When 1 g of anhydrous oxalic acid is burnt at $\text{25°C}$, the amount of heat liberated is 2.835 kJ. $\Delta$H combustion is (oxalic acid : ${\text{C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{O}}_{\text{4}}$)

We know,

$\Delta$Hcombustion = Amount of heat liberated when

1 mole of substance reacts with oxygen.

Mol. wt. of oxalic acid = 90 g mol^–1

1 g oxalic acid liberates 2.835 kJ

$\therefore$90 g (1 mole) oxalic acid liberates 2.835 × 90 kJ

mol^–1 = 255.15 kJ

$\therefore$Heat involved = – 255.15 Kj