Final answer:
To find the acid dissociation constant (Ka) and identify the unknown acid, we focus on the midpoint of the pH increase during titration, which is approximately where the pH equals the pKa. With a pH of 6.24 at the halfway point of the titration, we estimate the pKa as 6.24. Comparing the calculated molar mass of the acid with known values may help identify it.
Step-by-step explanation:
The titration data provided includes a significant variation in pH near the volume of 22.2 mL of NaOH, suggesting that this is around the equivalence point. To accurately determine the equivalence point and the acid dissociation constant (Ka) of the acid, we would look for the midpoint of the steep portion of the pH curve. The pH at the equivalence point for a monoprotic weak acid titration will be greater than 7 due to the formation of its conjugate base which is slightly basic.
The volume of NaOH at the midpoint is approximately 22.2 mL, which corresponds with a pH of 6.24. This pH is higher than the pH of 5.93 at 22 mL and significantly higher than the initial pH of 3.09. Since the pH equals the pKa at half the equivalence point in a titration of a weak acid, the pKa for the acid can be approximated as 6.24, which we obtained from the pH at half-neutralization.
By converting the mass of the acid (125.0 mg) to moles and considering the volume of NaOH added at the equivalence point, we can find the molar mass of the acid and potentially identify it based on known values of pKa and molar mass for various monoprotic acids. For example, if the molar mass and pKa correspond to acetic acid, we can identify the unknown acid as acetic acid.