Experiment_615_Titration of Vinegar_1_2_3
- Page ID
- 305052
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Laboratory Date: Date Report Submitted: |
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Experiment Number and Title |
Experiment 615: Titration of Vinegar |
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:
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A balanced chemical equation that describes the chemical reaction being used
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A way of indicating when the reaction has been completed
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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 |
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Acetic acid |
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Sodium hydroxide |
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Water |
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Sodium acetate |
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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)
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Part 2: Preparing the Vinegar Sample for Titration
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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:
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:
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Part 3: Cleaning and Filling the Buret (Refer to Technique G)
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Part 4: Titrating Vinegar
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Part 5: Data Table
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Determination |
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Section 4: Calculations
Part 1: Determining the molarity of the vinegar sample
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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:
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Notes: