Question

How can you experimentally determine the pK_a of acetic acid? Determine the pH of the solution 1/4 of the way to the end-point on the pH titration curve for acetic acid. Determine the pH of the solution half way to the end-point on the pH titration curve for acetic acid. Determine the pH at the end-point on the pH titration curve for acetic acid.

Answer 1

Answer:

Determine the pH of the solution half-way to the end-point on the pH titration curve for acetic acid.  

Explanation:

The equation for the ionization of acetic acid is

HA + H₂O ⇌ H₃O⁺ + A⁻

For points between the starting and equivalence points, the pH is given by the Henderson-Hasselbalch equation:

$\text{pH} = \text{pK}_{\text{a}} + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}$

At the half-way point, half of the HA has been converted to A⁻, so [HA] = [A⁻].  Then,

$\text{pH} = \text{pK}_{\text{a}} + \log\dfrac{1}{1} = \text{pK}_{\text{a}} + 0 \\\\\text{pH} = \text{pK}_{\text{a}}$

The pKₐ is the pH at the half-way point in the titration.