Acid-Base Titration Curves

Learning Objectives to understand the titration curves for the following solutions • a weak acid: acetic acid, CH3COOH. • • • • • a strong acid: hydrochloric acid, HCl. an acidic commercial cleanser. a basic commercial cleanser. to use the titration curves to calculate the percent of the active ingredients in the commercial cleansers. to determine the Ka of a weak acid. Procedure Overview • • • • after calibration of the pH electrode and determination of the flow rate for the automatic titration, a NaOH solution is standardized against HCl. a pH titration curve for acetic acid is obtained and its pKa is determined. an unknown sample of Lysol is analyzed for its HCl content using the standardized NaOH. an unknown sample of Liquid Plumr is analyzed for its NaOH and NaOCl content using HCl of known molarity.

ACID-BASE TITRATION CURVES Report Sheet Determination of flow rate Equation for conversion from time to volume: Titration of HCl pH at equivalence point volume of NaOH at equivalence point concentration of HCl volume of HCl Titration of acetic acid pH at equivalence point volume of NaOH at equivalence point volume at half-equivalence point pH at half-equivalence point Titration of Lysol weight of beaker with Lysol weight of empty beaker mass of Lysol pH at equivalence point volume of NaOH at equivalence point Titration of Liquid Plumr weight of beaker with Liquid Plumr weight of empty beaker mass of Liquid Plumr pH at first equivalence point volume of HCl at first equivalence point pH at second equivalence point volume of HCl at second equivalence point

ACID-BASE TITRATION CURVES Report Sheet Calculations 1. Titration of HCl (Standardization of NaOH) a. Determine the moles of HCl.

b. Calculate the molarity of NaOH.

2. Titration of acetic acid a. Calculate the Ka for acetic acid.

3. Titration of Lysol a. Calculate the percent HCl in Lysol.

4. Titration of Liquid Plumr a. Calculate the percent NaOH in Liquid Plumr.

b. Calculate the percent NaOCl in Liquid Plumr.

ACID-BASE TITRATION CURVES Questions/Problems 1. Calculate the pH of a solution in which the concentration of the hydronium ion equals 2.0 x 10-4 M.

2. If the pH at the half-equivalence point for a titration of a weak acid with a strong base is 4.60, determine the value of the Ka for the weak acid.

3. When a 0.5725 g sample of Lysol was titrated with 0.100 M NaOH, an endpoint was obtained at 15.00 ml. Calculate the percent (by weight) of hydrochloric acid in the Lysol sample.

4. When a 3.529 g sample of Liquid Plumr was titrated with 0.100 M HCl, two endpoints were obtained. The first endpoint occurred at 15.00 ml, the second at 42.00 ml. Calculate the percentages of both NaOH and NaOCl in Liquid Plumr.

5. If you did a “manual” titration, compare the graphically determined equivalence point and the indicator determined end point. If you did a “timed” titration, the indicator-determined end point MAY have passed too quickly to get meaningful data, but try to make a comparison as best you can.

ACID-BASE TITRATION CURVES Suggested Answers to Questions/Problems 1. (A) Calculate the pH of a solution in which the concentration of the hydronium ion equals 2.0 x 10-4 M. pH = log[H3O+] = pH = 3.70 (b) If the pH at the half-equivalence point for a titration of a weak acid with a strong base is 4.60, determine the value of the Ka for the weak acid. pKa = pH @ ½ equivalence point Ka = 10!4.60 = 2.5 x 10!5 2. When a 0.5725 g sample of Lysol was titrated with 0.100 M NaOH, an endpoint was obtained at 15.00 ml. Calculate the percent (by weight) of hydrochloric acid in the Lysol sample. %HCl = (15 ml HCl soln)(0.100 mmol NaOH)(36.5 g)(1 mol)(100) = 9.56 % (0.5725 g sample)(1.00 ml soln)(1 mol)(103 mmol)(100)

