Problem: Epsom salts is a hydrated ionic compound with the following formula: MgSO4 • xH2 O. A 4.93 g sample of Epsom salts is 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.

FREE Expert Solution

Step 1: Calculate the mass and moles of MgSO4 and H2O in the compound.


The problem states that the sample of Epsom salt is heated to drive off the water of hydration:

mass of hydrated (with water) Epsom salt = 4.93 g

mass of dehydrated (without water) Epsom salt = 2.41 g MgSO4


Therefore, the mass of water is then the difference in mass:

mass of water = mass of hydrated Epsom salt – mass of dehydrated Epsom salt

mass of water = 4.93 g – 2.41 g

mass of water = 2.52 g H2O



Now, we need to get the moles of each MgSO4 and H2O compound. We will need their molar mass for the calculation:

molar mass MgSO4:

MgSO4           Mg x 24.3 g/mol = 24.3 g/mol
                       1 S x 32.07 g/mol = 32.07 g/mol            
                       4 O x 16.00 g/mol = 64.00 g/mol
                     _____________________________
                                            Sum = 120.37 g/mol


molar mass H2O:

H2O                 H x 1.008 g/mol = 2.016 g/mol
                        1 O x 16.00 g/mol = 16.00 g/mol
                       ____________________________
                                            Sum = 18.016 g/mol


mass (molar mass) → moles

moles MgSO4=2.41 g MgSO4×1 mol MgSO4120.37 g MgSO4

moles MgSO4 = 0.0200 mol

moles H2O=2.52 g H2O×1 mol H2O18.016 g H2O

moles H20.1399 mol


Step 2: Determine the lowest whole number ratio of MgSO4 and H2O to get the formula of the compound.


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Problem Details

Epsom salts is a hydrated ionic compound with the following formula: MgSO4 • xH2 O. A 4.93 g sample of Epsom salts is 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.

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