To calculate the standard enthalpy change for the dissolution of 5.00 grams of solid magnesium chloride MgCl2 in 250.0 mL of water at 25 , we need to first determine the number of moles of MgCl2 and then use the molar enthalpy of hydration.1. Calculate the number of moles of MgCl2:MgCl2 has a molar mass of 24.31 g/mol Mg + 2 * 35.45 g/mol Cl = 95.21 g/mol.moles of MgCl2 = mass / molar massmoles of MgCl2 = 5.00 g / 95.21 g/mol = 0.0525 mol2. Use the molar enthalpy of hydration to calculate the standard enthalpy change:The molar enthalpy of hydration of MgCl2 is given as -641 kJ/mol. This means that when 1 mole of MgCl2 dissolves in water, the enthalpy change is -641 kJ.enthalpy change = moles * molar enthalpy of hydrationenthalpy change = 0.0525 mol * -641 kJ/mol = -33.65 kJTherefore, the standard enthalpy change for the dissolution of 5.00 grams of solid magnesium chloride MgCl2 in 250.0 mL of water at 25 is -33.65 kJ.