Materials Included In Kit

Additional Materials Required

Prelab Preparation

Potassium hydrogen phthalate, KHC₈H₄O₄
Dry the solid for at least two hours in an oven at 110 °C. Store the dry solid in a desiccator. It must be cool when its mass is measured.

Preparation for HCl 0.06 M, Unknown 1 

1. Measure 24 mL of 0.5 M hydrochloric acid in a graduated cylinder.
2. Measure 150 mL of distilled water in another graduated cylinder.
3. Pour the water in a 250-mL Erlenmeyer flask.
4. Add the 24 mL 0.5M hydrochloric acid to the water.
5. Add another 26 mL of distilled water to make 200 mL of 0.06M hydrochloric acid.
6. Label the flask Unknown 1.

Preparation for HCl 0.09 M, Unknown 2 

1. Measure 36 mL of 0.5 M hydrochloric acid in a graduated cylinder.
2. Measure 150 mL of distilled water in another graduated cylinder.
3. Pour the water in a 250-mL Erlenmeyer flask.
4. Add the 36 mL 0.5M hydrochloric acid to the water.
5. Add another 14 mL of distilled water to make 200 mL of 0.09 M hydrochloric acid.
6. Label the flask Unknown 2.

Preparation for HCl 0.12 M, Unknown 3 

1. Measure 48 mL of 0.5 M hydrochloric acid in a graduated cylinder.
2. Measure 150 mL of distilled water in another graduated cylinder.
3. Pour the water in a 250-mL Erlenmeyer flask.
4. Add the 48 mL 0.5M hydrochloric acid to the water.
5. Add another 2 mL of distilled water to make 200 mL of 0.12 M hydrochloric acid.
6. Label the flask Unknown 3.

Preparation for HCl 0.15 M, Unknown 4 

1. Measure 60 mL of 0.5 M hydrochloric acid in a graduated cylinder.
2. Measure 120 mL of distilled water in another graduated cylinder.
3. Pour the water in a 250-mL Erlenmeyer flask.
4. Add the 60 mL 0.5M hydrochloric acid to the water.
5. Add another 20 mL of distilled water to make 200 mL of 0.15 M hydrochloric acid.
6. Label the flask Unknown 4.

Preparation for HCl 0.18 M, Unknown 5 

1. Measure 72 mL of 0.5 M hydrochloric acid in a graduated cylinder.
2. Measure 120 mL of distilled water in another graduated cylinder.
3. Pour the water in a 250-mL Erlenmeyer flask.
4. Add the 72 mL 0.5M hydrochloric acid to the water.
5. Add another 8 mL of distilled water to make 200 mL of 0.18 M hydrochloric acid.
6. Label the flask Unknown 5.

Safety Precautions

The 1 M sodium hydroxide solution is moderately toxic by ingestion and skin absorption. It is corrosive to body tissues and causes severe skin burns and eye damage. Do not breathe mist, vapors or spray. The dilute (0.1 M) sodium hydroxide solution causes skin and eye irritation. Hydrochloric acid solution causes severe skin burns and eye damage. Do not breathe mist, vapors or spray. Potassium hydrogen phthalate may be harmful if swallowed or in contact with skin. Phenolphthalein is an alcohol-based solution and is flammable. It is moderately toxic by ingestion. Keep away from flames and other ignition sources. Avoid contact of all chemicals with eyes and skin and remind students to wash hands thoroughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemical-resistant gloves and apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The solid acid may be handled according to Flinn Suggested Disposal Method #24a. Excess sodium hydroxide solutions may be treated according to Flinn Suggested Disposal Method #10. Excess hydrochloric acid may be neutralized according to Flinn Suggested Disposal Method #24b. The titrated solutions may be rinsed down the drain according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The Prelaboratory Questions may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Before having students start the lab, demonstrate how to use a buret and how to titrate a solution. Show students how to fill a buret and also an example of when an endpoint is reached (the solution should be a very light pink). The Flinn Scientific Laboratory Techniques Guide (Catalog No. AP6248) provides thumbnail illustrations of these and 14 other common laboratory techniques.
• Remind students to slide the desiccator lid off.
• Instead of swirling the Erlenmeyer flasks, students can use a magnetic stir bar and stirrer.
• If solutions will be used the following day, cover solutions with Parafilm^® to avoid evaporation.

Teacher Tips

• Quantitative analysis represents a nearly invisible application of chemistry in our daily lives. To illustrate the importance of quantitative analysis, ask students how they would feel if they could not trust that the water they drink or the medicines they take had been tested to assure quality and safety.
• Additional dilute acids can be titrated if desired. For example, use a 0.08 M acetic acid solution.

