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Experiment: Titration of Acidic Household Substances

Objective
The purpose of this experiment is to determine the molarity and weight/volume percent of acetic acid in vinegar by titration with sodium hydroxide to a phenolphthalein endpoint. This procedure may be modified by titrating an acidic beverage such 7-up or Sprite.

 
Equipment and Material (Total Time 60 – 90 min.)
Variety of Berel Pipet
Long glass stirring rod
Wash bottle
10-mL graduated cylinder
100 mL beakers
Material
NaOH, Sodium hydroxide Titrant solution
White vinegar
phenolphthalein
Distilled water
 
Discussion
Acetic acid (CH3COOH) is the chemical compound which gives vinegar its pungent odor and sour taste.  It contains one ionizable hydrogen atom, CH3COOH, (shown in bold type)
 

In aqueous solution, when acetic acid reacts with a strong base like sodium hydroxide (NaOH), the hydroxide ion from the base reacts with the hydrogen ion from the acid to produce water.  The remaining sodium ion (Na+) and the polyatomic (CH3COO-) acetate ion form the soluble salt sodium acetate, NaCH3COO(aq), which remains dissociated in solution.  This acid-base reaction is called a neutralization reaction and has the general form:

Acid + base   a salt  +  water

For the reaction of acetic acid with sodium hydroxide, the chemical equation is written as:

HC2H3O2(aq) + NaOH(aq)  -(H2O)   Na+(aq) + C2H3O2-(aq) + H2O(l)

The progress of a neutralization reaction can be monitored by using a pH indicator, such as phenolphthalein (see Experiment 11).  This type of monitoring is called titration and involves the incremental addition of small amounts of base to the acid in the presence of a pH indicator.  An indicator is chosen which will change color at a pH very close to the equivalence point of the titration.  The equivalence point is defined as the point at which the number of moles of base added is equal to the number of moles of acid present in the solution being titrated.  When the indicator changes color, this is called the endpoint of the titration and indicates that the equivalence point has been reached.  By accurately knowing the molarity of the base and the volume of base added to reach the endpoint, the molarity of the acid can be calculated, providing the reaction has a one to one stoichiometry of acid to base.  (It is possible to do this for other stoichiometric ratios as well but that is beyond the scope of this experiment.)
The method for calculating the molarity of acid in the solution is based on the definition of the equivalence point.  At the equivalence point (endpoint):
moles of base added = moles of acid present

 
 
Further, it is known from solution chemistry that:

molarity • volume = number of moles

Now, in this titration we know the molarity and volume of the base added and the volume of acid titrated.  Therefore, to find the molarity of the acid we use the equation:

Mbase • Vbase = Macid • Vacid

The known values are then substituted to solve for the molarity of the acid.

Example 1:    A 25.00 mL sample of a monoprotic acid of unknown molarity was titrated to the equivalence point with 50.00 mL of 0.100 M sodium hydroxide.  What is the molarity of this acid?
 
Step 1: Write the equation.  Mbase • Vbase = Macid • Vacid
Step 2: Convert mL of base to L of base.

Step 3: Convert mL of acid to L of acid.


Step 4.  Substitute the appropriate values into the equation and solve for molarity.  

 

M acid = 0.200 M

 
For acids used in the laboratory, molarity is commonly used to express concentration. However, for common household acids, such as the vinegar you will be titrating, the concentration of the acid is usually reported on the label as the percentage of acid in the solution (weight/volume percent).  In other words, the weight of the acid in grams divided by the volume of solution in milliliters.  The weight/volume percent of the acid sample titrated can be calculated from the molarity of the acid as follows:
Step 1:  Macid (Calculated above) • Vacid (titrated) = # moles of acid titrated

Step 2:   # moles of acid titrated • molar mass of acid  = grams of acid titrated

Step 3:        = weight / volume percent of acid


Example 2:  If the acid titrated in Example 1 was HCN (hydrocyanic acid), what is the weight/volume percent of HCN in the sample?

Step 1:  0.200 M • 0.02500L = 0.00500 moles HCN

Step 2:

Step 3:   

Example 2:  If the acid titrated in Example 1 was HCN (hydrocyanic acid), what is the weight/volume percent of HCN in the sample?

