The standard enthalpies of formation of MgCl ₂(s) , Mg ²⁺(aq) and Cl ^-(aq) are -641 ~kJ/mol , -467 ~kJ/mol and -167 ~kJ/mol respectively. The total enthalpy change when 9.5 ~g of MgCl ₂(s) is dissolved in water is : [Given : Molar mass of MgCl ₂ = 95 ~g/mol ]

Step-by-step solution

The balanced chemical equation for the dissolution of magnesium chloride is: MgCl ₂(s) Mg ²⁺(aq) + 2 Cl ^-(aq) Using Hess's Law, the standard molar enthalpy of solution is: H_ sol ^ = [ H_f^ ( Mg ²⁺) + 2 H_f^ ( Cl ^-)] - H_f^ ( MgCl ₂) Substituting the given values (note the stoichiometric coefficient of 2 for the chloride ion): H_ sol ^ = [-467 + 2(-167)] - (-641) H_ sol ^ = [-467 - 334] + 641 H_ sol ^ = -801 + 641 = -160 ~kJ/mol Next, calculate the number of moles of MgCl ₂(s) dissolved: Moles = Mass Molar mass =