Find the number of waters of hydration in Epsom salts

Question

Epsom salt os a hydrated ionic compound with the following formula: MgSO₄•xH₂O. A 4.93 g sample of Epsom salts was heated to drive off the water of hydration. The mass of the sample after complete dehydration was 2.41 g. Find the number of waters of hydration (x) in Epsom salts.

Answer 1

The formula is MgSO₄ * 7 H₂O

 

First, find the moles of MgSO₄ by using the second piece of information, 2.41g to get the number of moles
Second, compute by difference the total mass of H₂O in the original sample (4.93-2.41)
Then compute the number of moles of water present in the original sample from this information.

Take the ratio of moles water/moles MgSO₄ and this gives you the number for X.