12. Analysis of Vinegar by Titration

Transcription

1 12. Analysis of Vinegar by Titration What you will accomplish in this experiment You ll use a chemical analysis method known as titration to determine the concentration of acetic acid in the aqueous solution that is common household vinegar. Concepts you need to know to be prepared Vinegar contains acetic acid, HC 2 H 3 O 2. This molecular compound is classified as a weak acid because when acetic acid molecules are dissolved in water, only a small fraction of those acid molecules dissociate. You should recall from Lab 11 that dissociation is the term used to describe the acid molecules donating their protons (H + ions) to the water molecules to create hydronium ions (H 3 O + ) and the conjugate base of the acid. In this case, the conjugate base is the acetate polyatomic ion (C 2 H 3 O 2 - ). HC 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 - (aq) Titration Titration is a method that s used to determine the concentration of an acid or a base in an aqueous solution. For an acidic solution (such as vinegar) a titration experiment would involve adding a base (such as sodium hydroxide, NaOH) to the vinegar until all of the acid in the vinegar solution has reacted with that added base. The acid-base reaction between the acetic acid molecules in the vinegar and the hydroxide ions in the NaOH solutions is a called a neutralization reaction. HC 2 H 3 O 2 (aq) + NaOH (aq) H 2 O (l) + NaC 2 H 3 O 2 (aq) In neutralization reactions, an acid molecule will donate a proton (H + ) to the hydroxide ion (OH - ) of the base to make a water molecule (H 2 O). The other product in a neutralization reaction is a salt an ionic compound. In this case, the cation is the sodium ion (Na + ) and the anion is the acetate ion (C 2 H 3 O 2 - ), the conjugate base of acetic acid. The strategy of our titration experiment is as follows: If we know both the volume and the molarity (# moles/liter) of the added base, then we know the number of moles of hydroxide ion needed to neutralize the acetic acid in the vinegar solution. # = C. Graham Brittain Page 1 of 7 12/2/2010

2 In a successful neutralization reaction, the number of moles of acid will be exactly equal to the number of moles of added base. # = # If we know the volume of the sample of acid, and we know the number of moles of the acid, we can calculate the molarity of the acid #, = Thus the experimental method used for a titration will require a method to accurately measure the volume of the sample of acid, as well as the volume of base added to that acid. As you consider the various volume measurements you ve made in lab, you should recall that the volumes of liquid you ve delivered from a buret were recorded to two places past the decimal. So you ll use a buret to carefully measure out a sample of vinegar. You ll then add the sodium hydroxide solution to that sample from a second buret. But you ll also need a way to detect the instant when the acetic acid in the vinegar sample has been completely neutralized meaning that all of the acetic acid molecules have reacted with the added base (and the number of moles of acid is exactly EQUAL to the number of moles of added base). You should recall from Lab 11 that acid-base indicators are molecular compounds that change color when the ph of the solution containing the indicator reaches a certain value. The phydrion paper you used in that experiment was filter paper that had been impregnated with a mixture of indicator compounds. These compounds changed color across a full range of ph values. By matching the color of the paper to the phydrion indicator color chart, you were able to determine the ph of various solutions. In this experiment, we ll use just one indicator compound that changes color at precisely the desired ph. The figure below illustrates how ph changes when a strong base (such as NaOH) is added to a weak acid (such as acetic acid). You can see that when the titration experiment approaches the equivalence point when the number of moles of acid is equal to (equivalent to) the number of moles of base there is a sudden change in ph from approximately ph 6 to ph 9. C. Graham Brittain Page 2 of 7 12/2/2010

3 Thus the appropriate indicator to detect this sudden ph change is phenolphthalein, which changes from colorless to a pinkish-red at ph 8 9. Procedure that you will follow A. Preparation of Vinegar Sample 1. Three different vinegars will be available in the reagent fume hood: white vinegar, red wine vinegar, and apple cider vinegar. Thus the fume hood will contain three burets one for each of these vinegars. Your lab instructor will refill these burets as needed. You re to select one of three vinegars for your titration experiment, and use this vinegar for both of the titrations you ll perform. 2. Each time you take a sample of vinegar, you ll need to record the initial volume from the buret. Then you ll open the stopcock and deliver a 2-mL sample of vinegar into your 125-mL Erlenmeyer flask. You ll then record the final volume from the buret. Realize that you may deliver slightly less than 2 ml of vinegar, or slightly more than 2 ml. Some variation in this volume is fine as long as you correctly record both the initial and final volume readings (to the appropriate number of significant figures), and so have accurately measured the volume of vinegar delivered from the buret. Realize this volume is essential to your post-lab calculations, so the measurement must be made carefully and recorded properly. 3. After obtaining your vinegar sample, add several drops of phenolphthalein indicator to the Erlenmeyer flask. Then use your graduated cylinder to add 10 ml of distilled water to the flask. This water is added to dilute the natural color that some commercial vinegars have, so that you ll not confuse the color of the vinegar with the color change of the phenolphthalein indicator. B. Titration of the Vinegar Sample with Sodium Hydroxide 1. Use an appropriate-sized beaker to take approximately 75 ml of the sodium hydroxide stock solution to your lab bench. Record the molarity of the sodium hydroxide solution on your Report Sheet; you ll need this concentration for your post-lab calculations. 2. As described in the Remedies document, first rinse and then fill your own buret with the sodium hydroxide solution. Then use the stopcock to lower the meniscus to the zero calibration mark (or below). Collect the runoff in a waste beaker, to be disposed of in the appropriate waste container. C. Graham Brittain Page 3 of 7 12/2/2010

