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#

1 Answer
anor277
May 30, 2016

#"Option 3"#

Explanation:

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

#HA(aq) + NaOH(aq) rightleftharpoons Na^+A^(-) + H_2O#

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 #HA_3#, i.e. its #K_a# value is the smallest, #A^-# will be the strongest base, #K_b=10^-7#, because #K_AxxK_B=10^-14#.

You can use this treatment to explain why #NaF# salts give a basic solution in water, whereas #NaCl# salts give a neutral solution. What does this say about the relative acid strengths of #HF# and #HCl#? Why?

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