During an investigation, students titrate an unknown monoprotic weak acid with 0.100 M KOH and generate the following verbal description of the pH versus volume curve:
• The curve begins at pH 2.90 with 0 mL of base added.
• A gentle, almost linear rise is observed until approximately 14.0 mL of base have been delivered, at which point the pH is 4.75.
• A steep rise follows, centering at 28.0 mL where the pH passes through 8.80.
• Beyond 32 mL the curve levels off above pH 11.
(a) Using the description of the curve, estimate as precisely as possible the volume of base that corresponds to the half-equivalence point.
(b) Using the estimated half-equivalence volume, determine pKa and calculate Ka for the unknown acid.
(c) The initial acid concentration is later found to be 0.050 M. Calculate the molar concentration of the conjugate base A⁻ in solution exactly at the half-equivalence point.
(d) Suppose the titration is repeated with the same initial acid concentration but the KOH concentration is reduced to 0.050 M. Predict whether the calculated pKa would change, decrease, or remain the same. Provide a chemical justification for your prediction.
(a) Estimate the half-equivalence volume from the curve description.
(b) Determine pKa and calculate Ka.
(c) Calculate [A⁻] at the half-equivalence point.
(d) Predict and justify the effect of using 0.050 M KOH on the calculated pKa.