Acid-Base Titration Laboratory Experiment: Preparation and Procedures, Assignments of Chemistry

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Teacher Notes

An Acid-Base Titration

Objectives

Use a titration to determine the concentration of a hydrochloric acid solution and the concentration of an acetic acid solution. Through this investigation, students:

♦ Differentiate between concentration and strength of acids and bases

♦ Perform neutralization reactions

♦ Describe and explain the shape of a titration curve

Time Requirement

♦ Preparation time 20 minutes

♦ Lab activity 60 minutes

Materials and Equipment

For each student or group:

♦ Data collection system ♦ Buret clamp

♦ Drop counter ♦ Funnel

♦ pH sensor ♦ Transfer pipet

♦ Magnetic stirrer ♦ Waste container

♦ Micro stir bar ♦ Wash bottle filled with distilled (deionized) water

♦ Beakers (2), 250-mL ♦ Buffer solution pH 4, 25 mL

♦ Beakers (2), 50-mL ♦ Buffer solution pH 10, 25 mL

♦ Graduated cylinder, 100-mL ♦ Distilled (deionized) water, 200 mL

♦ Volumetric pipet or graduated cylinder, 10-mL ♦ Hydrochloric acid (HCl) solution, 10 mL^1

♦ Buret , 50-mL ♦ Acetic acid (HC 2 H 3 O 2 ) solution, 10 mL^2

♦ Ring stand ♦ Standardized sodium hydroxide (NaOH)

♦ Right-angle clamp solution, 120 mL^3 (^1) The hydrochloric acid (HCl) solution should be approximately 0.1 M. To formulate this solution using

concentrated (12 M) or dilute (6 M) HCl, refer to the Lab Preparation section. (^2) The acetic acid (HC 2 H 3 O 2 ) solution should be approximately 0.1 M. To formulate this solution using concentrated (glacial, 17.4 M) acetic acid or 6 M acetic acid refer to the Lab Preparation section. (^3) The standardized sodium hydroxide (NaOH) solution should be approximately 0.1 M. Refer to the Lab

Preparation section on how to create and standardize the solution.

An Acid-Base Titration

Concepts Students Should Already Know

Students should be familiar with the following concepts:

♦ Balanced chemical equations

♦ Concentration (molarity)

♦ Molar ratios and stoichiometric calculations

♦ Properties of acids and bases

♦ pH

Lab Preparation

These are the materials and equipment to set up prior to the lab.

Follow these safety procedures as you begin your preparations:

♦ Wear eye protection, lab apron, and protective gloves when handling acids. Splash-proof goggles are recommended. Either latex or nitrile gloves are suitable. ♦ If acid solutions come in contact with skin or eyes, rinse immediately with a copious amount of running water for a minimum of 15 minutes. ♦ Diluting acids creates heat. Be extra careful when handling freshly prepared solutions and glassware because they might be very hot.

♦ Always add acids to water, not the other way around, because the solutions may boil vigorously. ♦ Handle concentrated acids in a fume hood; the fumes are caustic and toxic.

Prepare the following solutions:

1. Prepare 1000 mL of 0.1 M hydrochloric acid from either concentrated (12 M) or dilute (6 M) HCl. This is enough for 100 lab groups. Starting with concentrated (12 M) HCl:

a. Place a stir bar in a 1000-mL beaker, and add approximately 500 mL of distilled water.

b. Slowly add 8.3 mL of 12 M HCl to the beaker, stirring continuously.

c. Allow the solution to cool. Then carefully pour the solution into a 1000-mL volumetric flask, and dilute to the mark with distilled water.

d. Cap and invert three times carefully to ensure complete mixing.

Starting with dilute (6 M) HCl

a. Add approximately 500 mL of distilled water to a 1000-mL volumetric flask.

b. Add 16.7 mL of 6 M HCl to the water, and dilute to the mark with distilled water.

c. Cap and invert three times carefully to ensure complete mixing.

Teacher Notes

2. Prepare 1000 mL of 0.1 M acetic acid from either concentrated (glacial, 17.4 M) or dilute (6 M) acetic acid. This is enough for 100 lab groups. Starting with concentrated (glacial, 17.4 M) acetic acid:

a. Place a stir bar in a 1000-mL beaker, and add approximately 500 mL of distilled water.

b. Slowly add 5.7 mL of 17.4 M acetic acid to the beaker, stirring continuously.

c. Allow the solution to cool. Then carefully pour the solution into a 1000-mL volumetric flask, and dilute to the mark with distilled water.

d. Cap and invert three times carefully to ensure complete mixing. Starting with dilute (6 M) acetic acid:

a. Add approximately 500 mL of distilled water to a 1000-mL volumetric flask. b. Add 16.7 mL of 6 M acetic acid to the water, and dilute to the mark with distilled water. c. Cap and invert three times carefully to ensure complete mixing.

3. Prepare 1000 mL of 0.1 M sodium hydroxide (NaOH). This is enough for 8 lab groups.

a. Place a stir bar in a 1000-mL beaker and add approximately 500 mL of distilled water.

b. Slowly add 4.0 g of solid NaOH to the beaker; allow it to dissolve completely while stirring continuously.

c. Allow the solution to cool, then carefully pour the solution into a 1000-mL volumetric flask and dilute to the mark with distilled water.

d. Cap and invert three times carefully to ensure complete mixing.

e. The exact concentration of the NaOH solution will vary due to the hygroscopic (attracting water from the surrounding environment) nature of solid NaOH. The NaOH solution will absorb carbon dioxide from the air; this will produce a small amount of carbonic acid in the solution which will neutralize a small portion of the NaOH. For best results, accurately determine the concentration of (standardize) the 0.1 M NaOH solution by titrating it with potassium bitartrate (cream of tartar) as described in the next step. Report to the students the actual molarity of the NaOH solution.

4. To standardize NaOH using potassium bitartrate (KHC 4 H 4 O 6 ) follow the steps below.

a. Using a 100-mL beaker, dissolve approximately 0.5 g of potassium bitartrate (record actual amount) in exactly 50 mL of distilled water.

b. Using a calibrated pH sensor and a drop counter, find the volume of 0.1 M NaOH solution that is required to neutralize the potassium bitartrate solution.

c. Use the following calculation to find the actual concentration of the NaOH solution:

( 4 4 6 ) 4 4 6 4 4 6

1 mol 1 mol NaOH 1 grams KHC H O 188.177 g KHC H O 1 mol KHC H O L NaOH added

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An Acid-Base Titration

Safety

Add these important safety precautions to your normal laboratory procedures:

♦ Sodium hydroxide, hydrochloric acid, and acetic acid are corrosive irritants. Avoid contact with the eyes and wash hands after handling.

♦ Be sure that all acids and bases are neutralized before disposal down the drain.

♦ When mixing acids with water, always add the acid to the water, not the other way around, as the solutions may get hot enough to boil.

Answer Key

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