EXPERIMENT 8 Prepared by Edward L. Brown, Lee University and Verrill M. Norwood, Cleveland State Community Collee The student will become familiar with the techniques of titration and expressin a solution s concentration as molarity and mass percent. A P P A R A T U S Buret / Buret Clamp 250 ml Erlenmeyer Flask (3) Rin Stand Volumetric pipet (25 ml) / bulb Volumetric Flask (100 ml) Stir plate / stir bar OBJECTIVE APPARATUS AND CHEMICALS C H E M I C A L S Vinear Phenolphthalein (1%) Standardized NaOH from Experiment 7 Vinear 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 stron acids: HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3, and HClO 4. The difference between a stron acid and a weak acid is the deree of dissociation when placed in water. A stron 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 stron or weak does not affect its reaction with a base - stron and weak acids both react swiftly and completely with a base. By usin the NaOH solution that you prepared and standardized in Experiment 7, you will be able to determine the molar concentration of vinear (M) and the mass percent. The concentration seen on a vinear label is typically expressed as a mass percent (mass of solute / mass of solution 100). Vinear s molarity can be converted to mass percent once the density of the vinear solution is known [Fiure 1]. The molarity of the vinear provides the number of moles of acetic acid in 1 L vinear the moles of acetic acid can be converted to rams acetic acid (solute) while the 1 L vinear (solution) can be converted in rams of vinear (solution). Substitution, as shown, will provide the mass percent of acetic acid in a solution of vinear. Experiment 8 8-1
Mass % HC2H3O2 acetic acid = 100 vinear k H 2 O 1 k = 1E3 HC 2 H 3 O 2 + H 2 O = vinear Molar Mass solute moles HC 2 H 3 O 2 Molar Mass H 2O moles H 2 O Density vinear ml vinear 1 ml = 1E-3 L L vinear moles solute = L solution Molarity (M) Fiure1 Experiment 8 8-2
Density of Vinear PROCEDURE 1. Obtain a dry 100 ml volumetric flask from your instructor and determine its mass to the nearest 0.001 [Data Sheet]. 2. Usin a suction bulb and a 25 ml pipette, place 25.00 ml of vinear into the volumetric flask. 3. Determine the mass of the flask and vinear to the nearest 0.001 [Data Sheet]. 4. Dilute the 25.00 ml of vinear 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 for 1 minute by invertin it several times. 6. Usin a suction bulb and a different 25.00 ml pipette, remove 25.00 ml of the dilute vinear and place it in a 250 ml Erlenmeyer flask (FLASK 1). 7. Remove an additional 25.00 ml of the dilute vinear and place it in another 250 ml Erlenmeyer flask (FLASK 2). 8. Remove an additional 25.00 ml of the dilute vinear and place it in another 250 ml Erlenmeyer flask (FLASK 3). 9. Add ~ 75 ml distilled water to each flask. 10. Add 3 drops phenolphthalein to each flask, and a stirrin bar to Flask 1. 11. Obtain the sodium hydroxide solution (~ 0.1 M) that you standardized last week from your Instructor. 12. Obtain a clean buret and buret clamp. Make sure the stopcock on the buret is closed. 13. Add ~ 5 ml of the standardized NaOH solution to the buret (rotate the buret to wash down the sides with this NaOH solution) and empty it into a beaker desinated for waste. Repeat with another 5 ml of the NaOH solution. 14. Then, usin a funnel, fill the buret to the top with the NaOH solution. Place the beaker containin the waste NaOH under the stopcock and fully open the stopcock allowin ~1 ml of base to exit. Repeat openin and closin 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 stirrin plate. 16. Record the volume of NaOH in the buret [Data Sheet] and bein to slowly add the base to Flask 1. Experiment 8 8-3
17. As base drops onto the acid solution, a pink color appears that rapidly disappears with stirrin. As time passes, the color remains loner you should add the base dropwise at this point. Eventually, a sinle drop of base will chane the colorless solution to a persistent (for at least 1 minute) liht pink. 18. Record the final volume of base used [Data Sheet]. 19. Repeat Steps 16 18 for Flasks 2 & 3 [Data Sheet]. Waste Disposal: Dispose of all solutions in the sink flush with water. Return clean burets and any remainin 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 8 ). Enter your data accurately to avoid penalty. The lab proram will take you in order to each calculation. If there is an error, you will be iven additional submissions (the number and penalty to be determined by your instructor) to correct your calculation. Experiment 8 8-4
Density of Vinear Mass of 100 ml Volumetric Flask CHEM 1111 - Lab 8 Student Data Sheet Mass of Volumetric Flask + Vinear Trial 1 Trial 2 Trial 3 Initial buret readin ml ml ml Final buret readin ml ml ml Names: Density of Vinear Mass of 100 ml Volumetric Flask CHEM 1111 - Lab 8 Instructor Data Sheet Mass of Volumetric Flask + Vinear Trial 1 Trial 2 Trial 3 Initial buret readin ml ml ml Final buret readin ml ml ml Experiment 8 8-5