EXPERIMENT 12 Prepared by Edward L. Brown, Lee University and Verrill M. Norwood, Cleveland State Community College The student will become familiar with the techniques of titration and expressing a solution s concentration as molarity and mass percent. this experiment, you will use the NaOH solution that you standardized in Experiment 11 to determine the concentration of vinegar in units of molarity and mass percent. In OBJECTIVE APPARATUS AND CHEMICALS A P P A R A T U S Buret / Buret Clamp Ring Stand Volumetric pipet (25 ml) Erlenmeyer Flask (250 ml) Volumetric Flask (100 ml) Stir Plate / stir bar C H E M I C A L S Vinegar Phenolphthalein Vinegar is a dilute solution of acetic acid (HC 2 H 3 O 2 ) which is one of the many weak acids used in chemistry. In contrast, there are only seven common strong acids: HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3, and HClO 4. The difference between a strong acid and a weak acid is the degree of dissociation when placed in water. A strong acid is essentially 100% dissociated into H + (or H 3 O + ) and an anion when placed in water; a weak acid is less than 100% dissociated. Whether an acid is strong or weak does not affect its reaction with a base - strong and weak acids both react swiftly and completely with a base. The concentration seen on a vinegar label is typically expressed as a mass percent (mass of solute / mass of solution 100). The molarity of vinegar can be converted to mass percent once the density of the vinegar solution is known. The molarity of the vinegar provides the number of moles of acetic acid in 1 L vinegar the moles of acetic acid can be converted to grams acetic acid (solute) while the 1 L vinegar (solution) can be converted into grams of vinegar (solution). Substitution, as shown in the following example, will provide the mass percent of acetic acid in a solution of vinegar. CHEM 1010 - Experiment 12 12-1
Example 1: A sample of vinegar (25.0 ml) was placed in a 100-mL volumetric flask and distilled water was added to the line scored on the flask. The volumetric flask was capped and shaken before 25.0 ml of diluted vinegar was withdrawn and placed in a 250-mL Erlenmeyer flask. Approximately 50 ml of distilled water was added to the Erlenmeyer flask along with 3 drops of phenolphthalein indicator. A buret was filled with an NaOH solution that was previously standardized (0.0987 M). A light pink endpoint was reached after 28.4 ml of the NaOH solution was added to the Erlenmeyer flask containing the diluted vinegar. What is the Molarity of the diluted vinegar solution? M= -3 1 10 L NaOH solution 0.0987 moles NaOH solution 1 mole diluted Vinegar solution 28.4 ml NaOH solution 1 ml NaOH solution 1 L NaOH solution 1 mole NaOH solution -3 1 10 L diluted Vinegar 25.0 ml diluted Vinegar 1 ml diluted Vinegar moles diluted Vinegar = 0.112 = 0.112 M L diluted Vinegar solution What is the Molarity of the vinegar solution? Recall that we added 25.0 ml of vinegar to a 100 ml volumetric flask and then diluted the vinegar with ~ 75 ml water. So, if this diluted vinegar solution has a concentration of 0.112 M, then the undiluted vinegar s concentration must be 4 0.112M, or 0.448M. What is the Mass Percent of a 0.448 M Acetic Acid (Vinegar) solution if the density is 1.10 g/ml? First, break apart the 0.448 M into 0.448 moles solute and 1 L solution. Then, convert 0.448 moles into grams using 60.05 g/mole as the molar mass of acetic acid. This gives 26.9 grams acetic acid. Next, convert 1 L to 1000 ml and multiply by the density (1.10 g/ml) to find the mass of the solution (1100 g solution). Finally, the mass percent is calculated as follows: 26.9 g HOAc 100 = 2.45 % 1100 g Solution CHEM 1010 - Experiment 12 12-2
Density of Vinegar Procedure 1. Obtain a dry 100 ml volumetric flask from your instructor and determine its mass to the nearest 0.001 g [Data Sheet Q1]. 2. Using a suction bulb and a 25 ml pipette, place 25.00 ml of vinegar into the volumetric flask. 3. Determine the mass of the flask and vinegar to the nearest 0.001g [Data Sheet Q2]. 4. Dilute the 25.00 ml of vinegar in the 100 ml volumetric flask with distilled water until the final volume is 100.00 ml (add distilled water to the scored mark on the volumetric flask). 5. Stopper and mix this solution by inverting it several times. 6. Using a suction bulb and a different 25.00 ml pipette, remove 25.00 ml of the dilute vinegar and place it in a 250 ml Erlenmeyer flask (FLASK 1). 7. Remove an additional 25.00 ml of the dilute vinegar and place it in another 250 ml Erlenmeyer flask (FLASK 2). 8. Remove an additional 25.00 ml of the dilute vinegar and place it in another 250 ml Erlenmeyer flask (FLASK 3). 9. Add ~ 75 ml distilled water and 3 drops phenolphthalein to each flask. 10. Add a stirring bar to Flask 1. Use the same stirring bar for Flasks 2 and 3. 11. Obtain the sodium hydroxide (NaOH) solution (~ 0.1 M) that you standardized last week from your Instructor. 12. Obtain a clean buret and a buret clamp. Make sure the stopcock on the buret is closed. 13. Add ~ 5 ml of the NaOH solution to the buret (rotate the buret to wash down the sides with this NaOH solution) and empty it into the sink. Repeat with another 5 ml of the NaOH solution. 14. Then, fill the buret to the top with the NaOH solution. Place a small beaker or plastic cup under the stopcock and fully open the stopcock allowing ~1 ml of base to exit. Repeat opening and closing the stopcock until no more air bubbles exit the tip of the buret. 15. Adjust the buret so that it is directly above the flask and the stirring plate. 16. Record the starting volume of NaOH in the buret [Data Sheet Q3] and begin to slowly add the base to Flask 1. 17. As base drops onto the acid solution, a pink color appears that rapidly disappears with stirring. As time passes, the color remains longer you should add the base CHEM 1010 - Experiment 12 12-3
dropwise at this point. Eventually, a single drop of base will change the colorless solution to a persistent (for at least 30 seconds) light pink. 18. Record the final volume of base used [Data Sheet Q4]. 19. Repeat Steps 16 18 for Flasks 2 & 3. Waste Disposal: Dispose of all solutions in the sink flush with plenty of water. Return Clean Burets To Your Instructor. Return any remaining 0.1M NaOH to your Instructor. Lab Report: Once you have turned in your Instructor Data Sheet, lab attendance will be entered and you will be permitted to access the online data / calculation submission part of the lab report (click on Lab 12 ). Enter your data accurately to avoid penalty. The lab program will take you in order to each calculation. If there is an error, you will be given additional submissions (the number and penalty to be determined by your instructor) to correct your calculation. CHEM 1010 - Experiment 12 12-4
Density of Vinegar CHEM 1010 - Lab 12 Student Data Sheet 1. Mass of 100 ml Volumetric Flask g 2. Mass of Volumetric Flask + Vinegar g Trial 1 Trial 2 Trial 3 Initial buret reading 0 ml 0 ml 0 ml Final buret reading ml ml ml Names: Density of Vinegar CHEM 1001 - Lab 12 Instructor Data Sheet 1. Mass of 100 ml Volumetric Flask g 2. Mass of Volumetric Flask + Vinegar g Trial 1 Trial 2 Trial 3 3. Initial buret reading 0 ml 0 ml 0 ml 4. Final buret reading ml ml ml CHEM 1010 - Experiment 12 12-5