 

The term error refers to the absolute value of the numerical difference between the known value and the experimental value.  Errors are often expressed relative to the known value as a percent error.  Percent error is given by the equation:

The closer the percent error is to zero, then the more accurate your experimental value (i.e., the more closely the experimental value agrees with the known value).


Example 3:  If the label on the acid above indicated that the weight/volume percent was 0.600, then what was the percent error for the titration?

 


Procedure

Step1: If you are given an unknown acid solution write the unknown number, if you are given instructions to titrate a vinegar solution, write the brand of the vinegar. You must provide your own sample of white vinegar in this case. Heinz Distilled White Vinegar is the Vinegar and Berel Pipetpreferred choice.

Step2: Remove the three berel pipet from your chemical container kit and label one berel pipet “T” (titrant), another “A” (acid) and a third “W” (water).

Digital Picture1: Take a digital picture of acid (vinegar) sample you are to analyze If the sample is a household vinegar sample, show the label of the vinegar so it can be identified. Calibration of the berel pipet.
Grad Cylinder Pipet

Step 3: The berel pipet labeled with the letter “T” is your titrant pipet. You are to calibrate the drops of this berel pipet in this procedure. Pour about 20mL of deionized water into a clean glass beaker. Draw about 3.5 mL into the berel pipet. Then add the water into a clean dry 10mL graduated cylinder 1 drop at a time. Count the number of drops that is necessary to reach the 3.00mL mark. Recall that to read the volume in the cylinder correctly, you must be at eye level to the meniscus and the bottom of the meniscus must be on the 3.00mL graduated line of the cylinder. Write in your notebook the number of drops you counted to reach the 3.00 mL mark. Pour the water out of the graduated cylinder, dry the graduated cylinder and repeat the procedure two more times for a total of 3 trials. After this calibration procedure, dry the graduated cylinder.

Step 4: Add all the drops for the three trials and divide by 9mL. This is the number of drops necessary for 1.00mL. Show your calculation in your notebook as well as the result of your calculation.

Digital Picture2: Take a digital picture of acid (vinegar) in the graduated cylinder clearly showing the 3.00mL volume

 

Recording solution information
Step 5: If you are analyzing a vinegar sample, (not an unknown) read the label and record the weight/volume concentration of acetic acid in the vinegar solution. HydroIon Paper

Step 6: You are to test the pH of the vinegar solution with Hydro-ion paper. Pour about 5 mL of the vinegar into a clean dry beaker. Using your glass stirring rod, touch the vinegar solution and then touch it to the Hydro-ion paper. Record the pH of the solution in your notebook.

Step7: If you have not already done so, remove the NaOH (titrant solution) from the baggie and then record the concentration in your notebook as written on the label for the NaOH titrant solution.

Digital Picture3: Take a digital picture of result of the hydro-ion paper test. Clearly show the test strip results with and pH scale to show the pH of the solution.

 


Titration of Vinegar (prepare these samples in triplicate)
Step 8: Pour approximately 0.90 mL of vinegar into the clean dry graduated cylinder from step 2. Using the berel pipet with the label “A”, add more vinegar solution dropwise so the volume is precisely 1.00 mL. Pour this solution into a clean test tube. The test tube you are to use should be the largest in your equipment box. Rinse the inside of the graduated Test tubescylinder with 0.5mL water and add this washing with the 1.00 ml vinegar solution. Set this aside.

Step 9: Repeat step 7 for the other two vinegar solutions. Label the test tubes, 1, 2 and 3. Note that the total volume in each of the three test tube should be approximately a quarter (or less) of the total volume capacity of that test tube.

Step 10: To each of the 3 vinegar solutions in the test tube, add one drop of phenolphthalein indicator.
.

Step 11: Using the berel pipet that is labeled “T”, draw as much NaOH(aq) as possible in the berel pipet. First titrate test tube 1. Counting the drops, add NaOH(aq) titrant to this test tube 1 drop at a time. You can go ahead and quickly add the first 40 drops and then pause to stir the vinegar solution with a clean dry glass stirring rod so that the NaOH(aq) is allowed to mix thoroughly. If there are any NaOH(aq) along the side of the test tube, Titration in Test Tubethis will be shown as a magenta color. Using your berel pipet labeled “W”, draw up some deionized water and rinse the side of the test tube with this water. Remember that at this point the amount of water added to the test tube will not change the moles of acid that was originally present. Try to use a minimum amount of water when washing down the residual NaOH(aq) along the wall of the test tube.  Continue to add NaOH(aq) to the test tube drop wise (counting only the drops of NaOH(aq) added to the solution). As the endpoint approach, the pink color will persist for a longer period. Use the glass stirring rod to mix the solution. When the pink color persist for more than 1 minute then you have neutralized the acid completely (endpoint). Record the number of drops required to reach the end point.

Step 12: Repeat step 10 for the second and third trials. These are the vinegar solution labeled 1 and 2. Record the volume of NaOH titrant necessary to reach the end point. The results for second and third trial should be fairly close to each other.

Step 13: If you have time and enough NaOH(aq) titrant, repeat Steps 7-10 until you have three titrations that agree within 0.50 ml to each other.

Digital Picture4: Take a digital picture of all three acid (vinegar) samples at the endpoint of the titration. The solutions should be pink or magenta to demonstrate the color at the endpoint of the experiment.



Clean up and Reflection
Step 14: Clean your work area and pour the solutions you created in this experiment into your waste container. Pour any excess residual chemicals also in the waste container. Any unused chemicals should remain in their original container kit and reseal. Be sure to turn in these chemicals at the end of the semester for proper disposal by Miramar College.

Step 15: In your lab notebook, write a detailed summary of what you did in this experiment as part of your observations and data.


 

 
Prelab Questions 

LastName ______________First________________

Note that these questions may be slightly different from those found in the Blackboard assessment.  Numerical questions will have different data but the concept of the question is the same and if you can answer these questions here with the given data, then you can use a similar strategy in solving the problems in BlackBoard using different numbers.

1. Phenolphthalein is one of the most common indicator used to test acidity or basicity. What is the color of a solution that contains two drops of phenolphthalein in acidic solutions? What is the color in basic solutions?

 

2. pH concept:
a) What does “pH” mean?

 

b) What is the pH of a neutral solution

 

3. If a solution contains 0.01 mole of HCl in 10.0 L of solution-

a) What is its molarity?

b) What is the H+ ion concentration for this solution?

c) What is the pH for this solution?

 

4. H+ and OH- relationship:
a) What relationship exists between H+ ion concentration and OH- ion concentration?

 

b) What is the OH- concentration of an aqueous solution with the H+ concentration of 1•10-4 M?

 

c) Calculate the pOH of the concentration in 4b.

 

 

5. How many pH units change are usually required for an indicator to undergo a color transition?

 

 

6. What are the colors and the transition pH for methyl red, bromothymol blue, and alizarin yellow? (See indicator color chart)

 

Experiment: Titration of Acid (Vinegar) with NaOH

___ / __ pts score      Last Name _________________First_________________

Lab Partner _______________ Date _____

You will titrate three acid (vinegar or unknown acid) samples and then take the average of the results.

I. Acid (Analyte) information:

If you are given an unknown acid solution, write the unknown number here.

If you are analyzing a vinegar sample, write "0" as your unknown number.

 ___________ Unknown #
Write the brand of the Vinegar that is to be analyze if you are analyzing vinegar. (Brand of Vinegar)
_______________ Brand
   


  

II. Calibrate your Berel pipet:
Draw water in your berel pipet and count the number of drops necessary to fill a 10mL graduated cylinder to the 3-mL mark



Trial #1 Number of drops to reach 3-mL


_______________ Trial #1


Trial #2 Number of drops to reach 3-mL


_______________ Trial #2


Trial #2 Number of drops to reach 3mL


_______________ Trial #3


Average  for 3 mL


_______________ Average


Number of drop per 1-mL


___________ Drops in 1-mL
   


III. Sample preparation:


Weight/Volume %.
From Label of Vinegar sample.

