Question

Consider the following two acids:

In two separate experiments, the pH was measured during the titration of 5.00 mmol of each acid with 0.200. Each experiment showed only one stoichiometric point when the data were plotted. In one experimentthe stoichiometric point was at 25.00 mL added,and in the other experiment the stoichiometric pointwas at 50.00 ml. Explain these results. (SeeExercise 85.)

Short answer

There is only 25 mL of NaOH spent for salicylic acid when half of proton present and titrated with NaOH. Both proton type can be titrated and there is equal initial concentration of both acids. Twice the NaOH volume spent for salicylic acid and 50 mL spent for adipic acid.

Step-by-step solution

Given Information

The acidity for salicylic acid and adipic acid can be given as:

$$\begin{gathered} \mathrm{Salicylic} \mathrm{acid}:{\mathrm{pK}}_{\mathrm{a}1}=2.98,{\mathrm{pK}}_{\mathrm{a}2}=13.4 \\ \mathrm{Adipic} \mathrm{acid}: {\mathrm{pK}}_{\mathrm{a}1}=4.41, {\mathrm{pK}}_{\mathrm{a}2}=5.28 \end{gathered}$$

$$\begin{gathered} \mathrm{salicylic} \mathrm{acid}:{\mathrm{K}}_{\mathrm{a}1}={10}^{-2.98},{\mathrm{K}}_{\mathrm{a}2}={10}^{-13.4} \\ \mathrm{For} \mathrm{adipic} \mathrm{acid}: {\mathrm{K}}_{\mathrm{a}1}={10}^{-4.41}, {\mathrm{K}}_{\mathrm{a}2}={10}^{-5.28} \end{gathered}$$

First and second dissociation constant 

The salicylic acid’s second dissociation constant is very smaller compared to the first dissociation constant. So, the first stoichiometric point can only be detected. The two points of dissociation constant for adipic acid are nearly close to each other. Both the protons can be titrated at same time to give only one stoichiometric point.

There is only 25 mL of NaOH spent for salicylic acid when half of proton present and titrated with NaOH. Both proton type can be titrated and there is equal initial concentration of both acids. Twice the NaOH volume spent for salicylic acid and 50 mL spent for adipic acid.