Titrating the Acetic Acid in Vinegar...4. Acetic acid solutions called “cleaning vinegar” are...

Titrating the Acetic Acid in Vinegarprepared by M. L. Gillette, Indiana University Kokomo,H. A. Neidig, Lebanon Valley College, and J. N. Spencer, Franklin and Marshall College

Purpose of the Experiment

Determine the molar concentration of acetic acid in vinegar by titrating it with astandard sodium hydroxide solution.

Background Required

You should understand the concepts associated with stoichiometry andacid–base chemistry.

Background Information

Vinegar, a familiar ingredient in foods, is an aqueous (water) solution of aceticacid (HC2H3O2, which we will symbolize as HOAc), often with added flavorings.Acetic acid can be neutralized by sodium hydroxide (NaOH) solution, as shown inEquation 1.

HOAc (aq) + NaOH (aq) → NaOAc (aq) + H2O ( l ) (Eq. 1)acid base salt water

We can determine the exact HOAc concentration in a vinegar sample by ti-trating a measured volume of the vinegar with a standard NaOH solution, one ofknown concentration. A titration is the measurement of the volume of standardsolution required to completely react with a measured volume or mass of thesubstance being analyzed. When performing a titration, we often add a thirdsubstance, called an indicator, which changes color when the titration is com-plete. The titration end point is signaled by this color change. For convenience,we will express HOAc concentration in terms of molarity(M), the number ofmoles of solute per liter of solution.

Phenolphthalein is a good indicator for this titration. Phenolphthalein is col-orless in acidic solution and pink in basic solution. As soon as any excess NaOH isadded to the titration mixture, the phenolphthalein turns pink, signaling the endpoint of the titration.

Example

Problem Find the molarity of HOAc in a vinegar sample when 22.34 mL of 0.4916M NaOHare required to neutralize 20.00 mL of vinegar.

Solution (1) Calculate the number of moles of NaOH required to neutralize the HOAc.

A N A L

619m o d u l a r • l a b o r a t o r y • p r o g r a m • i n • c h e m i s t r ypublisher: H. A. Neidig editor: M. L. Gillette

Copyright © 1999 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, includingphotocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.Printed in theUnited States of America 01 00 99 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

number of moles ofNaOH required, mol

volume of NaOH=solution used, mL

1 L

1000 mLconcentration

ofNaOH solution, mol / L

Eq.

mL )1 L

10

=

( )

( .

2

22 3400 mL

mol / L )

1.098 10 mol NaOH–2

= ×

( .0 4916

(2) Calculate the number of moles of HOAc present in the vinegar sample titrated.

number of moles of HOAc intitrated vinegar sample, mol

number of molesof NaOH required, mol

1 mo=

l HOAc1 mol NaOH

Eq.

mol NaOH1mol

= ×

( )

. –

3

1 098 10 2 HOAc

1 mol NaOH

mol HOAc

= ×1 098 10 2. –

(3) Calculate the molarity of HOAc in the vinegar sample.

molarity of HOAc invinegar sample, mol / L

number of= moles of HOAc in vinegar sample, mol

volume of vinegar sample analyzed, LEq.

mol H

= ×

( )

. –

4

1 098 10 2 OAc

mL )1 L

1000 mL

mol / L = 0.5491

( .

.

20 00

0 5491

= M

In This Experiment

You will titrate the HOAc in a vinegar sample with standard NaOH solution andthen safely dispose of your discarded solutions. From your titration data, you willcalculate the molarity of HOAc in the vinegar.

Procedure

Note: • The general set-up for this experiment is shown in Figure 1.• Dispose of your reaction mixtures and rinses according to your

laboratory instructor’s directions.• Record all of your data on your Data and Observations sheet.

2 ANAL 619/Titrating the Acetic Acid in Vinegar

©19

99C

hem

ical

Edu

catio

nR

esou

rcesCaution: Wear departmentally approved safety goggles while doing this experiment.

Always use caution in the laboratory. Many chemicals are potentially harmful.Prevent contact with your eyes, skin, and clothing. Avoid ingesting any of the reagents.

I. Preparing the Vinegar Sample for Titration

1. Obtain about 125 mL of a vinegar sample in a dry 250-mL beaker. Recordthe identification code of your sample.

Label a 400-mL beaker “Discarded Solutions”.

2. Rinse the buret with vinegar solution. Collect the rinses in the Discarded So-lutions beaker. Clamp the buret to the support stand. Label the buret “vinegar”.

3. Fill the buret with vinegar solution to a level above the 0-mL calibrationmark.

Note: Make sure that the buret tip is completely filled with vinegar solution.There should not be any air bubbles in the buret barrel or tip. This is because thevolume of any such bubbles will be included in the volume of vinegar titrated,which will lead to an erroneously low concentration of HOAc in the vinegar.

