Correct order of acidic strength. Please help!
how a>b>c?!?

Dear Student,
Acidic nature is directly proportional to strength of conjugate base.
In oxalic acid, the hydrogen oxalate ion and the oxalate ion are resonance stabilized.
The negative charge is on oxygen atom( more electronegative atom) which is in conjugation throughout the molecule (with two pi bonds). Hence it is more stable due to conjugation with 2 pi bonds... In malonic acid, the conjugate base is relatively less stable, as the conjugation is restricted to only one pi bond...
Resonance effect is more stronger than inductive effect and moreover, resonance effect in the conjugate base of oxalic acid takes place throughout the molecule.(there is a negative charge, pi bond, pi bond, lone pair conjugation )....
Hence oxalic acid is relatively more acidic as compared to malonic acid.
The strongest acid among oxalic, malonic, succinic acid is OXALIC ACID.
It is because in this there is no Carbon atom in between the two COOH groups.More the number of Carbon atoms between the carboxylic acid groups more is the +I effect and hence lesser the acidic strength.So the final order of acidic strength is :Succinic acid<Malonic acid <Oxalic acid.....

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