Experiment_615_Titration of Vinegar_1_2_3

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Student Name

Laboratory Date:

Date Report Submitted:

___________________________

Student ID

Experiment Number and Title

Experiment 615: Titration of Vinegar

Section 1: Purpose and Summary of Experiment

In this experiment, students will determine the precise concentration of a weak acid solution that has an unknown molarity. Students will do this by performing a titration. A titration is an experimental technique for determining the concentration of a solution by using a chemical reaction with another solution.

Titration is a versatile and relatively inexpensive investigative procedure to determine a concentration of a substance. Accuracy and precision better than 1% are commonly achieved. It is used daily in many industries, sometimes with automation, to monitor manufacturing as part of quality control. Dozens of variations of titrations exist.

To do a titration, three things must be known:

A balanced chemical equation that describes the chemical reaction being used

A way of indicating when the reaction has been completed

A standard, known amount of one reactant

To perform a titration, a carefully measured amount of one reactant is added to a reaction flask. Usually an Erlenmeyer flask is used as a reaction flask. An indicator is added that will signal the endpoint of the titration by a visible color change when the reaction is complete. The other reactant, called a titrant is added slowly to the flask using a buret just until one drop of the titrant causes the indicator to change color. When the indicator changes color, the reaction is complete and the volume of the titrant added is measured. This is called the equivalence point, because it occurs when the reaction is complete. If you know the volumes of both solutions used and the concentration of one of the solutions, you can calculate the concentration of the other solution using stoichiometry.

NOTE: Equivalence point (when the reaction is complete) and endpoint (when the indicator has changed color in a titration) have slightly different meanings. Indicators are selected so that the endpoint occurs close to the equivalence point. The terms are often used interchangeably.

This is a key point: it is important to add only just enough titrant to change the indicator. The indicator changes at the endpoint. Once the indicator has changed, it should not change back. Addition of additional titrant will introduce error in the determination.

Vinegar is a complex mixture that contains acetic acid as its acidic component. Using a vinegar sample, you will determine the precise concentration of an acetic acid (CH₃COOH) solution by titration. Because acids will react with bases, you will use a solution of sodium hydroxide (NaOH). The concentration of the NaOH standard solution has been determined by a procedure known as standardization and is known very precisely, usually to four significant figures.

Students will use a standardized NaOH solution to titrate a sample of vinegar. The reaction is as follows:

CH₃COOH(aq) + NaOH(aq) → H₂O(l) + NaCH₃COO(aq)									Reaction A
	Acetic acid		Sodium hydroxide		Water		Sodium acetate		Reaction A

Use the titration data to calculate the molarity of acetic acid in the vinegar.

Try a sample calculation. Refer to Section 4 for sample calculations:

A 5.00 ml sample of vinegar was titrated with 0.08751 M NaOH. The sample requires 22.31 mL of NaOH to reach the endpoint. Calculate the molarity of the acetic acid (CH₃COOH) solution in the vinegar sample. The density of the vinegar solution has been determined to be 1.0052 g/ml.

If the vinegar is being manufactured, and is supposed to have a molar concentration between 0.45 and 0.60, can this batch be sold?

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Wear your safety goggles.

If any acid or base solution splashes on you, rinse it off immediately.

The indicator used in this experiment is called phenolphthalein. Phenolphthalein has laxative properties and was once used medically for that purpose. Wash your hands and do not eat in the laboratory.

Never use your mouth to provide suction to a pipet. Always use a suction device like a rubber bulb.

Waste Disposal:

Waste from this experiment may be safely discarded down the drain using plenty of running water.

Section 3: Procedure

Note: Transfer pipets (sometimes called volumetric pipets) and burets are usually made of glass and are precision instruments. Please use gently, because they can and do break. If you are trying to put force on glass, stop and ask your instructor for assistance.

In this lab, as in any precise titration, you must be very careful not to alter the concentration of the solutions in any way before their volumes have been measured. Glassware used to measure solutions must be both absolutely clean and dry or rinsed with 3 small portions of the solution to be used in it (being sure to wet the entire inner surface of the glassware each time). If you accidentally use a piece of glassware that is already wet with laboratory water, the small amount of water on the inside of the glassware will dilute the solution slightly and alter its concentration. This may introduce error and you might have to start the process over. This is true for precision pipets and burets.

