Arrange in the order of increasing acidic strength :Formic acid, Acetic acid, Propionic acid. Give reason for the order.

Dear student,

Acidic strength of organic acids can be compared on the basis of stability of conjugate base thus formed on removing H from it.

$$\begin{gathered} HCOOH\to HCO{O}^{-} + H^{+} \\ C{H}_{3}COOH\to C{H}_{3}CO{O}^{-} + H^{+} \\ C{H}_{3}C{H}_{2}COOH\to C{H}_{3}C{H}_{2}CO{O}^{-} + H^{+} \end{gathered}$$

The negative charge on the conjugate base is destabilised by the electron releasing group attached to it.

Since electron releasing tendency is in order -CH₃CH₂ > -CH₃ >_​ -H

Therefore Formic acid is stronger than acetic acid which is stronger than propionic acid.

Hope this helps,

Regards