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Just a reminder that everything you do related to lab should be entered directly into your lab notebook.
Determination of Acetic Acid in Vinegar Objectives: After completing this lab, you should be able to -
Calculate the mass of a solid reagent required to prepare a solution of a given concentration. Use the appropriate glassware to mix and store aqueous solutions. Use a buret to perform an acid-base titration. Calculate the molarity and mass percent of an unknown analyte based on the results of a titration.
Safety: Safety glasses must be worn at all times in the lab. Sodium hydroxide and oxalic acid dihydrate are corrosive in case of skin or eye contact and toxic when ingested. Avoid contact with the dry reagents and solutions. MSDS sheets for all of the compounds used in this procedure are available in the lab.
Resources: Lab Text Chapter IV Titration Video Solution Process at the Atomic Level
Introduction Titrations are among the most commonly used and accurate methods of determining the amount of analyte in an unknown solution. In this procedure you will be performing two titrations, one to determine the concentration of the sodium hydroxide solution prepared in Part I and another to determine the concentration of acetic acid in a sample of vinegar. Each of these is an example of an acid-base titration. The net ionic equation for any acid-base titration is shown below
H+ (aq) + OH (aq)-
H2O (l)
When the number of equivalents of titrant is equal to the number of equivalents of analyte, a visual indicator, in this case phenolphthalein, changes color signaling the endpoint of the titration.
Give It Some Thought: What is the complete ionic equation for the reaction between sodium hydroxide and oxalic acid dihydrate? Can you identify the spectator ions?
Procedure: I Solution Preparation Preparation of solutions always begins with one or more calculations. In this case, you will need to calculate the mass of solid sodium hydroxide needed to prepare 250 mL of a 0.1 M aqueous solution of NaOH.
Give It Some Thought: Considering the number of significant figures in the molar concentration of this solution, should you spend time trying to adjust the weight of the solid NaOH to the nearest 0.001 g?
Use a 250 mL volumetric flask to prepare the NaOH solution. When the solution has cooled to room temperature, fill to the line and mix thoroughly. After mixing, transfer the solution to a properly labeled 250 mL plastic bottle for storage.
II Standardization The aqueous NaOH solution you prepared in Part I should be approximately 0.1 M assuming there were no errors in your calculations or lab procedures. In Part II, you will standardize your solution in order to determine the concentration much more accurately by titrating samples of primary standard grade oxalic acid dihydrate. Calculate the mass of solid oxalic acid dihydrate required to react with 30 – 40 mL of your NaOH solution, assuming the concentration is 0.1 M.
Give It Some Thought: How does the fact that oxalic acid dihydrate is a diprotic weak acid affect your calculation?
Use a 125 mL Erlenmeyer flask for each titration. Add enough water to completely dissolve the oxalic acid dihydrate and titrate to the phenolphthalein endpoint.
Give It Some Thought: Does it matter how much water is used to dissolve the solid oxalic acid dihydrate before titrating?
Perform three replicate titrations of H2C2O4 · 2 H2O. Determine the average molarity of the NaOH solution and calculate the standard deviation. III Determination Now that you know the molar concentration of your NaOH solution to three significant figures, you are ready to use it for the determination of acetic acid in a sample of vinegar. Use an appropriate pipet to transfer enough vinegar into a 125 mL flask to require 30 – 40 mL of your NaOH solution, assuming the vinegar contains 5% (w/w) acetic acid. Add additional water to the flask and titrate to the phenolphthalein endpoint.
Give It Some Thought: Does it matter how much additional water is added to the sample of vinegar before titrating?
If necessary, adjust the volume of vinegar to require 30 – 40 mL of NaOH solution.
Give It Some Thought: Can you think of a reason why you should use most of the volume of the buret for a titration rather than using only a few mL? (Hint: this is not a question about chemistry!)
Perform three replicate titrations and determine the average molarity of the acetic acid and standard deviation. Calculate the mass percent of acetic acid in the vinegar sample.
Clean Up: Excess solid reagents should be placed in the appropriate waste container. Waste acid or base solutions should be neutralized, as indicated by phenolphthalein, and flushed down the sink with plenty of water. Rinse all glassware thoroughly with deionized water. The buret should be left filled with deionized water with the dust cap in place.
Assessment: Pre-Lab Preparation responses – 40 pts. (Due at the beginning of the lab period) Successful completion of Parts I and II – 20 pts. Final Lab notes – 40 pts. (Due at the end of the lab period) In addition, you will be submitting the “Introduction” section of a formal lab report – 20 pts. (Due Oct. 6)
Pre-Lab Preparation: 1) Review Chapter IV of your lab text for a refresher on measuring liquid and solution volumes. 2) Calculate the mass of solid sodium hydroxide needed to prepare 250 mL of a 0.1 M aqueous solution of NaOH. 3) Write a balanced chemical equation for the reaction of sodium hydroxide and oxalic acid dihydrate. 4) Calculate the mass of solid oxalic acid dihydrate needed to react with 30 – 40 mL of 0.1 M NaOH. 5) Write out a procedure for this lab (in your lab book) in your own words and with enough detail so that you can perform Parts I and II of this experiment, from beginning to end, without referring to this handout.