 A.__________Wt/Vol % conc.

pH of Acid (Vinegar) sample.
B._____________pH of Acid

Molarity of NaOH(aq) titrant as written on label
C.__________NaOH (M) Conc


 

 

IV Table of Titration Results:
 
Titration #1
Titration #2
 Titration #3
Average


Initial Volume of Acid (Vinegar) Analyte
(mL)

       


Drops of phenolphthalein
added

       


Drops of NaOH to equivalent point

       


Volume of NaOH to equivalent point
(mL)


        

 

 

V.  Observation and Summary of Titration Experiment:

 

 

 

 

 

 

 

 

 

 

 

 

VI Calculations for Molarity of Acid:

i)  Concentration of NaOH(aq) Titrant solution

Find this in section III

 

 

 

____________Molarity NaOH
   

ii) Average Volume of NaOH (mL) to reach endpoint

Find this from section IV

 

 

 

_______________mL NaOH
   

iii) Average Volume of NaOH (L) to reach endpoint

Calculation: 

 

 

 

 

________________L NaOH
   

iv) Moles of NaOH 

Calculation: 

 

 

 

 

_______________mol NaOH
   

iv) Moles of Acid

 

 

  

 

_______________mol Acid
   

v) Volume of Acid (L)

This is from section IV, (use liters)

 

 

 

_______________L Acid
   

vi)Molarity of Acid

Calculation:

 

 

_______________Molarity Acid

 


VII Calculations for Weight/Volume Percent of Acid

 
 

Moles of acid titrated

This is from section IV

 

 

_________________mol Acid
   

vii) Molar mass of acetic acid (g/mol)
(Chemical formula of acetic acid HCH3O2)

Calculations

 

 

 

 

___________Molar Mass Acid
 

viii) Grams of acid titrated 

Calculation: 

 

 

 

 

___________________g Acid
 

x) Volume of acid analyzed (mL)

This is from section IV

 

 

 

 

 

__________________mL Acid

xi) Weight / volume  Percent 

Calculation: 

 

 

 

_______________% w/v Acid
 
 
 
VIII Post Lab Questions

Answer these post lab questions and turn in to your instructor before you leave lab unless told otherwise by instructor.

1.  What other indicator can be used beside phenolphthalein can be used for this experiment?
a) Aliarin
b) o-Cresolphthalein
c) 2,4-Dinitrophenol
d) Thymol Blue
e) Methyl Orange
f) No correct choice

 

2. Other than vinegar, name a household product that is acidic and can be used in this experiment.  Explain why you have selected this product. (Check all that applies)
a) Soda Beverage
b) Tomato juice
c) Baking soda
d) Windex cleaner
e) Aspirin
f) Draino

 

 

 

3. A 15.0mL volume of 0.0750M NaOH is required to reach a phenolphthalein endpoint in titrating a solution of vinegar sample. Calculate the moles (mol) of acetic acid in the vinegar sample?

 

 

 

4. A vinegar sample contains 2.00e-3 moles of acetic acid. If a 4.0 g sample of vinegar was used in the titration, calculate the percent mass (m/m) % of this vinegar solution. The molar mass of acetic acid is 60.05 g/mol.

 

 

 

5.  Complete the following table
pH values should be to the thousandth
[H3O+] and [OH- ] should be in exponential notation and with 3 significant figures

  [H3O+] [OH-] pH Acidic, Basic or Neurtal
i.e., 1.00 e-10 1.00 e-4 10.000 Basic

A

 

      

Neutral

B

 

     12.450  

C

 

 1.55e-6      

D

 

   1.66e-10    

IX.  Filing Report
In addition, be sure to turn in the following with your report.

Digital Picture1: Take a digital picture of acid (vinegar) sample you are to analyze If the sample is a household vinegar sample, show the label of the vinegar so it can be identified. Calibration of the berel pipet.

Digital Picture2: Take a digital picture of acid (vinegar) in the graduated cylinder clearly showing the 3.00mL volume

Digital Picture3: Take a digital picture of result of the hydro-ion paper test. Clearly show the test strip results with and pH scale to show the pH of the solution.

Digital Picture4: Take a digital picture of all three acid (vinegar) samples at the endpoint of the titration. The solutions should be pink or magenta to demonstrate the color at the endpoint of the experiment.

Digital Picture5, 6...: In addition, take digital photos of your lab notebook.

Lastname_ex11_pic1.jpg,  lastname_ex11_pic2.jpg, lastname_ex11_pic3.jpg ...