sir i did not understand what is meant by common ion effect in acids and bases concept .so please explain about that

see this example

let we have aqueous solution of acetic acid ....

CH₃COOH +H₂O=CH₃C00^- + H₃O⁺ 

its K_a=[CH₃COO^-][H₃O⁺]/[CH₃COOH]       ...............1

       K_a =CA²/1-A                       (A is degree of dissotion)

but if we have solution which containes acetic acid &  its salt (sodium acetate) then its degree of dissosiation is supressed...

CH₃COONa =CH₃COO^-+Na⁺

 now we have CH₃C00^- as common ion so eq 1 becomes

K_a=[CH₃COONa][H₃0⁺]/[CH₃COOH]

K_a=A¹[CH₃COONa]/1-A¹

A¹ is new degree of dissotion

change in degree of dissociation by the presence of common ion in the solution is known as common ion effect..