To calculate the enthalpy change, we can use the formula:q = mcTwhere q is the heat absorbed or released, m is the mass of the substance, c is the specific heat capacity, and T is the change in temperature.First, we need to find the mass of water. Since the density of water is approximately 1 g/mL, the mass of 250 mL of water is:m_water = 250 gNow, we can calculate the heat absorbed or released by the water:T = T_final - T_initial = 18.5C - 25C = -6.5Cq_water = m_water * c * T = 250 g * 4.184 J/gC * -6.5C = -6806 JSince the heat absorbed by the water is negative, it means that the water lost heat to the ammonium nitrate. Therefore, the heat absorbed by the ammonium nitrate is:q_ammonium_nitrate = -q_water = 6806 JNow, we need to find the moles of ammonium nitrate:moles_ammonium_nitrate = mass / molar_mass = 50.0 g / 80.04 g/mol = 0.6249 molFinally, we can calculate the enthalpy change per mole of ammonium nitrate:H = q_ammonium_nitrate / moles_ammonium_nitrate = 6806 J / 0.6249 mol = 10887 J/molThe enthalpy change when 50.0 grams of ammonium nitrate dissolves in 250 mL of water at 25C is approximately 10.9 kJ/mol.