Consider the following reactions: \[\mathrm{H}^{+}(\mathrm{aq}) + \mathrm{OH}^{-}(\mathrm{aq}) = \mathrm{H}_2\mathrm{O}(l), \Delta H = -\mathrm{X}_1\mathrm{kJmol}^{-1}\] \[\mathrm{H}_2(\mathrm{g}) + \frac{1}{2}\mathrm{O}_2(\mathrm{g}) = \mathrm{H}_2\mathrm{O}(l), \Delta H = -\mathrm{X}_2\mathrm{kJmol}^{-1}\] \[\mathrm{CO}_2(\mathrm{g}) + \mathrm{H}_2(\mathrm{g}) = \mathrm{CO}(\mathrm{g}) + \mathrm{H}_2\mathrm{O}, \Delta H = -\mathrm{X}_3\mathrm{kJmol}^{-1}\] \[\mathrm{C}_2\mathrm{H}_2(\mathrm{g}) + \frac{5}{2}\mathrm{O}_2(\mathrm{g}) = 2\mathrm{CO}_2(\mathrm{g}) + \mathrm{H}_2\mathrm{O}(l) \Delta H = +\mathrm{4X}_4\mathrm{kJmol}^{-1}\] Enthalpy of formation of \(\mathrm{H}_2\mathrm{O}(l)\) is