Refer to the balanced chemical equation of this system. Notice that water is not a reactant in Reaction A, although it is produced as a product. The addition of extra water does not change the number of reactant molecules in the reaction flask once the volumes of the solutions have been precisely measured and dispensed. For this titration, then it is perfectly fine to use laboratory water as necessary for washing the sides of the reaction flask.

Part 1: Cleaning a Volumetric or Transfer Pipet (Refer to Technique E)

Pour approximately 50 ml of a soap solution in a 100-ml beaker.

Using a rubber suction bulb, lower the pipet tip beneath the surface of the soap solution and draw enough soap solution into the pipet to fill it about one-third full. Remember to keep the pipet tip below the surface of the solution. Be careful as you get to the top of the middle section of the transfer pipet--the solution uptake will get faster!!

Release the pressure on the bulb slowly to avoid drawing solution into the bulb. Quickly and carefully disconnect the bulb from the pipet. Immediately place your index finger over the open top of the pipet in order to prevent the solution from draining out. Hint: If you use your index finger, not your thumb, you will have more control over the level of the liquid in the transfer pipet.

Hold the pipet almost horizontally. Carefully rotate the pipet so that the soap solution contacts the entire inner surface of the pipet. Remember to remove your index finger from the top of the pipet to allow the soap solution to fill the upper stem of the pipet.

Drain the soap solution through the pipet tip into the sink.

Rinse the pipet several times with laboratory water using the same procedure as in steps 2-5.

Fill a beaker with laboratory water. Fill the whole pipet one time as a final rinse of the interior surface of the pipet.

Repeat for any and all pipets you are using for this experiment.

Part 2: Preparing the Vinegar Sample for Titration

Obtain about 100 ml of a vinegar sample in a clean, dry 250-ml beaker.

Pour approximately 40 ml of the vinegar sample into a clean, dry 150 ml beaker. Rinse your clean pipets from Part 1 two times with about 5 ml of your vinegar sample each time using techniques similar to Part 1 steps 2-5. Your vinegar rinse waste can be discarded down the drain with running water.

Pipet 25.00 ml of your vinegar sample into a clean 250-ml Erlenmeyer flask. Record the volume of vinegar transferred on Part 5: Data Table line 1. If you use a transfer pipet with a different volume (10.00 ml, 15.00 ml and 20.00 ml are common), record the actual volume used. An example of the proper use of a transfer pipet is here:

Use a rubber bulb for suction. There are different styles of rubber bulbs. The style shown does not stay on the pipet. Use it for suction and then quickly use your finger to control the height of the column of solution in the transfer pipet. Pipet the fluid above the calibration mark. The fluid in the photos has been dyed purple to make it easier to see:

$\\lmcpitpfs1.lmc.local\homedirs\pwest644\Documents\A_ZeroCostTextbooks\Chem_6_Rewrite_2018\In Process Experiments\20181129_152450.jpg$

Position your eye at the same level as the calibration mark. If the calibration mark looks like an oval, your eye is not at the correct level. Let fluid fall until the meniscus just touches the paint line of the calibration mark:

$\\lmcpitpfs1.lmc.local\homedirs\pwest644\Documents\A_ZeroCostTextbooks\Chem_6_Rewrite_2018\In Process Experiments\20181129_152936.jpg$ Correct

Not Correct

(Paint line is an oval and meniscus is not level)

Leave the last drop

Add about 30-40 ml of laboratory water from a clean 50-ml graduated cylinder to the vinegar sample in the Erlenmeyer flask. This is just to add volume to this solution.

Add 1 drop of phenolphthalein indicator solution to the Erlenmeyer reaction flask. Gently swirl the flask and its contents to thoroughly mix the solution.

Part 3: Cleaning and Filling the Buret (Refer to Technique G)

Place the buret on a support stand using a buret clamp. A photo of the typical setup looks like this:

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Close the stopcock and place a buret funnel (a small funnel that fits loosely in the top of the buret) at the top of the buret. Add 10 ml of laboratory water to the buret through the funnel. Make sure the buret does not leak. Notify your laboratory instructor if the buret leaks.

Remove the buret from the support stand. Hold the buret almost horizontally. Carefully rotate the buret so that the water contacts the entire surface.

Drain the water through the tip into the sink.

Repeat steps 1-3 twice using 10 ml of laboratory water each time.

Obtain about 100 ml of the NaOH standardized solution in a clean, dry 250-ml Erlenmeyer flask. Record the molarity of the NaOH solution in Part 5: Data Table line 2. It usually takes some time to standardize a solution, so take what you need, but use the solution conservatively.