Answers to Prelab Questions

1. What is the difference between equivalence point and endpoint?

   The equivalence point is the point at which stoichiometric amounts of the acid and base have combined, whereas the endpoint of the titration is the point at which the indicator changes color. For a titration to work well, an indicator must be picked that will change colors near the equivalence point (i.e., an endpoint that is near the equivalence point of the reaction). In this lab, phenolphthalein was picked because it changes color near the equivalence point of the reaction.

2. Calculate the molarity of a solution of sodium hydroxide, NaOH, if 23.64 mL of this solution is needed to neutralize 0.5632 g of potassium hydrogen phthalate.
   {14101_PreLabAnswers_Equation_3}
3. Calculate the molarity of hydrochloric acid, if 24.68 mL of 0.1165 M NaOH is needed to titrate 20.0 mL of the acid.

   (0.1165 M)(0.02468 L) = M_a(0.0200L)
   M_a = 0.144 M

Sample Data

Standardization Data Table

Molarity NaOH (average) ___0.104___ M

Unknown Concentration Data Table Concentration of Unknown (average) ___0.0606___M

Answers to Questions

1. From the standardization data, calculate the molarity of the sodium hydroxide solution for each trial. Average the values and enter the molarity of NaOH above.
   {14101_Answers_Equation_4}

   Molarity (Average): (0.104 + 0.102 + 0.106)/3 = 0.104 M NaOH

2. From the unknown concentration data, calculate the molarity of the hydrochloric acid solution for each trial. Average the values and enter the concentration of unknown acid average above.

   Trial 1

   (0.104 M)(0.01190 L) = M_a(0.0205 L)
   M_a = 0.0604 M

   Trial 2

   (0.104 M)(0.01165 L) = M_a(0.0200 L)
   M_a = 0.0606 M

   Trial 3

   (0.104 M)(0.01140 L) = M_a(0.0195 L)
   M_a = 0.0608 M

   Molarity (Average): (0.0604 + 0.0606 + 0.0608)/3 = 0.0600 M

3. Why must the KHP samples be dried? If they are not dried, how would the results change (high or low)?

   If the KHP is moist when weighed, a higher mass of KHP would be measured. Therefore, the calculated molarity of the NaOH would be too high.

4. Why must NaOH be standardized? Why can’t an exact solution of NaOH be prepared?

   Sodium hydroxide pellets rapidly absorb water from the air, therefore, the precise mass of NaOH cannot be measured on an analytical balance.

Introduction

Acids and bases are all around us! They’re in food, household cleaners and more! Acid–base titrations can be used to measure the concentration of an acid or base in solution. In this lab, the concentration of a sample of hydrochloric acid will be determined through an acid–base titration.

Concepts

• Molarity
• Acid–base titration
• Indicator

Background

In acid–base chemistry, titration is most often used to analyze the amount of acid or base in a sample or solution. To perform a titration, the concentration of the sodium hydroxide must be known, this is called a standard solution. To “standardize” the NaOH, that is, to find its exact molarity, the NaOH is titrated against a solid acid, potassium hydrogen phthalate (abbreviated KHP). The KHP is chosen because it is easily dried and weighed and has a relatively high equivalent mass. The formula of KHP is shown in Figure 1.

KHP contains one ionizable H⁺. The titration is followed using phenolphthalein as an indicator.

In this lab, a known volume of the hydrochloric acid solution will be “titrated” by slowly adding dropwise a standard solution of a strong standardized base. The titrant, sodium hydroxide in this case, reacts with and consumes the acid via a neutralization reaction (Equation 1). The exact volume of base needed to react completely with the acid is then measured. This is called the equivalence point of the titration—the point at which stoichiometric amounts of the acid and base have combined. Indicators are usually added to acid–base titrations to detect the equivalence point. The endpoint of the titration is the point at which the indicator changes color and signals that the equivalence point has indeed been reached. For example, in the case of the neutralization reaction shown in Equation 1, the pH of the solution would be acidic (< 7) before the equivalence point and basic (> 7) after the equivalence point. The pH at the equivalence point should be exactly 7, corresponding to the neutral products (sodium chloride and water). An indicator, such as phenolphthalein, that changes color around pH 7 is therefore the suitable indicator for the titration of a strong acid with a strong base.

When the solution turns light pink, the volume of the titrant (base) added will be recorded. Knowing the exact concentration and volume added of the titrant gives the number of moles of sodium hydroxide. The latter, in turn, is related by stoichiometry to the number of moles of hydrochloric acid initially present in the unknown.

In this reaction, the ratio is 1:1. When the molarity of the base is known, as well as the volume of the acid and base, the unknown concentration of the acid can be determined using Equation 2. M_a is the molarity of the acid and i is the volume of the acid. M_b is the molarity of the base and V_b is the volume of the base.

Experiment Overview

The purpose of this experiment is to standardize a sodium hydroxide solution and use the standard solution to titrate an unknown concentration of hydrochloric acid solution.