4. If there are any air bubbles in the buret tip or barrel, eliminate them by open-ing the stopcock for 1–2 s. Then slowly drain vinegar solution into the DiscardedSolutions beaker until the bottom of the meniscus is at, or slightly below, the0-mL calibration. Touch the inner sidewall of the beaker to the buret tip to re-move any vinegar solution that may be clinging to the tip.

Note: A 50-mL buret is calibrated in units of 0.1 mL, but measurements to thenearest 0.01 mL can be reproducibly estimated. Record every reading to thenearest 0.01 mL.

When reading the buret, hold a white card marked with a dark stripe directlybehind and slightly below the meniscus, as shown in Figure 2 on the next page.Keep your line of sight level with the bottom of the meniscus.

5. Read the buret and estimate to the nearest 0.01 mL and record this initial vin-egar buret reading in the column headed “determination 1”.

ANAL 619/Titrating the Acetic Acid in Vinegar 3

©19

99C

hem

ical

Edu

catio

nR

esou

rces

Figure 1 A titration assembly

6. Deliver 20–25 mL of your vinegar sample into a 125-mL Erlenmeyer flask.Record the final vinegar buret reading for determination 1.

7. Add 40 mL of distilled or deionized water and three drops of phenol-phthalein solution to the vinegar in the flask. Gently swirl the flask to mix thesolution.

II. Preparing the NaOH Buret

8. Obtain about 150 mL of standard NaOH solution in a dry 250-mL Erlenmeyerflask. Stopper the flask with a clean cork, and keep the flask stoppered when notin use. Record the molarity of the NaOH solution to four significant figures.

9. Rinse a second 50-mL buret with standard NaOH solution. Collect the rinsesin the Discarded Solutions beaker. Clamp the buret to the support stand. Labelthe buret “NaOH”.

10. Fill this buret with NaOH solution to a level above the 0-mL calibration. Fol-low the procedure of Step 4 to fill and remove air bubbles from the buret tip.

Read the buret and estimate to the nearest 0.01 mL and record this initialNaOH buret reading in the column headed “determination 1”.

11. Position the NaOH buret and the 125-mL Erlenmeyer flask containing thevinegar solution as shown in Figure 3.

III. Titrating Vinegar

Note: As the titration proceeds, you will observe a pink coloration at the pointwhere NaOH solution contacts the surface of the vinegar solution. As you ap-proach the end point, pink will begin to momentarily flash through the titrationmixture and you should start adding the NaOH solution dropwise.

Stop the titration when pink persists throughout the titration mixture for 30 safter you have thoroughly swirled the flask.


©19

99C

hem

ical

Edu

catio

nR

esou

rces

Figure 2 Reading the liquid level in a buret

Caution: NaOH solution is toxic and corrosive, and it can cause skin burns.

12. Titrate the vinegar sample, adding 1–2 mL of NaOH solution at a time, untilyou begin to see pink flashing through the mixture. Then start adding NaOH indrops. Gently swirl the flask after each NaOH addition.

At the end point, read the meniscus level and estimate to the nearest 0.01 mL.Record this final NaOH buret reading in the column headed “determination 1”.

13. Refill the buret with your NaOH solution. Titrate a second vinegar sample ina second 125-mL Erlenmeyer flask, repeating Steps 5–12. Record your data in thecolumn headed “determination 2”.

14. Drain the remaining NaOH solution from your buret into the Discarded Solu-tions beaker. Rinse the buret with tap water. Transfer the rinses to the DiscardedSolutions beaker.

Rinse the buret twice with distilled water. Transfer the rinses into the Dis-carded Solutions beaker. With the stopcock open, invert the buret, and clamp itto the support stand for drying.

15. Repeat Step 14 with your vinegar buret.

16. Empty the titration mixtures from the Erlenmeyer flasks into the drain. Rinsethe flasks with tap water and then with distilled water.

Pour the remaining vinegar into the Discarded Solutions beaker. Rinse thevinegar beaker with tap water and then with distilled water. Transfer the rinsesinto the Discarded Solutions beaker.

17. Unless your laboratory instructor indicates otherwise, pour any remainingstandard NaOH solution into the Discarded Solutions beaker. Rinse the flaskwith tap water and then with distilled water. Discard the rinses into the DiscardedSolutions beaker.

IV. Neutralizing Your Discarded Solutions


©19

99C

hem

ical

Edu

catio

nR

esou

rces

Figure 3 Positioning the NaOH buret for titration and manipulating the stopcock

Caution: 6M HCl is a toxic, corrosive solution that can cause skin burns.

18. Add 2–3 drops of phenolphthalein solution to the Discarded Solutionsbeaker. Gently swirl the beaker to mix. If the resulting solution is not pink, con-sult with your laboratory instructor.

