Titration of Vinegar

Acid-Base Titration of a Solution Found in the Home

Introduction:

Much of the chemistry that we do at home makes use of solutions. A solution is a homogeneous mixture composed of a (usually solid) solute, which has been dissolved in a liquid solvent. In a solution, the solute is usually the component present in lesser quantity, while the solvent is the component present in greater quantity. We often use solutions without paying much attention to the concentration. Yet, concentration is important to us as consumers and to the manufacturers who must sell a reliable product. (Can you think of three examples of the importance of concentration in the home?)

We have three different ways to express the concentration of solutions. They are:

mass of solute 1. % weight = mass of solution x 1000%

mass of solute 6 2. parts-per-million (ppm) = mass of solution x 10

moles of solute 3. molarity = liters of solution

If we know the mass of the solute and either the mass or the volume of the entire solution, we can express the concentration of any solution by one (or more) of these three expressions.

Sometimes, however, we don't know enough data about a solution to express the concentration. This situation is especially common when we work with solutions that we did not make up ourselves, such as sea water, fruit juice, blood, industrial waste, etc. For these solutions, we need a method by which we can collect enough information to express the concentration by one of our three equations.

An extremely common technique by which concentration data can be collected is titration. A well-done titration will provide sufficient data to permit calculation of the molarity of the solution. Here's the "strategy" that makes titration work:

A measured volume of the solution (A), whose concentration will be measured, is placed in a flask. A second solution (B), containing a reagent that reacts with the solute in solution A, is added to the flask from a buret until all of the solute in A is reacted with the reagent in B. Solution B is called a “standard solution”, which means that its concentration is known. If we know the balanced equation for the reaction of A with B, we can tell how many moles of A will react with a mole of B. If we also know the molarity of B, and we know the volume of B required to react the sample of A in the flask, we can perform a calculation to determine how many moles of A were originally in the flask, before the titration:

Volume B used x molarity B x moles A reacting per mole B = moles A in the flask

Once moles of A in the flask are known, we can easily calculate the molarity of solution A:

moles of A in the flask molarity of A = measured volume of solution A in the flask

In summary, you need 3 items of data: Acid-Base Titration of a Solution Found in the Home

1. the volume of A in the flask (measured by the pipet) 2. the volume of standard solution B (measured by the buret) 3. the molarity of standard solution B (read from the stock bottle)

You also need a factor from the balanced equation, aA + bB  cC + dD

a moles A The factor is: b moles B , where b and a are the coefficients from the balanced equation.

One last consideration before we look at the specific acid-base titration that you will do: Recall that you will add B from the buret until all of the A is used up. How do you know when all the A is used up? Even our old standby observations, such as color changes, generation of an odor, and formation of a precipitate can only tell us that a reaction has occurred, and not that a reaction has stopped occurring. To know when all of A has been consumed, we must add an indicator to the flask. An indicator is a small amount of a chemical that changes color when the contents of the flask change from basic to acidic. The point where this change occurs is called the endpoint of the titration.

In this experiment, you will determine the concentration of acetic acid in a sample of household vinegar. Vinegar is a product of the continued fermentation of alcoholic liquid, such as wine or hard cider. Acetic acid is the main ingredient in vinegar. The concentration of acetic acid will be determined by titrating with a standard solution of NaOH, a base. Write the balanced equation for this neutralization in the space below:

+ - H3C C O H + Na O H Acetic Acid Sodium Hydroxide

Now look at your balanced equation. How many moles of acetic acid are used up when one mole of NaOH reacts? Write the factor below, with the moles of acetic acid in the numerator:

You will use this factor, along with: 1) the volume of vinegar in the flask, 2) the volume of NaOH used to react with the acetic acid in the vinegar, and 3) the molarity of the NaOH to calculate the molarity of the acetic acid solution:

liters of NaOH X M of NaOH X factor = moles of acetic acid

Moles of Acetic Acid = M of acetic acid in vinegar Liters of Vinegar

You will use phenolphthalein indicator to help you detect the titration’s endpoint. Procedure: Acid-Base Titration of a Solution Found in the Home

1. From the stockroom check out a burette and a 25.00 mL pipette with bulb.

2. Obtain about 100 mL of vinegar in a clean, dry 250 mL beaker. Pipette 25.00 mL of the vinegar into a clean 125 mL Erlenmeyer flask, using the procedure demonstrated by the instructor.

