First, we need to determine the number of moles of MgSO4 in 45 grams. The molar mass of MgSO4 is:Mg = 24.3 g/molS = 32.1 g/molO = 16.0 g/mol and there are 4 oxygen atoms Molar mass of MgSO4 = 24.3 + 32.1 + 4 16.0 = 24.3 + 32.1 + 64.0 = 120.4 g/molNow, we can calculate the number of moles of MgSO4 in 45 grams:moles = mass / molar mass = 45 g / 120.4 g/mol 0.374 molesNext, we can use the molar heat of hydration to find the heat of hydration for 0.374 moles of MgSO4:Heat of hydration = moles molar heat of hydration = 0.374 moles -138.5 kJ/mol -51.8 kJSo, the heat of hydration of 45 grams of magnesium sulfate in 250 mL of water is approximately -51.8 kJ.