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Titration of Vinegar PRE-LAB ASSIGNMENTS: To be assigned by your lab instructor. STUDENT LEARNING OUTCOMES: • • •
Learn how to use the stoichiometry of a reaction to relate moles of one thing to moles of something else. Learn how to perform an acid-base titration. Be able to work stoichiometry problems involving molarity.
EXPERIMENTAL GOALS: In this procedure, the molarity of acetic acid in vinegar and the percentage of acetic acid in vinegar will be determined by a reaction with a solution of sodium hydroxide. INTRODUCTION: This lab is another procedure designed to introduce the concept of reaction stoichiometry. In this procedure, the molarity of acetic acid in a solution of vinegar will be determined by titration with an aqueous solution of sodium hydroxide. A titration is a procedure that is often used for determining the concentration of a solution. Most commonly, a standard solution of known concentration is reacted with a solution of unknown concentration. By measuring the volume of standard solution that reacts with a known volume of the unknown solution, the concentration can be calculated from the reaction stoichiometry. A common example of this process is an acid-base titration, in which an acid or base of unknown concentration reacts with a base or acid of known concentration in a neutralization reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) In the example in Figure 1, base (OH -) of known concentration from the buret is added to the acid (H+) of unknown concentration in the flask until the equivalence point is reached, when the number of moles of OH- added equals the number of moles of H+ from the acid originally present.
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83
OHOHOHOHOHOHOH-
Base of known concentration
Acid of unknown concentration
H+ +
H
H+
OHOHOHOHOH-
H2O
H+
OHOH-
H2O
+
H
H2O
H2O
H+
H+
beginning of titration no OH- added
H2O
H+
some OH- added; some H+ has become H2O
H2O
H2O
equivalence point all H+ has become H2O
Figure 1. A typical acid-base titration. At the equivalence point, moles HCl = moles NaOH, so the concentration of the unknown HCl solution can be calculated from the reaction stoichiometry: L NaOH ×
mol NaOH 1 mol HCl × = mol HCl L 1 mol NaOH mol HCl = M HCl L HCl soln.
84 Burets are usually marked in increments of 0.1 mL, starting with 0.0 mL at the top and 50.0 mL at the bottom. The volume of the liquid in the buret can be measured very precisely to the nearest 0.01 mL, which is enough to achieve a reasonable accuracy in most volumetric analyses. The end point of the titration, where we experimentally estimate the equivalence point to be, is usually signaled by the color change of an acid-base indicator. The indicator is chosen in such a way that the end point occurs as closely as possible to the equivalence point of the titration. In many strong acid-strong base or weak acid-strong base titrations, phenolphthalein is a good indicator. Phenolphthalein is an organic dye which is colorless in an acidic environment, but pink in a basic environment (more precisely, it changes from colorless to pink over a pH range of 8.0 to 9.6). As the sodium hydroxide titrant is added to the unknown acid solution, faint swirls of pink may be observed, which disappear quickly as the base is neutralized. As the titration nears the end point, the pink swirls take longer and longer to disappear. The end point is reach when one excess drop of the titrant reacts with the phenolphthalein, producing a permanent pink color which does not disappear when the solution is swirled. When performing a titration, it is necessary to first determine the concentration of the known solution as accurately as possible. This process is referred to as standardization. In this experiment, the sodium hydroxide solution has already been standardized, and you will be provided with the solution’s concentration. In this titration, the concentration of acetic acid, HC 2H3O2, in commercial vinegar will be determined by titration against a standard solution of NaOH. The equation for this neutralization reaction is: HC2H3O2(aq) + NaOH(aq) → NaC2H3O2(aq) + H2O(l) From the known concentration of the sodium hydroxide, and the volume of the solution that emerges from the buret, the number of moles of NaOH can be determined, which allows the number of moles of HC2H3O2 originally present and the concentration and weight percent of the acid to be determined. PROCEDURE: 1. Pour approximately 75 mL of the NaOH solution into a 100 mL beaker. 2. Rinse the buret well with tap water, then rinse twice with 5 mL portions of deionized water and twice more with 5 mL portions of the NaOH solution. 3. Attach a buret clamp to a ring stand and place the buret in the clamp. Fill the buret with the NaOH solution. It is important to ensure that the buret tip does not contain an air bubble, so open the valve to allow some of the solution to run out of the buret until there are no more bubbles of air running out of the buret tip.
