c) Molarity of Acetic Acid = (Molarity of NaOH) x (Vol of NaOH) (Vol of Acetic Acid)

Transcription

1 CH105 LAB ANALYSIS OF Acetic Acid in VINEGAR Name Purpose: Theory: 1) Explain the chemical reactions used in titrations to analyze acid solutions. 2) Explain how standards are used in titrations. 3) Describe the function of visual acid/base indicators such as phenolphthalein. 4) Use a known amount of Potassium Hydrogen Phthalate to standardize the concentration of a sodium hydroxide solution. 5) Use the standardized sodium hydroxide solution to determine the concentration of household vinegar. NOTE: Be sure to complete the pre-lab calculations before coming to lab. 1. Acids such as acetic acid [HC2H3O2] react with bases such as NaOH to form salt and water. This is illustrated in the following equation: a) HC2H3O2 + NaOH NaC2H3O2 + H2O 2. If the acetic acid content of a vinegar solution is to be determined, the following relationship is used: This equation rearranges to: b) (M Acetic Acid) x (V Acetic Acid) = (M NaOH) x (V NaOH) c) Molarity of Acetic Acid = (Molarity of NaOH) x (Vol of NaOH) (Vol of Acetic Acid) 3. Thus there are three measurements that are made to determine the Molarity of acetic acid in vinegar. a. Molarity of NaOH must first be determined by a preliminary titration. During this preliminary titration, small amounts of the NaOH are added to a measured amount of a standard reagent that has been dissolved in water. Standards are chosen based on their purity. We will use Potassium Hydrogen Phthalate (KHP: FW ). KHP is a monoprotic solid available in very pure crystalline form. It will react in a 1:1 mole ratio with the NaOH. The number of moles of NaOH is equal to the number of moles of KHP. The NaOH solution and the KHP are both colorless in solution. A special compound called an acidbase indicator is required to reveal the point when all of the KHP has been neutralized. The indicator of choice in this type of titration is Phenolphthalein. Phenolphthalein added to solution remains colorless until the ph rises above 7.0. At this point it turns pink. The appearance of the pink color during itration reveals when equivalent amounts of acid and base are present (i.e., the solution is neutral). This is called the endpoint of the titration. To calculate the molarity of the NaOH solution, you divide the number of moles of KHP by the volume of the NaOH used to reach the endpoint of the titration. eqn. d) Molarity of NaOH = Moles of KHP Liters of NaOH The NaOH solution has been officially "standardized" when the concentration of the NaOH solution has been determined by using a standard reagent such as KHP. b. The Volume of Acetic Acid is set by using a volumetric pipet to deliver an exact amount of diluted vinegar to the titration flask. The vinegar is diluted before the titration using volumetric pipets and volumetric flasks. c. The Volume of NaOH is measured during the actual titration of vinegar by recording the initial volume of NaOH at the start of the titration, and the final volume when the solution just turns pink. The Standardized NaOH solution will be added to the reaction flask using a buret. -1-

2 PreLab Calculations. To be checked by the instructor at the beginning of lab. 1. Calculate the number of moles in a gram sample of KHP. (MW ) 2. Calculate the molarity of NaOH if ml are used to neutralize the above sample of KHP. (Molarity = moles Liters) 3. A ml sample of 6.00 M HCl is diluted to a new volume of ml Calculate the new concentration. ( C1 x V1 = C2 x V2 ) 4. A ml sample of an unknown acid requires ml of M NaOH to reach the endpoint. Calculate the molarity of the unknown acid. ( C1 x V1 = C2 x V2 ) 5. Calculate the % Mass/Vol concentration of a M Acetic Acid solution. (MW 60.06) 6. Calculate the mass of a 1000 ml sample of vinegar. Density = 1.05 grams/ ml -2-