Repeat steps 1-3 three times using 5 ml of the NaOH standard solution each time.

Finally, put a little of the NaOH solution in the buret and flush out any air from between the stopcock and the buret tip.

Clamp the empty buret to the support stand.

Fill the buret with NaOH solution to the 0.00 ml line at the top of the buret. The meniscus should be blocked by the paint-line of the 0.00 ml mark. If you miss the 0.00 ml mark, it is not a problem. Just read the meniscus to the nearest 0.01 ml and record this initial reading on Part 5: Data Table line 3. You can subtract this initial volume out later.

Part 4: Titrating Vinegar

Place the 250-ml Erlenmeyer flask containing the vinegar solution under the buret tip. Lower the buret tip so the tip extends about 1 cm into the mouth of the flask. Keeping the buret tip below the level of the flask reduces the likelihood of any droplets splashing out of the flask.

Open the stopcock and release 1-2 ml volumes of the NaOH solution from the buret to the vinegar solution while gently swirling the flask. You should see a pink coloration in the solution at the point where the NaOH solution contacts the surface of the vinegar solution. That is the color of the indicator. As you approach the endpoint of the titration, the pink will begin to momentarily flash through the entire solution. When you see this happening, slow down and begin adding the NaOH solution one drop at a time. You can even add 1/2 drop by dispensing a little of the NaOH on the pipet tip and washing it in using laboratory water.

Stop the titration when pink persists throughout the entire solution for 30 seconds after you have thoroughly swirled the flask. You have now reached the endpoint. Record the reading on the buret to the nearest 0.01 ml on Part 5: Data Table line 4.

Refill the buret with your NaOH solution to the 0-ml line at the top of the buret. Do a second determination by preparing a new vinegar sample as in Part 2 Steps 3-5. Then repeat Part 4 steps 1-3 to record a second set of data in column #2.

When finished, discard the remaining NaOH solution in the buret into the sink with running water.

Rinse the empty buret twice with 10 ml of laboratoy water each time.

Discard all remaining solutions into the sink with running water.

Part 5: Data Table

	Determination
	1	2
Volume of vinegar sample (ml)
Molarity of NaOH standard solution (M)
Initial buret reading (ml)
Final buret reading (ml)

Section 4: Calculations

Part 1: Determining the molarity of the vinegar sample

Volume of NaOH used (ml)
# of moles of NaOH required for titration (mol)
# of moles of CH₃COOH in vinegar sample (mol)
Molarity (mol/L) of CH₃COOH in vinegar sample (M)
Mean (the average of multiple values) molarity of CH₃COOH in vinegar sample (M)

Calculating # of moles from molarity and volume

#moles of NaOH required for titration = $\left(\begin{array}{l}\text { volume of } \mathrm{NaOH} \\ \text { solution used, } \mathrm{ml}\end{array}\right)\left(\frac{1 L}{1000 \mathrm{ml}}\right)\left(\begin{array}{c}\text { concentration of } \\ \mathrm{NaOH} \text { solution, } \mathrm{M}\end{array}\right)$

# moles of CH₃COOH in vinegar sample = $\left(\begin{array}{c}\text { number of moles of } \\ \text { NaOH required, mol }\end{array}\right)\left(\frac{1 \text { mol } C H_{3} \text { COOH }}{1 \text { mol } N a O H}\right)$

Molarity of CH₃COOH in vinegar sample = $\left(\frac{\text { number of moles of } \mathrm{CH}_{3} \mathrm{COOH}, \text { mol }}{\text { volume of vinegar sample, } L}\right)$

NOTE: The fraction 1 mol CH₃COOH / 1 mol NaOH is derived from the ratios in the balanced chemical equation for this reaction.

Post Lab Questions:

Briefly explain why adding laboratory water to the vinegar sample in the reaction flask prior to beginning the titration does not affect the results of your determination of the molarity of CH₃COOH in your vinegar sample.

A 25.00 mL sample of a sulfuric acid solution (H₂SO₄) is titrated with sodium hydroxide (NaOH). If it requires 35.88 mL of 0.1127 M NaOH to reach the endpoint, what is the molarity of the sulfuric acid solution? (All of the hydrogens of the sulfuric acid react with the sodium hydroxide.) Hint: Write a balanced chemical equation of this reaction first.

The vinegar in this titration was distilled vinegar and is clear and colorless. What modifications would you need to do to this procedure in order to test solutions such as cider vinegar or pickling liquor, both of which have significant color?

Notes: