In this question, law of conservation of mass is used to calculate mass of water present. MgSO4.XH20 →MgSO4+H20 Mass of water of hydration= Mass of hydrated Magnesium Sulfate – Mass of Anhydrous Magnesium Sulfate Mass of H20 =3.210g-1.567g=1.643g Moles of MgSO4= Mass of MgSO4/ Molecular Mass of MgSO4 = 1.567/ 120= 0.0131 mol Moles of MgSO4.XH20= Moles of MgSO4= 0.0131 mol Moles of H20 present in 0.0131 mol of Hydrated Magnesium Sulfate = Mass of Water/ Mr of H20 = 1.643/18 = 0.0912 mol Moles of Water present in 1 mol of Hydrated Magnesium Sulfate = 0.0912mol/0.0131 =6.96 X=7