Add 6M hydrochloric acid (HCl) dropwise to the Discarded Solutions beaker,mixing well with a glass stirring rod after each addition. Continue adding HCl so-lution dropwise just until the solution becomes colorless. Pour the colorless solu-tion into the drain. Rinse the beaker with tap water, then distilled water, pouringthe rinses into the drain.


©19

99C

hem

ical

Edu

catio

nR

esou

rces

Caution: Wash your hands thoroughly with soap or detergent before leaving thelaboratory.

Post-Laboratory Questions

Use the spaces provided for the answers and additional paper if necessary.

1. Would the following procedural errors cause your calculated molarity of HOAc in vinegar to be toohigh, too low, or unchanged? Briefly explain your answer.

(a) You forgot to rinse your wet buret with vinegar before you delivered the vinegar into theErlenmeyer flask.

(b) You delivered the vinegar into a wet Erlenmeyer flask.

(c) You used a buret with a broken tip.

2. A student performing this experiment forgot to add phenolphthalein solution to the vinegar solutionbefore beginning the titration. After adding 27 mL of NaOH solution, he realized his error and added theindicator. The solution turned bright pink. Suggest a procedure the student could follow to salvage the ti-tration.

3. In Part IV of the Procedure, you added 6M HCl to the Discarded Solutions beaker containing excessNaOH solution. HCl reacts with NaOH as shown in Equation 5.

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O ( l ) (Eq. 5)

Briefly explain why it would be hazardous to dispose of either your NaOH solution or 6M HCl in the drain,yet it is safe to pour the reaction mixture you prepared in Part IV into the drain.


name partner section date

©19

99C

hem

ical

Edu

catio

nR

esou

rces

4. Acetic acid solutions called “cleaning vinegar” are effective in dissolving calcium carbonate depositson shower tiles. What sample size of 0.9181M cleaning vinegar would require 27.50 mL of 0.5185MNaOH for neutralization?

5. Muriatic acid is a concentrated HCl solution available in the hardware department of some stores.What mass of baking soda (sodium hydrogen carbonate, NaHCO3) would be required to neutralize10.00 mL of 12.1M muriatic acid remaining in a nearly empty bottle, so that the solution could be safelypoured into the drain, and the bottle recycled? The reaction of NaHCO3 with HCl is shown in Equation 6.

HCl(aq) + NaHCO3(aq) → NaCl(aq) + CO2(g) + H2O( l ) (Eq. 6)


©19

99C

hem

ical

Edu

catio

nR

esou

rces

Data and Observations

I. Preparing the Vinegar Sample for Titration

vinegar sample identification code _____________________

determination1 2

initial vinegar buret reading, mL _____________________ _____________________

final vinegar buret reading, mL _____________________ _____________________

volume of vinegar sample transferred, mL _____________________ _____________________

II. Preparing the NaOH Buret

concentration of NaOH solution, mol/L _____________________

III. Titrating Vinegar

determination

1 2

initial NaOH buret reading, mL _____________________ _____________________

final NaOH buret reading, mL _____________________ _____________________

volume of NaOH used for titration, mL _____________________ _____________________



©19

99C

hem

ical

Edu

catio

nR

esou

rces

Calculations and Conclusions

Show your calculations in the spaces provided. Remember to include units with all calculated results.

1. Calculate the number of moles of NaOH required for the titration, using Equation 2.

determination 1 _________________ determination 2 _________________

2. Calculate the number of moles of HOAc titrated, using Equation 3.


3. Calculate the molarity of HOAc in the vinegar sample, using Equation 4.


4. Calculate the average molarity of HOAc in the vinegar sample, using Equation 7.

average molarity, mol / L =molarity from determination 1 + molarity from determination 2

2(Eq. 7)

average molarity _________________


©19

99C

hem

ical

Edu

catio

nR

esou

rces

Pre-Laboratory Assignment

1. With regard to the solutions you will use in this experiment, briefly explain why you are required towear departmentally approved safety goggles while doing this experiment.

2. Briefly define the following terms:

(a) standard solution

(b) titration

(c) molarity



©19

99C

hem

ical

Edu

catio

nR

esou

rces

3. Why is it important that

(a) there are no bubbles in the buret tip prior to beginning a titration of vinegar with NaOH solution?

(b) you add phenolphthalein indicator solution to the vinegar solution prior to beginning thetitration?

4. A student performing this experiment titrates 22.32 mL of vinegar with 0.5172M NaOH. The initialNaOH buret reading is 1.18 mL and the final NaOH buret reading is 21.35 mL. What is the molarity ofHOAc in the vinegar sample?


©19

99C

hem

ical

Edu

catio

nR

esou

rces

ISBN 0-87540-619-X

Titrating the Acetic Acid in Vinegar...4. Acetic acid solutions called “cleaning vinegar” are...

Documents

Transcript of Titrating the Acetic Acid in Vinegar...4. Acetic acid solutions called “cleaning vinegar” are...