Which acid of each pair shown here would you expect to be stronger?
$(i)$ $CH_3CO_2H$ or $CH_2FCO_2H$
$(ii)$ $CH_2FCO_2H$ or $CH_2ClCO_2H$
$(iii)$ $CH_2FCH_2CH_2CO_2H$ or $CH_3CHFCH_2CO_2H$
$(iv)$ $F_3C-C_6H_4-COOH$ or $H_3C-C_6H_4-COOH$

(N/A) $(i)$ The $+I$ effect of the $-CH_3$ group increases the electron density on the $O-H$ bond,making the release of a proton difficult. Conversely,the $-I$ effect of $F$ decreases the electron density on the $O-H$ bond,facilitating proton release. Therefore,$CH_2FCO_2H$ is a stronger acid than $CH_3CO_2H$.
$(ii)$ $F$ has a stronger $-I$ effect than $Cl$. Therefore,$CH_2FCO_2H$ can release a proton more easily than $CH_2ClCO_2H$. Hence,$CH_2FCO_2H$ is a stronger acid than $CH_2ClCO_2H$.
$(iii)$ The inductive effect decreases with an increase in distance. The $-I$ effect of $F$ in $CH_3CHFCH_2CO_2H$ is closer to the carboxyl group than in $CH_2FCH_2CH_2CO_2H$. Hence,$CH_3CHFCH_2CO_2H$ is a stronger acid than $CH_2FCH_2CH_2CO_2H$.
$(iv)$ Due to the strong $-I$ effect of the $-CF_3$ group,it is easier to release a proton in $p-(trifluoromethyl)benzoic$ acid. In contrast,the $+I$ effect of the $-CH_3$ group in $p-toluic$ acid makes the release of a proton more difficult. Hence,$F_3C-C_6H_4-COOH$ is a stronger acid than $H_3C-C_6H_4-COOH$.