For which of the given acids, whose pK_a data are listed, will pH be HIGHEST at the equivalence point when titrated by NaOH(aq)?

1. HA_1; pK_a=-2.0. 2. HA_2; pK_a=0.052 3. HA_3; pK_a=1.13 4. HA_4; pK_a=-3.56

May 30, 2016

$\text{Option 3}$

Explanation:

The reaction between the weak acid and strong base is quantitative:

$H A \left(a q\right) + N a O H \left(a q\right) r i g h t \le f t h a r p \infty n s N {a}^{+} {A}^{-} + {H}_{2} O$

After the reaction has taken place, we have the conjugate base of a weak acid, which is by definition a moderate to strong base. Again, by definition, the weakest acid will have the strongest conjugate base, because the conjugate base competes most strongly for the proton.

Given that the weakest acid is $H {A}_{3}$, i.e. its ${K}_{a}$ value is the smallest, ${A}^{-}$ will be the strongest base, ${K}_{b} = {10}^{-} 7$, because ${K}_{A} \times {K}_{B} = {10}^{-} 14$.

You can use this treatment to explain why $N a F$ salts give a basic solution in water, whereas $N a C l$ salts give a neutral solution. What does this say about the relative acid strengths of $H F$ and $H C l$? Why?