Question

Epsom salt has the formula of MgSO4·7H2O. A 5.45 g sample was heated and dried completely. After cooling the mass was 2.69 g. Using calculations, show that the experimental results confirm the formula for the hydrate (i.e., seven moles of water per MgSO4 formula unit). What is the mass percent of water in the original salt sample?

Answer 1

mass of MgSO4 = mass of sample after heating
= 2.69 g

mass of H2O = (mass of sample) - (mass of MgSO4)
= 5.45 - 2.69
= 2.76 g

molar mass of MgSO4 = 120.4 g/mol
molar mass of H2O = 18 g/mol
number of mol = (given mass)/(molar mass)
number of mol of MgSO4 = 2.69/120.4
= 0.0223 mol
number of mol of H2O = 2.76/18
= 0.153 mol

approximately
(number of mol of H2O)/(number of mol of MgSO4) = 6.86
nearly equal to 7
so,
there is 7 moles of water per mol of MgSO4
so,
formula is MgSO4.7H2O

mass % H2O = {(mass of H2O)/(mass of sample)}*100
= (2.76/5.45)*100
= 50.6 %

Answer: 50.6 %