4. If epsom salt MgSO4.zH2O is heated to 250 degrees celsius all of the water of hydration is lost. After heating a 2.04 g sample of the hydrate, 0.989 g of anhydrous MgSO4 remains. How many molecules of water are there per formula unit of MgSO4?
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Z = mole ratio of H2O molecules to MgSO4 formula units.
1) Calculate moles of MgSO4
0.989 g x 1 mol / 120.4 g = 0.008214 mol
2) Calculate moles of H2O lost
2.04 g - 0.989 g = 1.051 g H2O
1.051 g H2O x 1 mol / 18.02 g = 0.05832 mol
3) Calculate the mole ratio of H2O to MgSO4
0.05832 mol / 0.008214 mol = 7.1 / 1
So ...the most likely number for Z is 7
1) Calculate moles of MgSO4
0.989 g x 1 mol / 120.4 g = 0.008214 mol
2) Calculate moles of H2O lost
2.04 g - 0.989 g = 1.051 g H2O
1.051 g H2O x 1 mol / 18.02 g = 0.05832 mol
3) Calculate the mole ratio of H2O to MgSO4
0.05832 mol / 0.008214 mol = 7.1 / 1
So ...the most likely number for Z is 7