium sulfate. the formula for epsom salt is mgso4·7h2o. if a 7.834 g sample is heated until constant mass is obtained, what is the mass of the anhydrous magnesium sulfate? what percent of the hydrate is water?

Answer 1

1. For the first question, we must find the mass of the anhydrous salt, MgSO₄. The molar mass for MgSO₄·7H₂O is 246.47 g/mol, while that of MgSO₄ is 120.37 g/mol.

7.834 g MgSO₄·7H₂O * 1 mol MgSO₄·7H₂O/246.47 g * 1 mol MgSO₄/1 mol MgSO₄·7H₂O * 20.37 g MgSO₄/mol = 0.647 g MgSO₄

2. Mass of Water = Mass of sample - Mass of anhydrous sale
Mass of water = 7.834 - 0.647 = 7.19 g
Percent Water in Hydrate = 7.19/7.834 * 100 = 91.74%