Acidic strength is directly proportional ot stability of conjugate base.

Note:  *Acidic strength  stability of conjugate base

i.e. More stable conjugate base more acidic character of corresponding acid.

Example:-

1.  H₃C-CH₃         <     H₂C=CH₂       <      HC≡CH

-> H₃C-C^-H₂              H₂C=C^-H                HC≡C^-         {Conjugate base}

-> -ve on less EN (SP³ C)                      -> -ve on more EN (SP C)

-> less stable conjugate base                -> more stable conjugate base

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

-> H₃C-CH₂-O^-              (CH₃)₂-CH-O^-               (CH₃)₃-C-O^-     {Conjugate base}

-> Less +I effect of ethyl group                       -> more +I effect of butyl group

-> Less –ve charge on O-atom                        -> more –ve charge on O-atom

-> more stable conjugate base                        -> less stable conjugate base

3. NO₂-CH₂-COOH   >  H-CH₂-COOH   >   CH₃-CH₂-COOH

-> NO₂-CH₂-COO^-        H-CH₂-COO^-         CH₃-CH₂-COO^-     {Conjugate base}

                                                                   -> More +I effect

                                                                   -> More –ve charge

                                                                   -> Less stable conjugate base

                                                                    So, cross pounding acid is less acidic