Acidic strength is directly proportional to -I effect and inversion-ally proportional to +I effect.

Example:

a. H₃C-CH₂-COOH      >       (CH₃)₂-CH-COOH       >       (CH₃)₃-C-COOH

                               +I Effect increases

          ========================================>

                             So, acidic strength decreases

b.   Cl₃C-CH₂-COOH      >     Cl₂HC-CH-COOH      >      ClH₂C-C-COOH

                               -I Effect decreases

         =========================================>

                         So, acidic strength decreases

c. CH₃-CH₂-CHF-COOH     >     CH₃-CHF-CH₂-COOH   >   F-(CH₂)₃-COOH

             Distance of Fluorine atom increases so, -I Effect decreases

         ==========================================>

                         That’s why acidic strength decreases

d. COOH-COOH    >    COOH-CH₂-COOH   >   COOH-CH₂-CH₂-COOH

   {Oxalic acid}               {Malonic acid}                 {Succinic acid}

         ==========================================>

              +I Effect increases so, acidic strength decreases

e. CH₃-(CH₂)₂-COOH   <    CH₂=CH-CH₃-COOH   <   CH≡C-CH₂-COOH

          ===================================================>

                   -I Effect increases

                S-Character increases 

                So, EN increases 

                So, -ve charge decreases 

                   That’s why stability increases