CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
An unknown monoprotic acid, HA, is analysed by titration with 0.150 M NaOH at 25 °C. A 20.0 mL aliquot of the acid solution is titrated and the following data are collected:
• Initial pH: 2.48
• After addition of 10.0 mL NaOH: pH 4.74
• After addition of 25.0 mL NaOH (equivalence point): pH 8.88
Answer the following questions.
(a) Determine the molarity of the original HA solution.
(b) Use the pH at 10.0 mL of added NaOH to calculate the acid dissociation constant, Ka, for HA.
(c) Using the pH at the equivalence point, independently calculate Ka for HA and compare this value to the result from part (b). Comment on any discrepancy.
(d) Among the indicators methyl red (pH 4.4–6.2), bromothymol blue (pH 6.0–7.6), and phenolphthalein (pH 8.2–10.0), select the most appropriate indicator for this titration and justify your choice using quantitative evidence from parts (b) and (c).