2 beakers, 100-mL
250-mL beaker (for discards)
Erlenmeyer flask, 125-mL
2 burets (50 mL)
Double buret clamp
Sheet of white paper
**Standard solution of NaOH (0.600 M)
* If you do this experiment with several brands of vinegar, you can compare the quality of the brands. Be sure to have enough standard NaOH solution so you can do all the tests with the same solution.
** Use a volumetric flask to prepare the NaOH standard solution for accuracy. Add some distilled water to the volumetric flask. Mass out the NaOH in a plastic weighing dish. Carefully rinse the chemical into the flask with distilled water in a rinse bottle to get all the chemical into the flask. Swirl the flask until the chemical is completely dissolved, then add distilled water until the bottom of the meniscus is exactly on the etched mark. Stopper the flask and invert at least 25 times to mix. Be sure you make enough solution to do the complete lab.
To prepare a 0.600 M solution of sodium hydroxide:
***If you do not have phenolphthalein indicator already prepared, dissolve 10 g of powdered phenolphthalein in 500 mL denatured alcohol and add 500 mL distilled water.
Preparing for titrations:
1. Obtain approximately 80 mL of the standard NaOH solution in a clean, dry, labeled 100-mL beaker.
NOTE ON BURET TECHNIQUE: When filling a buret, check to make sure the stopcock is closed. Hold the buret in your hand with a paper towel wrapped around it to catch any spill, and fill slowly. Wipe the buret up to the top with the paper towel.
2. Pour directly from the beaker into the buret a 5 mL portion of the NaOH solution. Rinse the walls of the buret thoroughly with the NaOH, allow it to drain through the stopcock, and discard it. Rinse the buret two more times in a similar fashion, using a new 5 mL portion of the NaOH solution each time. Discard all rinse solutions. Fill the buret with the standard NaOH solution above the zero mark. and place the buret in one side of the double buret clamp (see drawing). Withdraw enough solution from the buret to remove the air from the jet tip and bring the liquid level into the graduated region of the buret. Your starting volume does not have to be zero. Discard what you withdrew. Label the buret.
3. Follow steps 1 and 2 for the vinegar, and place it on the other side of the double buret clamp. Be sure to label the buret.
4. Read the buret with the vinegar as the initial volume and record it in the DATA TABLE for trial 1, as the initial volume for vinegar. Obtain approximately 10 mL of vinegar in the flask. Read the buret and record it in the DATA TABLE for trial 1, as the final volume for vinegar.
5. Add 10 or 15 mL of distilled water to the flask to increase the volume and make reading the equivalence point easier to read. Add 1 or 2 drops of phenolphthalein solution to the flask to serve as an indicator.
6. Read the initial volume of the NaOH solution in the buret and record it in the DATA TABLE for trial 1, as the initial volume for NaOH.
7. Place the flask with the vinegar sample, on a sheet of white paper, under the buret. Now begin the titration by adding the hydroxide solution slowly, while swirling the flask. A pink color will appear in the center, but will go away as you swirl. When the pink color begins to linger, add the NaOH one drop at a time, swirling between each drop. When one drop is added, and the faint pink color does not disappear, you have reached the equivalence point (the point where all the acid is just neutralized by the base, but no extra base is added). Read the volume in the buret, and record it in the DATA TABLE for trial 1, as the final volume for NaOH.
8. Discard the liquid in the flask and rinse the flask with 4 or 5 rinses of distilled water to be sure it is clean. Repeat steps 4-7 for trials 2 and 3. Record volumes for the burets in the DATA TABLE in the appropriate places.
NOTE: You do not have to refill the buret, if you are sure it will not go past the 50 mL mark in a titration, just read the initial and final volumes. If you do go below the graduations, you will have to re-do that titration, as you will not be able to read a final volume.
|Buret Readings (mL)|
in vinegar (mole)
9. Calculate the volumes of vinegar and NaOH used for each of the three trials. Record in the CALCULATIONS TABLE.
10. Record the molarity of the NaOH solution used in the titration.
11. Determine the moles of NaOH used in each of the three trials. Record in the CALCULATIONS TABLE.
|By definition molarity =|| moles solute
1 L solution
|X||liters of solution|
12. The balanced equation of the reaction between acetic acid (vinegar) and sodium hydroxide is:
|1 mole||1 mole||1 mole||1 mole|
|moles NaOH||X|| 1 mole acetic acid
1 mole NaOH
|=||moles acetic acid|
13. Given the moles of acetic acid and the volumes of the vinegar sample for each trial, calculate the molarities for the three trials. Record in the spaces provided.
Don't forget to convert the volume to liters.
| moles of acid
volume of vinegar
|=||molarity of vinegar|
Molarity of vinegar for trial 1 = _______ M
Molarity of vinegar for trial 2 = _______ M
Molarity of vinegar for trial 3 = _______ M
| molarity trial 1 + molarity trial 2 + molarity trial 3
Average molarity of vinegar = _______ M
| moles HC2H3O2
1 L vinegar
|X|| 60.0 g HC2H3O2
1 mole HC2H3O2
|=||g / liter of HC2H3O2|
Mass of HC2H3O2 / 1 L vinegar = _______ g
| mass of HC2H3O2 / 1 L
1000 g vinegar
|X||100%||=||% HC2H3O2 (by mass)|