Question

Find the enthalpy of dissociation of $H_{2}C_{2}O_{4}$ from the given data:

Enthalpy of neutralisation of strong acid and strong base is $-13.7 \, kcal \, mol^{- 1}$ and that of oxalic acid by a strong base is $-25 \, kcal \, mol^{- 1}$ .

• A. $-11.3\, kcal \, mol^{- 1}$
• B. $2.4\, kcal \, mol^{- 1}$
• C. $1.2\, kcal \, mol^{- 1}$
• D. $11.7\, kcal \, mol^{- 1}$

Answer 1

Answer: (B)

One equivalent of oxalic acid gives two moles of $H^{+}$ ions, as the basicity of the acid is two.

Now enthalpy of neutralisation is defined for one equivalent of acid as well as base.

For weak acids:

Enthalpy of neutralisation $=$ Basicity $\times $ enthalpy of neutralisation for strong acid and base $+$ enthalpy of dissociation

Thus, enthalpy of dissociation for oxalic acid $=\left(- 25\right)-\left(2 \times - 13 . 7\right)=27.4-25=2.4kcal\left(mol\right)^{- 1}$