Above, you wrote kJ/kg, not kJ/mol, which was confusing - which is it? In either case, it's the wrong value to use. ΔH in the van't Hoff equation is not
the difference between the enthalpy of water at 25 and 40°C. It is the enthalpy change for the reaction whose equilibrium constant is under consideration, i.e. for the reaction
(aq) + OH-
57 kJ/mol was a value I used from memory. Using the values of enthalpy of formation from Johnson, "Thermodynamic aspects of inorganic chemistry", I get a value of 55.84 kJ/mol.