Question

A 10.00 g sample of a hydrate of magnesium chloride (MgCl2•x H2O) contains 5.317 g of H2O. What is the value of x?

Answer 1

Ans. Given,     Total mass of hydrate sample = 10.00 g

            Mass of water in hydrated sample = 5.317 g

Now,

Mass of pure MgCl₂ in sample = Total mass of hydrated sample – Mass of water in it

                                                = 10.00 g – 5.317 g

                                                = 4.683 g

Moles of pure MgCl₂ in sample = Mass of pure MgCl₂ / Molar mass of MgCl₂

                                                = 4.683 g/ (95.2104 g/ mol)

                                                = 0.04919 mol

# Moles of water in hydrated sample = Mass /Molar mass

                                                = 5.317 g / (18.01528 g/ mol)

                                                = 0.29514 mol

# Molar ratio of H₂O and MgCl₂ = Moles of H₂O / Moles of MgCl₂

                                                = 0.29514 mol / 0.04919 mol

                                                = 6.0005 : 1

                                                = 6 : 1 (nearest whole number)

The molar ratio indicates that there are 6 moles of water per mol of MgCl₂ in the hydrated sample.

Hence, value of x = 6

Formula of the hydrate = MgCl₂.6H₂O