Question

a 3.592 g sample of hydrated magnesium bromide, MgBr2. xH20, is dried in an oven. when the anhydrous salt is removed from the oven, it's mass is 2.263 g. what is the value of x?

Answer 1

Molar mass of MgBr₂. xH₂O = At.mass of Mg + (2*At.mass of Br) + x[(2*At.mass of H ) + At.mass of O]

                                          = 24.3 + ( 2*79.90)+x[(2*1)+16]

                                          = 184.1+18x

Initial mass of MgBr₂. xH₂O = 3.592 g

Final mass of MgBr₂ = 2.263 g

So mass of water evoporated is m = 3.592 - 2.263 = 1.329 g

No . of moles of water , n = mass / Molar mass

                                       = 1.329 / 18                    

                                       = 0.074 moles

No.of moles of anhydrous salt , n = mass / Molar mass of MgBr₂.

                                                 = 2.263 / 184.1

                                                 = 0.0123 mol

ration to the no . of moles of water to the anhydrous salt is = 0.074/ 0.0123

                                                                                     = 6.02

                                                                                     ~ 6

So x = 6