Question

The following plot shows the pH curves for the titrations of various acids with 0.10 M NaOH (all of the acids were 50.0mL samples of 0.10 M concentration)

1. Which pH curve corresponds to the weakest acid?
2. Which pH curve corresponds to the strongest acid?

Which point on the pH curve would you examine to see if this acid is a strong acid or a weak acid (assuming you did not know the initial concentration of the acid)?

c. Which pH curve corresponds to an acid with${\mathit{K}}_{\mathbf{a}}\mathbf{\approx }\mathbf{1}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{6}}\mathbf{?}$

Short answer

1. The pH curve corresponds to the weakest acid is f
2. The pH curve corresponds to the strongest acid is a
3. The pH curve corresponds to an acid with $K_a\approx 1\times {10}^{-6}?$is d

Step-by-step solution

Definition of pH curve

The plot of the pH of the analyte solution versus the volume of the titrant added as the titration progresses is defined as a pH curve.

Explanation

a. The equivalence point of an acid-base titration can be obtained by the halfway point of the vertical solution of the pH curve. For a strong acid-strong base titration, the equivalence point lies at pH 7.0, and the vertical portion has a wide range. For a weak acid, this vertical portion is narrower than that of a strong acid. Weaker the acid, the narrower the vertical portion. This is because the weaker the acid, the larger its initial pH

According to the given pH curves, the f curve corresponds to the weakest acid.

b. The equivalence point of an acid-base titration can be obtained by the halfway point of the vertical solution of the pH curve. For a strong acid-strong base titration, the equivalence point lies at pH 7.0, and the vertical portion has a wide range.

According to the given pH curves, a curve corresponds to the strongest acids. It has a wider vertical portion, and the halfway point of the vertical portion lies at pH 7.0

When the initial concentration of the acid is unknown, the halfway point of the vertical portion of the pH curve should be examined to see if the acid is a strong or a weak acid.

c. In a weak acid-strong base titration, when OH- is added, the weak acid is consumed, and its conjugate base is formed. At the halfway point of the titration, the concentration of weak acid and concentration of its conjugate base become equal. At this point $\text{pH}=\text{pKa}$,

For a weak acid with $K_a\approx 1\times {10}^{-6},\text{pH}=6.0$at the halfway point of the titration. The pH of the halfway point of titration can be obtained from the horizontal portion of the curve.

According to the given pH curves, the curve corresponds to an acid with${\mathit{K}}_{\mathbf{a}}\mathbf{\approx }\mathbf{1}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{6}}$ is d.