4 3. When you re ready to begin titrating a vinegar sample, carefully read and record the initial volume reading from your sodium hydroxide buret. 4. With one hand, you ll hold the neck of the Erlenmeyer flask and swirl the flask. With the other hand, you ll need to open the stopcock of the buret slightly, to permit dropwise addition of the base to the acid solution in the flask. At the spot where the base first hits the vinegar solution, the color may temporarily turn pink, but this color will disappear as you mix the solution by swirling. 5. You ll continue the titration, adding base as needed until a faint, PERMANENT pink color appears. (Placing a white sheet of paper beneath the Erlenmeyer flask will help you to see this color change; the faint pink color is difficult to discern when the flask is on the black surface of the lab bench.) You want to be careful not to add too much sodium hydroxide. This error is called over-titration. If the indicator color becomes a deep pink or purple, you have over-titrated, and passed the desired end-point. If this happens, you ll need to repeat the entire titration with a new sample of vinegar. 6. When you ve reached this titration end-point, carefully read and record the final volume reading from your sodium hydroxide buret. 7. Now repeat all of these procedures: Obtain/prepare a second sample of vinegar and titrate it with sodium hydroxide. It s helpful to keep the flask from the first titration close by, so you can compare the colors of the titration end-points, and try to achieve the same pink color in your second titration. 8. When you ve successfully titrated two samples of vinegar to a consistent end-point color, your experimental work is complete. Be sure to empty your buret into your bench-top waste beaker, and then rinse the buret thoroughly with distilled water. Dispose of all liquid waste in the appropriate waste container. 9. Following the strategy-of-titration discussion in the lab handout, calculate the molarity of the vinegar for each titration. Determine the average molarity, and the percent difference of the two titration results. C. Graham Brittain Page 4 of 7 12/2/2010

5 Report Sheet 12: Analysis of Vinegar by Titration Student Lab Partner Date Lab Performed Section # Lab Instructor Date Report Received Lab Notebook: Data and Observations A. Preparation of Vinegar Sample Volume of Vinegar Sample 1: Initial volume reading for vinegar in buret Final volume reading for vinegar in buret Volume of vinegar sample (from subtraction) Volume of Vinegar Sample 2: Initial volume reading for vinegar in buret Final volume reading for vinegar in buret Volume of vinegar sample (from subtraction) B. Titration of the Vinegar Sample with Sodium Hydroxide Stock concentration of sodium hydroxide solution (moles/liter, M) Volume of Sodium Hydroxide used to Titrate Vinegar Sample 1: Initial volume reading for sodium hydroxide in buret Final volume reading for sodium hydroxide in buret Volume of sodium hydroxide delivered from buret (from subtraction) Volume of Sodium Hydroxide used to Titrate Vinegar Sample 2: Initial volume reading for sodium hydroxide in buret Final volume reading for sodium hydroxide in buret Volume of sodium hydroxide delivered from buret (from subtraction) C. Graham Brittain Page 5 of 7 12/2/2010

6 Formal Report: Results and Conclusions Calculation of # MOLES of NaOH used to neutralize the vinegar sample Vinegar Sample 1: Vinegar Sample 2: Determination of # MOLES of Acetic Acid in the vinegar sample Vinegar Sample 1: Vinegar Sample 2: Calculation of Molarity of Acetic Acid in the vinegar sample Vinegar Sample 1: Vinegar Sample 2: C. Graham Brittain Page 6 of 7 12/2/2010

7 Average of the two Acetic Acid molarities: Show calculation: Percent Difference of the two Acetic Acid molarities: Show calculation: Additional Questions: 1. In the Pre-Lab Assignment, you converted the concentration of acetic acid in vinegar from 5.0% w/v to molarity units. Using that molarity as the accepted value, calculate the Percent Error of your experimentallydetermined concentration. Accepted Value (from Pre-Lab) = Experimental Value (from Average above) = Show calculation of Percent Error: 2. You ll recall that distilled water was added to each vinegar sample to dilute the natural color of the vinegar, so it would not be confused with the color change of the phenolphthalein indicator. This added volume was not considered in the calculations. Explain why this volume could be ignored. C. Graham Brittain Page 7 of 7 12/2/2010