3. Obtain about 100 mL of the NaOH standard solution in a clean, dry 150 mL beaker. Rinse the inside of your burette with distilled water from your wash bottle. Let the water drain out the tip of the burette into a beaker labeled "waste". Then rinse the inside of the burette with about 5 mL of the NaOH from the 150 mL beaker. Let the NaOH solution drain through the burette tip into the waste beaker after rinsing is complete. Rinse with NaOH twice more, using 5 mL NaOH each time.

4. Fill the burette with the NaOH standard solution. Drain off a little of the NaOH, until no air bubbles remain in the burette tip. Record the volume. You do not need to fill the burette exactly to the 0.00 mL mark, but you do need to read the burette to two digits to the right of the decimal point. Filling the burette exactly to the 0.000 mL mark, while not necessary, makes your work much easier.

For example, consider the drawing on the right. The meniscus is between 26 and 27 mL: counting the small, unnumbered divisions indicates that the meniscus is between 26.4 and 26.5 mL. Careful observation reveals that the meniscus is about 2/10 of the way between 26.4 and 26.5 mL, or 26 26.42 mL. The volume is therefore 26.42 mL. Holding a piece of white paper behind the meniscus makes The the burette much easier to read. meniscus 27 5. Add 3-4 drops of indicator to the flask containing the vinegar. Place a piece of white paper on the ring 0.1 mL { stand’s base, and then set the flask on the paper so 1 mL that the mouth of the flask is directly under the burette 28 } tip.

6. As you swirl the contents of the flask with one hand, use your other hand to open the stopcock to allow NaOH solution to drain into the flask from the burette. It is best to drain about 1 mL at a time into the flask at the beginning, swirling between additions of NaOH. You may wish to do a “rough” or trial titration to give you an idea where the endpoint will occur before you titrate for your data. Steadily decrease the amount of NaOH draining into the flask from 1 mL at a time to just a drop or two at a time near the end of the titration. You have reached the endpoint when the pink color does not disappear after 15 seconds of swirling. The fainter the pink color, the closer you are to the endpoint, i.e., the less you have "overshot" the endpoint. Try to obtain as faint a pink as possible. Record the volume in the burette at the endpoint.

7. Repeat the entire titration procedure for a second trial. Note that, since you now know about how much NaOH solution is required to react with the vinegar, you can quickly drain about 80% of the required NaOH volume into the flask at the beginning of the titration, then drain the last few mL into the flask at the slower pace used in trial 1. Acid-Base Titration of a Solution Found in the Home

8. Return your equipment to the stockroom and complete the calculations. Acid-Base Titration of a Solution Found in the Home

Date ______Name ______

Trial 1 Trial 2 1. Volume of pipette (gives volume of vinegar) 25.00 mL 25.00 mL 2. Molarity of NaOH (from stock bottle) M M 3. Initial buret reading mL mL 4. Endpoint buret reading mL mL 5. Volume of NaOH mL mL 6. Moles acetic acid in flask moles moles Show a sample calculation here:

7. Molarity of acetic acid in vinegar M M Show a sample calculation here:

8. Average Molarity of acetic acid in vinegar, based on your two trials. ______M

9. Report the average molarity of acetic acid, as calculated by 5 other teams in the class. Include your average value as the last entry in the list.

a. ______M d. ______M

b. ______M e. ______M

c. ______M f. ______M

10. Compute the average of the six values listed in step 9.

______M