3. When a 3.529 g sample of Liquid Plumr was titrated with 0.100 M HCl, two endpoints were obtained. The first endpoint occurred at 15.00 ml, the second at 42.00 ml. Calculate the percentages of both NaOH and NaOCl in Liquid Plumr. 1st equivalence point = HCl reacting with NaOH = 15.00 ml 2nd eq. point = HCl reacting with NaOCl = 42.00 ml ! 15.00 ml = 27.00 ml 4. If you did a “manual” titration, compare the graphically determined equivalence point and the indicator determined end point. If you did a “timed” titration, the indicator-determined end point MAY have passed too quickly to get meaningful data, but try to make a comparison as best you can. The indicator determined end point is noticeable before the equivalence point. A faint pink color begins to appear before the equivalence point is reached. log(2.0 x 10-4)

ACID-BASE TITRATION CURVES Tips and Traps 1. 2. Lysol and Liquid Plumr should be dispensed in a fume hood, using an automatic dispenser, if possible. Students should determine the flow rate three times and average the result. Flow rate should be determined for each titration separately.

ACID-BASE TITRATION CURVES Sample Data Determination of flow rate Equation for conversion from time to volume: Titration of HCl pH at equivalence point volume of NaOH at equivalence point concentration of HCl volume of HCl Titration of acetic acid pH at equivalence point volume of NaOH at equivalence point volume at half-equivalence point pH at half-equivalence point Titration of Lysol weight of beaker with Lysol weight of empty beaker mass of Lysol pH at equivalence point volume of NaOH at equivalence point 67.5794 65.6321 1.9473 7.0 40.56 ml g g g 8.4 21.50 10.75 4.7 ml ml 7.0 20.6 0.100 25.00 ml M ml

ACID-BASE TITRATION CURVES Sample Data (page 2) Titration of Liquid Plumr weight of beaker with Liquid Plumr weight of empty beaker mass of Liquid Plumr pH at first equivalence point volume of HCl at first equivalence point pH at second equivalence point volume of HCl at second equivalence point Calculations Titration of HCl (Standardization of NaOH) 1. Determine the moles of HCl. 0.100 M x 0.02500 L = 0.00250 mol 2. Calculate the molarity of NaOH. at equivalence point: mol NaOH = mol HCl [NaOH] = 0.00250 mol = 0.121 M 0.0206 L Titration of acetic acid 1. Calculate the Ka for acetic acid. pKa = pH1/2 = 4.7 Ka = 10-4.7 = 2.0 x 10! 5 67.8074 65.6321 2.1753 9.2 9.20 4.7 13.6 ml ml g g g

Titration of Lysol 1. Calculate the percent HCl in Lysol. %HCl = (40.56 ml NaOH)(0.121 mmol NaOH)(36.5 g)(1 mol)(100) = 9.23 % (1.9743 g sample)(1.00 ml soln) (1 mol) (103 mmol)(100)

ACID-BASE TITRATION CURVES Sample Data (page 3) Titration of Liquid Plumr 1. Calculate the percent NaOH in Liquid Plumr. %NaOH = (9.20 ml HCl)(0.100 mmol HCl)(40.0 g)(1 mol)(100) = 1.69 % (2.1753 g sample)(1.00 ml soln)(1 mol)(103 mmol)(100)

Calculate the percent NaOCl in Liquid Plumr. %NaOCl = ((13.6-9.2) ml HCl)(0.100 mmol NaOH)(74.4 g)(1 mol)(100) = 1.51 % (2.1753 g sample)(1.00 ml soln)(1 mol)(103 mmol)(100)

Sample Titration Curves

HOAc titration curve with first derivative.

Combined HCl & HOAc idealized titration curves

ACID-BASE TITRATION CURVES Laboratory Preparation (per student station) Equipment • two 250-ml beakers • 150-ml beaker • 50-ml buret • buret clamp • 25-ml graduated cylinder • 10-ml graduated cylinder • glass stirring rod • glass funnel (short stem) • pH electrodes + stands w/ clamp • timer Supplies • towel Chemicals Exact quantities needed are listed below. A minimum 50% excess is recommended. • 2.5 M NaOH, 10 ml • 0.l M HCl, 50 ml • 0.l M CH3COOH, 25 ml • pH 4, 7 and 10 buffer solutions, 15 ml each, reusable • Lysol, 2 ml • Liquid Plumr, 2 ml Safety and Disposal • Lysol: handle with care – strongly acidic. Can be flushed down drain with lots of water • Liquid Plumr: handle with care – strongly basic.