Due to stronger-$I$-effect of $F$ than that of $Cl, CHF_3$ should be more acidic than $CHCl_3 .$ But actually reverse is true. This is due to : $CCl^{3-}$ left after the removal of a proton from $CHCl_3$ is stabilised due to presence of d-orbitals in $Cl$ than: $CF _{3}^{-}$ left after the removal of a proton from $CHF _{3}$ which is not stabilised due to the absence of d-orbitals on $F$.