The heat of neutralization of oxalic acid is -25.4 k.cal. ${\mathrm{mol}}^{-1}$ using strong base NaOH then the enthaphy change of the process is

${\mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4\underset{ }{\rightleftharpoons }2{\mathrm{H}}^{+}+{\mathrm{C}}_2{\mathrm{O}}_4^{-2}$

Oxalic acid has two ionisable ${\mathrm{H}}^{+}$ hence expected heat of neutralization, If it behaves as a strong acid would have been $=-13.7\times 2=-27.4\mathrm{k}.\mathrm{cal}{\mathrm{mol}}^{-1}$

But experimental value $=-25.4k.cal.mo{l}^{-1}$

Heat of ionization $=27.4-25.4=2k.cal/mol$