Question

Strongest conjugate base is:

• A. $C{l}^{-}$
• B. $B{r}^{-}$
  
• C. $F^{-}$
  
• D. $I^{-}$

Answer 1

The correct option is C $F^{-}$


It's a fact that weakest acids have, strongest conjugate base.

If you add H-atom to each base the acids are $HF,HCl,HBr,HI$. As the $F^{-}$ ion is the smallest halide; therefore, its electrons are concentrated around its nucleus, and as a result, the $H-F$ bond is relatively short. Shorter bonds are more stable, and thus the $H-F$ bond is more difficult to break due to which $H^{+}$ donation is difficult for $H-F$. So $H-F$ is the weakest acid which means its conjugate base $\left(F^{-}\right)$ is the strongest conjugate base.


Alternate solution:

$F^{+}$ has the smallest size in a group and gain proton due to the most electronegative capacity. So it is strongest conjugate base.
$H^{+}+F^{-}\to HF$