Materials

Prelab Questions

1. What is the difference between equivalence point and endpoint?
2. Calculate the molarity of a solution of sodium hydroxide, NaOH, if 23.64 mL of this solution is needed to neutralize 0.5632 g of potassium hydrogen phthalate.
3. Calculate the molarity of hydrochloric acid, if 24.68 mL of 0.1165 M NaOH is needed to titrate 20.0 mL of the acid.

Safety Precautions

All the acids and bases used in this lab are irritating to eyes, skin and other body tissues. Phenolphthalein is an alcohol-based solution and is flammable. It is moderately toxic by ingestion and a possible carcinogen. Keep away from flames and other ignition sources. Avoid contact of all chemicals with eyes and skin and wash hands thoroughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemical-resistant gloves and apron.

Procedure

Part A. Preparing a 0.1 M NaOH solution.

1. Measure 25 mL of 1.0 M sodium hydroxide solution in a graduated cylinder.
2. Place the 1.0 M sodium hydroxide solution into a 250-mL beaker.
3. Dilute the sodium hydroxide to 250 mL by adding 225 mL of distilled or deionized water.

Part B. Standardization of a Sodium Hydroxide Solution

4. Obtain a sample of potassium hydrogen phthalate (KHP) that has been previously dried in an oven and stored in a desiccator.
5. On an analytical balance, accurately weigh approximately 0.4 g of KHP in a previously tared weighing dish. Record the mass of the KHP in the Standardization Data Table on the Titration Worksheet.
6. Transfer the KHP into an Erlenmeyer flask—pour the solid through a funnel into the flask. Use water from a wash bottle to rinse all of the remaining solid in the weighing dish or in the funnel into the flask as well.
7. Add about 40 mL of distilled water to the flask and swirl until all the KHP is dissolved.
8. Prepare a 50-mL buret by rinsing it with distilled or deionized water. Then rinse it with three small portions (about 7 mL each) of the 0.1 M NaOH solution.
9. Fill the buret to above the zero mark with the 0.1 M NaOH solution.
10. Open the buret stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level is between the 0- and 10-mL marks.
11. Measure the precise volume of the solution in the buret and record this value in the Standardization Data Table as the “initial volume.” Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus, remembering to include the appropriate number of significant figures (see Figure 2).
    {14101_Procedure_Figure_2}
12. Position the buret over the Erlenmeyer flask so that the tip of the buret is within the flask but at least 2 cm above the liquid surface.
13. Add three drops of phenolphthalein solution to the KHP solution in the flask.
14. Place a white piece of paper under the Erlenmeyer flask to help view the color change.
15. Begin the titration by adding 1.0 mL of the 0.1 M NaOH solution to the Erlenmeyer flask, then closing the buret stopcock and swirling the flask.
16. Repeat step 15 until 15 mL of the 0.1 M NaOH solution have been added to the flask. Be sure to continuously swirl the flask.
17. Reduce the incremental volumes of NaOH solution to ½ mL until the pink color starts to persist. Reduce the rate of addition of NaOH solution to drop by drop until the pink color persists for 15 seconds. Remember to constantly swirl the flask and to rinse the walls of the flask with distilled water before the endpoint is reached.
18. Measure the volume of 0.1 M NaOH remaining in the buret, estimating to the nearest 0.01 mL. Record this value as the “final volume” in the Standardization Data Table.
19. Repeat the standardization titration two more times. Rinse the Erlenmeyer flask thoroughly between trials with deionized water.

Part C. Determining the Molarity of a Sample of Hydrochloric Acid with an Unknown Concentration

20. Obtain 60.0 mL of hydrochloric acid with an unknown concentration. Record the unknown number in the Unknown Concentration Data Table.
21. Measure 20.0 mL of the hydrochloric acid into a clean Erlenmeyer flask.
22. Add 20 mL of distilled water to the flask.
23. Add 3 drops of phenolphthalein to the flask.
24. Fill the buret with 0.1 M sodium hydroxide to between the 0- to 10-mL marks.
25. Measure the precise volume of the solution in the buret and record this value in the Unknown Concentration Data Table as the “initial volume.” Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus, remembering to include the appropriate number of significant figures (see Figure 2).
26. Place a white piece of paper under the Erlenmeyer flask to help view the color change.
27. Slowly titrate the hydrochloric acid with the 0.1 M sodium hydroxide solution.
28. When the solution in the Erlenmeyer flask is light pink, stop the titration and record the final volume of 0.1 M NaOH in the buret in the Unknown Concentration Data Table.
29. Repeat steps 21–28 one more time. A third trial is optional.
30. Dispose of the solutions and any solid acid as directed by your instructor.

Student Worksheet PDF

14101_Student1.pdf