85 4. Record the volume reading on your sodium hydroxide buret at the starting point for the titration (2). The buret is calibrated in milliliters to the nearest 0.1 mL, so the volume readings should be estimated to the nearest 0.01 mL. The 0.0-mL mark is at the top of the buret and the 50.0-mL mark is near the bottom. To measure the volume delivered from a buret, take the difference between the volume reading at the starting point of the titration, and the final volume reading after the titration. It is not necessary that the starting point be exactly zero, as long as it is accurately known. 5. Clean two 250 mL Erlenmeyer flasks, and rinse them thoroughly with deionized water. Dry the outside of the flasks, and weigh each flask (10). (It is not necessary for the inside of the flask to be dry.) 6. Select one of the available vinegar solutions, and record its identity on the report sheet. Record the initial vinegar buret reading (6), deliver 10-12 mL of the vinegar into one of the Erlenmeyer flasks, and record the final vinegar buret reading (5). Reweigh the flask and record the mass (9). Repeat the process with the second Erlenmeyer flask; making sure to use a different volume of the same vinegar solution as was used in the first flask. 7. Add 2 drops of phenolphthalein indicator to each of the samples of vinegar. 8. Position one of the Erlenmeyer flasks containing a vinegar sample under the buret. Make sure the buret tip is inside the flask, so no drops of sodium hydroxide solution are lost. Place a sheet of white paper underneath the flask to make the color changes more obvious. 9. Open the valve on the buret and begin to add sodium hydroxide solution to the vinegar sample, while constantly swirling the flask. The addition may be rapid at first, but as soon as you start to see swirls of pink color in the vinegar solution, slow the rate of addition down to a fast dropwise rate. Swirl the solution continuously, and observe the color which develops as each drop is added to the vinegar solution. As the titration gets closer and closer to the end point, the swirls of pink color will take longer and longer to disappear — as long as the color disappears between drops, however, you have not yet reached the end point. When the pink color is no longer cleared by swirling, stop the fast drip. Make sure the pink color still disappears,1 and continue the titration one drop at a time, swirling the solution after each drop, until one drops turns the vinegar solution to a solid pink color which does not disappear on swirling. This is the end point of the titration. Record the volume on the buret (1) and rinse the contents of the flask out in the sink. 10. Repeat the titration with the other vinegar sample. 11. Dispose of the sodium hydroxide solution in the beaker and buret in the sink. Rinse the buret well with tap water, and then rinse it again a couple of times with deionized water (making sure to run some water through the buret tip) before returning it to the stockroom. Rinse the beakers and flasks with tap water and deionized water.
1
If the pink color persists at this point, you have overshot the end point, and need to repeat the titration with a fresh sample of vinegar.
86 CALCULATIONS: 1. Calculate the volume of the sodium hydroxide solution used in mL (3) and L (4). From the concentration of sodium hydroxide you have been given, determine the number of moles of sodium hydroxide used (12), and from the stoichiometry of the reaction, determine the moles of HC2H3O2 present in the vinegar sample (13). 2. Calculate the volume of the vinegar solution used in mL (7) and L (8). From the number of moles of HC2H3O2 present in the vinegar sample and the volume of the vinegar sample, calculate the molarity of acetic acid in vinegar (14). Record the average molarity of the two experiments (15). 3. Determine the mass of the vinegar sample (11). From the number of moles of HC 2H3O2 in the solution (13), determine the mass of HC 2H3O2 in the vinegar sample (16). From the mass of HC2H3O2 and the mass of the vinegar (14), determine the mass % of HC2H3O2 in the vinegar sample (17). Record the average mass % of the two experiments (18).
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88 LAB REPORT Titration of Vinegar Name ________________________________
Date _________
Partner ________________________________
Section _________
Report Grade ______
Vinegar sample used: ______ Concentration of sodium hydroxide solution: ____________ First Determination
Second Determination
1.
Final reading of NaOH
____________
____________
2.
Initial reading of NaOH
____________
____________
3.
Volume of NaOH used (mL)
____________
____________
4.
Volume of NaOH used (L)
____________
____________
5.
Final reading of vinegar
____________
____________
6.
Initial reading of vinegar
____________
____________
7.
Volume of vinegar used (mL)
____________
____________
8.
Volume of vinegar used (L)
____________
____________
9.
Weight of flask + vinegar
____________
____________
10 .
Weight of empty flask
____________
____________
11 .
Weight of vinegar
____________
____________
89
90
First Determination
Second Determination
12 .
Moles of NaOH used (show calculations)
___________ _
___________ _
13 .
Moles of HC2H3O2 in vinegar (show calculations)
___________ _
___________ _
14 .
Molarity of HC2H3O2 in vinegar (show calculations)
___________ _
___________ _
15 .
Average molarity of HC2H3O2 in vinegar
16 .
Mass of HC2H3O2 in vinegar (show calculations)
____________
___________ _
___________ _
91
17 .
Mass % HC2H3O2 in vinegar (show calculations)
18 .
Average Mass % of HC2H3O2 in vinegar
___________ _
___________ _
____________
