CHEM L110 - Chemistry and Society Laboratory
INTRODUCTION
Acids and bases are two important classes of chemical compounds. Aqueous solutions of acids contain excess H+ ions, while aqueous solutions of bases contain excess OH- ions. Acids and bases readily react with each other.
This type of reaction serves as the basis for an analytical procedure that we call a titration. In a titration, a measured volume of acid solution is used. Then, just enough base solution of known concentration is added to completely react with the acid. The color change of an indicator is used to signal the end of the titration (the "end point"). At the end of the titration, you can calculate the concentration of the acid because you know its volume as well as the volume and concentration of the base that reacted with it. Solution concentrations are expressed as molarity (M), which is defined as the number of moles of solute in 1 liter of solution.
Calculation of the amount of acetic acid in a vinegar sample normally requires that exact volumes of acid and base be measured. For this experiment we will assume that the plastic pipets we use deliver drops of uniform size. This means, then, that we can count drops of acid and base used instead of measuring volumes.
Vinegar is a dilute solution of acetic acid (CH3CO2H) in water. Sodium hydroxide (NaOH) is a very common base. In this experiment you are going to use the reaction below to analyze a sample of vinegar for the concentration of acetic acid that it contains.
Use the following equation to calculate the concentration of acetic acid in your vinegar sample:
EXPERIMENTAL PROCEDURE
You will be using a white ceramic wellplate for all the reactions (titrations) in this experiment. Make sure it is clean and dry before you start. Clean and dry three small test tubes.
You must use the same plastic pipet for both the vinegar and NaOH solutions. The pipet must be rinsed with deionized H2O anytime you switch between the two solutions.
I. Practice Titration
1. Place 1 mL of "vinegar practice solution" in a small clean and dry test tube. Place 3 mL of NaOH solution in a second small clean and dry test tube. Note the concentration of the NaOH solution and record it on your report sheet.
2. Using a plastic pipet, add 10 drops of "vinegar practice solution" to one well in the wellplate.
3. Using the dropper bottle itself, add one drop of phenolphthalein indicator to your solution. The solution is colorless at this point.
4. Using a plastic pipet, add NaOH slowly, one drop at a time. Count the drops of NaOH as you add them. With a small stirring rod, stir the solution after the addition of each drop. As each drop of NaOH is added, you may see a splash of pink color that should disappear after stirring. Continue adding NaOH dropwise until, after stirring, the pink color remains. This signals the end of the titration. Record the number of drops of NaOH used. Check this number with your instructor to see if you did this practice titration correctly.
II. Vinegar Titrations
You will be assigned a vinegar sample of unknown acetic acid (CH3CO2H) concentration. These samples are labeled A, B, or C. Record the letter of your vinegar sample on your report sheet.
1. Place 3 mL of your assigned vinegar solution in a small clean and dry test tube.
2. Using a plastic pipet, add 20 drops of vinegar solution to each of six different wells in the wellplate. Be careful not to contaminate the solution in one well with solutions from any other well.
3. Using the dropper bottle itself, add one drop of phenolphthalein indicator to each of the six solutions. They all should be colorless.
4. Using a plastic pipet, add NaOH slowly, one drop at a time to the vinegar solution in the first well. Count the drops of NaOH as you add them. Stir the solution after the addition of each drop. Continue adding NaOH dropwise until, after stirring, the pink color remains. This signals the end of the titration. Record the number of drops of NaOH used. Repeat this procedure with the vinegar solution in the next well.
5. Do the titration at least six times. The number of drops of NaOH used for each of the six titrations should agree with each other within three drops. If they do not, repeat the titration(s) until you have six results that agree with each other within this limit. Cross out the results that are too high or too low. Do not use them in your calculations.
6. Calculate the concentration of acetic acid (CH3CO2H) in each of the six vinegar samples that you titrated. Calculate the average acetic acid concentration for your six samples. Repeat these calculations for your lab partner's titration data. ALL CALCULATIONS FOR BOTH SETS OF TITRATION DATA MUST BE SHOWN ON PAGE 6 - 6 OF YOUR REPORT SHEET.
