The sodium salt of an organic acid $\mathrm{X}$ produces effervescence with concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$. $\mathrm{X}$ reacts with the acidified aqueous ${\mathrm{CaCl}}_2$ solution to give a white precipitate which decolourises acidic solution of ${\mathrm{KMnO}}_4$. $\mathrm{X}$ is