Factors affecting on acidity of carboxylic acids

1. Electron releasing alkyl groups decrease the acidity :-  

This is because the electron releasing groups increase the negative charge on the carboxylate ion and destabilize it. The loss of proton becomes more difficult also as the size of the alkyl group increases acidity decreases 

 2. Electron-withdrawing substituents(Cl, Br, F, OH, CN) increase acidity :-

This is because the electron withdrawing substituents decrease the negative charge on the carboxylate ion and stabilize it the loss of proton becomes relatively easy for example, chloroacetic acid is about 100 times stronger than acetic acid.

The strength of the electron with-drawing substituents determines the magnitude of the effect on acidity. For example, fluoro  acetic acid is  stronger than chloroacetic acid since F is more electronegative than Cl .

As the number of electron withdrawing substituents increases acidity also increases 

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