Vinegar - Is the Label Truthful?

 

We’ve just been hired as consultants for the ‘Truth on Labels’ Division of the Food and Drug Administration. Our job is to determine whether the concentration of acid in a bottle of vinegar is accurately displayed on the bottle’s label. Vinegar is a mixture (solution) of acetic acid (HC₂H₃O₂) and water. Vinegar labels generally claim that vinegar is ‘5% acidity’. This means that the vinegar should be an aqueous 5% (w/v%) acetic acid solution. We will determine the amount of acetic acid in the vinegar by reacting it with a known quantity of sodium hydroxide (NaOH). Unfortunately, our sodium hydroxide is only approximately 0.1 M, so we will have to determine its concentration more accurately first. To determine the molarity of sodium hydroxide, we will react it with a known quantity of potassium hydrogen phthalate (KHC₈H₄O₄ - we’ll abbreviate this KHP- its molar mass is 204.23 grams per mole). Fortunately, KHP is a pure solid, and we can measure its mass on a balance to determine its moles. Here are the chemical equations representing the reactions we will use:

 

KHC₈H₄O₄ (aq) +  NaOH (aq) ® NaKC₈H₄O₄ (aq) + H₂O

 

NaOH (aq) + HC₂H₃O₂ (aq) ® NaC₂H₃O₂ (aq) + H₂O

 

One other problem. We will need to know when the quantities (moles) of the reactants are equal. This can be done using phenolphthalein. Phenolphthalein indicates when the quantities (moles) of these reactants are equal by changing colors. We call substances like phenolphthalein indicators. Here’s what we’ll need to do:

 

1.       Place 25.00[1] mL of vinegar into a 250 mL volumetric flask. Dilute to the mark with distilled water.

2.       Place 3.0 millimoles[2] of KHP into four 250 mL Erlenmeyer flasks.  Dilute with 50 mL[3] of distilled water, and add a few drops of phenolphthalein.

3.       Titrate to the first permanent faint pink color[4] by adding NaOH from the buret. Your instructor will show you proper technique for using the buret. The buret should be read to the nearest 0.01 mL.

4.       Repeat the titration using the KHP in the other Erlenmeyer flasks. Clean and rinse the Erlenmeyer flasks.

5.       Place 25.00 mL[5] of diluted vinegar solution in each of four 250 mL Erlenmeyer flasks. Dilute with 25 mL[6] of distilled water.

6.       Titrate to the first permanent faint pink color by adding NaOH from the buret.

 

You should now have sufficient data to determine whether the label on the vinegar bottle is truthful.  Coordinate with two other groups so that you know the concentration of acetic acid for three different brands of vinegar.  Are all the labels truthful?  You will be performing these titrations several times so that you can make a statement about the confidence you have in your results. 

Use our usual lab write-up format – purpose, brief procedure, results, and conclusion.  Be sure to report all your data (to the proper significant digits, with units!) and the appropriate results from the groups you work with in neatly formatted tables.  Show all your calculations – from mass of KHP all the way to weight/volume of acetic acid in vinegar – but not the calculations of your collaborating groups’; just show their results.  Part of your grade will depend on the accuracy and precision of the data you report and the results you calculate, so work very carefully.  Be sure to comment (in your conclusion) on any data/results you feel are suspicious.  Give reasons why you think the data/result are poor.  Hint:  Here’s the part many groups forget.  The solution titrated is not vinegar from the bottle – it is diluted vinegar.  Don’t forget to account for this.