Epsom salts are magnesium sulphate heptahydrate. Suppose the lid was left off a 5oog container for several months and all the water evaporated from the hydrate.

Question

Calculate the new mass of the anhydrous salt that is left behind in the container 

Answer 1

Epsom salt = MgSO_4*7H₂O

molar mass = 246.47 g/mole

500 g x 1 mole/246.47 g = 2.029 moles

2.029 moles epsom salt x 7 moles H₂O/mole epsom salt = 14.203 moles H₂O

mass of H₂O present = 14.203 moles x 18 g/mole = 255.65 g H₂O

mass of MgSO₄ left after removal of all water = 500 g - 255.65 g = 244 g

 

You can do this another simpler way, by considering % by mass of H₂O

molar mass H₂O = 18 g/mole x 7 moles H₂O = 126 g H₂O

molar mass of heptahydrate = 246.47 g/mole

126/246.47 = 51.1% water

Removing 51.1% of the 500 g mass leaves one with 48.9 % of the original mass

48.9% x 500 g = 245 g (close to the 246 g calculated above - difference is due to rounding)