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Acid-Base Titration Curves

INSTRUCTOR RESOURCES
The CCLI Initiative

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.

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Name ______________________________ Section _______

Date ____________________

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

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Name ______________________________ Section _______

Date ____________________

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.

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Name ______________________________ Section _______

Date ____________________

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.

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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...
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