3 Procedure: Work in Groups of two or three. Each person should do at least three titrations. Report results as a group average. I. Prepare buret. 1. Obtain approximately 150 ml of the 0.1 N NaOH solution in a beaker. 2. Obtain a 25mL or 50mL buret. a. Use a beaker to pour approx. 20 ml of tap water into the buret. b. Open the valve and allow 5-10mL to run into the sink. If the valve is not working, bring to the attention of the instructor and obtain another buret. c. After allowing some water to run out the valve, shut the valve and roll the buret, allowing the water to coat the insides of the barrel while draining into the sink. d. Repeat steps a-c, once more with tap water. e. Repeat steps a-c with distilled (deionized) water. f. Repeat steps a-c with 20 ml of the 0.1 N NaOH solution. 3. Mount the buret on the lab stand using a buret clamp. 4. Use a funnel and add 0.1N NaOH solution to approximately the ml level of the buret. 5. Drain 2-3 ml of the 0.1N NaOH solution into a waste beaker. II. Standardize the 0.1N NaOH solution 1. Use three 125mL or 250 ml flasks. Label "A," "B," and "C." 2. Tare a piece of weighing paper on the balance and add grams of Potassium Hydrogen Phthalate (KHP). Record the exact reading. 3. Transfer the KHP into the sample "A" flask. Record each mass reading. 4. Repeat the weighing procedure for samples "B" and "C." Use a slightly different mass each time. Mass of KHP "A" "B" "C" 5. Add approximately 25mL of distilled (D.I) water to each flask and swirl until completely dissolved. 6. Add 2 drops of phenolphthalein to each flask. 7. Read the bottom of the meniscus and record the initial volume of NaOH solution in the buret. 8. Add 15mL of the 0.1N NaOH to the flask with constant stirring. Record description of the process. 9. Assuming the solution is still clear, begin adding the NaOH solution in 1mL amounts with constant stirring until you have reached 20mL total volume added. If the solution changes to complete pink, record the final volume. Record descriptions. 10. If the solution has remained clear after adding the 20 ml of NaOH solution, begin adding NaOH 5-10 drops at a time until you reach the endpoint and the solution remains pink. You may wish to reduce the number of drops added as you get closer to the endpoint. The ideal endpoint is the lightest shade of pink that remains visible for at least 30 seconds. Record the final volume. 11. Repeat the titration of KHP with the NaOH solution until all members of the group have had a chance to standardize the NaOH. Refill the buret with NaOH to the 10-15mL mark and record the initial volume for each run. The group should have at least 3 standardization runs. A initial B initial C initial final final final -3-

4 12. All solutions may be disposed of in the sink. Rinse the flasks with tap water and d.i. water. III. Prepare dilutions of the Vinegar Obtain: 25mL of household vinegar, Record the concentration from the label: 10mL volumetric pipet, 25mL volumetric pipet, 100mL volumetric flask, pipet bulb 1. Rinse the 100mL volumetric flask and the 10mL volumetric pipet: twice with tap water and once with distilled (deionized) water. the flask does not need to be dry. 2. Rinse the 10mL volumetric pipet twice with vinegar, dumping the liquid into the sink each time. 3. Use the 10mL volumetric pipet to transfer ml of household vinegar into the 100mL volumetric flask. 4. Dilute with distilled (d.i.) water up to the calibration mark. Read the bottom of the meniscus. 5. Stopper the flask and invert times to ensure complete mixing. 6. Pour the diluted vinegar into a clean, dry, labeled beaker. 7. Rinse a 25mL volumetric pipet with tap water, distilled (d.i.) water, and finally twice with 5-10 ml of the diluted vinegar solution. 8. Use the 25mL volumetric pipet to transfer ml of the diluted vinegar solution to each of three clean flasks, labeled "A," "B," and "C." 9. Add 2 drops of phenolphthalein to each flask. 10. Make sure the buret is filled to the 10-15mL mark with NaOH. Record the initial volume. 11. Titrate each flask to the proper endpoint, and record the final volume. Init. Vol. Flask A Flask B Flask C Final Vol. 12. Each group member should have performed at least two titrations. IV. Clean up 1. All solutions may be emptied into the sink. 2. Rinse all burets, pipets, and flasks with tap water and distilled (d.i.) water, blot the outsides dry with a paper towel, and return to their proper places. 3. Return key to the board and obtain instructor's initials before leaving. V. Analysis ( Refer to Pre-Lab Calculations. Show only one set of calculations. Put results in a table.) 1. Standardize NaOH a. Calculate the number of moles of KHP in each of the three samples. b. Calculate the molarity of the NaOH solution. c. Select the two values that are closest to each other and calculate the average Molarity. -4-

5 2. Vinegar Calculations a. Calculate the number of moles and the molarity of acetic acid [CH3COOH, MW 60.06] in the diluted vinegar samples. b. Calculate the Molarity of the undiluted vinegar. c. Calculate the mass/volume % of acetic acid in the undiluted vinegar. d. Calculate the mass/mass % of acetic acid in the undiluted vinegar. (Density of vinegar = 1.05 g/ml) e. Calculate the % error based on the information on the bottle label. TABLE OF RESULTS 1. Standardized NaOH Sample A B C Molarity Average Molarity 2. Acetic Acid in Vinegar Sample A B C Molarity of Dilute Sample Average Molarity Molarity of Undilute Vinegar % Mass/Volume % Mass/Mass Actual % (From Label) % Error VI. Be sure to write a conclusion based on the purpose, and relate the lab to daily life. -5-