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 1 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 2 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 = 0.0200 mol
moles H2O = 0.1399 mol
Step 2: Determine the lowest whole number ratio of MgSO4 and H2O to get the formula of the compound.
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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