When of anhydrous oxalic acid is burnt at $25°\mathrm{C}$, the amount of heat liberated is $2.835 \mathrm{kJ}.\mathrm{\Delta H}$ combustion is (oxalic acid: ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{O}}_4$ )

$\mathrm{HA}+{\mathrm{OH}}^{-}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{A}}^{-}+{\mathrm{q}}_1 \mathrm{kJ}$

${\mathrm{H}}^{+}+{\mathrm{OH}}^{-}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{q}}_2 \mathrm{kJ}$
The enthalpy of ionisation of $\mathrm{HA}$ is

${\mathrm{H}}_2\left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)⟶{\mathrm{H}}_2\mathrm{O}\left(\mathrm{\ell }\right)$

${\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)};\mathrm{\Delta H}={\mathrm{x}}_4$

Given,

${\mathrm{E}}_{\mathrm{H}-\mathrm{H}}={\mathrm{x}}_1$

${\mathrm{E}}_{\mathrm{O}=\mathrm{O}}={\mathrm{x}}_2$

${\mathrm{E}}_{\mathrm{O}-\mathrm{H}}={\mathrm{x}}_3$

${\mathrm{\Delta H}}_{\mathrm{F}}$ of ${\mathrm{H}}_2\mathrm{O}$ vapour is