Chapter 17 Practice Problems: Acid-Base Titrations and Buffers

Chapter 17 Practice Problems: Acid-Base Titrations and Buffers1. Consider the following titration curves for three weak acids titrated with NaOH:a. Explain whether bromthymol blue is an appropriate indicator for each weak acid titrated above.i. Bromthymol blue is yellow for pH below 6 and blue for pH greater than 7.6.Bromthymol blue would not be appropriate for the HOCl-NaOH or the KHP-NaOHtitration since the color change at pH around 6 would be too early for both of them. Itmight work for the HF-NaOH titration since the color change might occur close thesame volume as the equivalence point for that titration.ii. Cresol red is yellow for pH below 7.2 and reddish-purple for pH greater than 8.8.Cresol red would not be appropriate for the HOCl-NaOH or the KHP-NaOH titrationsince the color change at pH around 6 would be too early for each of them, but it wouldwork for the HF-NaOH titration since the color change would occur at the same volumeas the equivalence point for that titration.iii. Thymolphthalein is colorless for pH less than 9.3 and blue for pH greater than 10.5.Thymolphthalein would not be appropriate for the HOCl-NaOH titration since the colorchange at pH around 6 would be slightly too early for it, but it would work for the KHP-NaOH and HF-NaOH titrations since the color change would occur at the same volumeas the equivalence points for these titrations.

. Calculate the molarity for the NaOH used in the titrations above if 50.0 mL of each acid wasused, and all the acids had a molarity of 0.050M.0.050 mol HA 1 mol HA 1L0.0500 L××× = 0.10M NaOHL 1 mol NaOH 0.0250 Lc. Indicate the major species at the equivalence point for each weak acid-NaOH titration shownabove, then write the relevant dissociation or hydrolysis reaction for the species that regulates thepH at the equivalence point.i. major species: OCl − reaction: OCl − (aq) + H 2 O(l) HOCl(aq) + OH − (aq)ii. major species: Phthalate − reaction: Phth −2 (aq) + H 2 O(l) HPhth − (aq) + OH − (aq)iii. major species: F − reaction: F − (aq) + H 2 O(l) HF(aq) + OH − (aq)d. Explain whether the following solutions will be acidic, basic, or neutral at the equivalence pointby indicating the major species that regulates the pH for each solution:i. a strong acid titrated with a strong baseiv. a strong base titrated with a strong acidThe solution would be neutral since the conjugate base of a strong acid is a weakerbase than water, so the major species (conjugate base of strong acid and cation ofstrong base) remaining in solution are neutral.ii. a weak acid titrated with a strong baseThe solution would be basic since the conjugate base of a weak acid is a stronger basethan water, so it hydrolyzes in water to produce hydroxide ions.iii. a weak base titrated with a strong acidThe solution would be acidic since the conjugate acid of a weak base dissociates inwater to produce H + (or hydronium) ions.