I. II. OBJECTIVE To prepare a standard acid solution from solid KHP. To learn how to condition and use a buret. To determine the end point of a solution using

an indicator. To standardize a basic solution using a standard acid solution. To determine the amount of acid in a vinegar unknown using the standardized base solution. 1. 2. 3. 4. 5. Dase Intration and Unknown Analysis Percent Vinegar THEORY (Reference: Zumdahl, Chapters 15 and 16) Volumetric analysis is a chemical method that measures the volumes of reactants in order to determine the amount of a substance present in a sample. In this experiment, the amount of acid in two unknown acids will be determined by titrating with a measured volume of base of known concentration. The point in the titration (the process of measuring the volume of a certain substance needed to react with another) where the moles of base delivered is equal to the moles of acid present is called the equivalence point. The equivalence point is detected by the use of an indicator, such as phenolphthalein. The point at which the indicator undergoes a color change is called the endpoint. We always try to pick an indicator whose endpoint is as close as possible to the actual equivalence point for the particular reaction. Phenolphthalein changes from colorless to pink as soon as EXCESS hydroxide ions are present (this occurs after all the acid has been neutralized). In this experiment you will use a prepared solution of sodium hydroxide (lye or caustic soda) that is approximately 0.2 M. This solution has been made from solid NaOH or by dilution a more concentrated STOCK solution. It is necessary to standardize the prepared base solution so that its actual concentration is known to three or more significant figures. This base can then be used to measure the acidic properties of other substances - such as the concentration of acetic acid in a vinegar sample or the molar mass weight of an unknown solid acid. Describe how your standardized base can be used for these purposes and the chemical reactions involved in your Theory for part 2. Include balanced chemical reactions as well. For part 1, a solution of base having an approximate molarity concentration is prepared and its concentration to more significant figures is determined using a known amount of acid in a neutralization reaction. The solid base used to make this solution, NaOH, cannot be accurately weighed due to its hygroscopic behavior when exposed to air. Thus, a very stable solid organic acid called potassium hydrogen phthalate is used as a primary standard. Potassium hydrogen phthalate has the formula KHC8H404 and is usually abbreviated KHP where the P = C8H4O4, not phosphorus. KHP is monoprotic, meaning it is able to donate only one hydrogen ion per molecule of KHP in aqueous solutions. The reaction between NaOH and KHP is as follows: NaOH + KHP NaKP + HOH Lab Experiment #10- Acid/Base Titration 181 3 III. 182 with titration experiments: VOCABULARY- Define the following vocabulary words commonly as they are used Emon 3. condition, end point, equivalence point, molarity, indicator, phenolphthalein, aliquot, neutralization reaction, volumetric analysis, titration, titrant, hygroscopic, primary standard, monoprotic, diprotic, triprotic, molarity IV. EXPERIMENTAL PROCEDURE VITE A. EQUIPMENT buret, ring stand, buret clamp, 125 or 250-mL Erlenmeyer flasks, solid potassium hydrogen phthalate (KHP), stockroom-prepared ~ 0.2 M NaOH solution, balance, phenolphthalein indicator, 10-mL pipet, pipet bulb, 100-mL graduated cylinder, 150-mL beaker, 800 or 1000-mL waste beaker, unknown vinegar samples, unknown acid samples B. SAFETY/DISPOSAL: Determine any health concerns (symptoms of overexposure) and safety requirements you should be aware of for all identified reagents used in this experiment. Disposal: All solutions containing phenolphthalein, KHP, or NaOH must be disposed in a labeled container indicated by your professor. C. PROCEDURE Part 1 Standardizing the Sodium Hydroxide Solution 1. Take three clean 125 or 250-ml flasks and label them "1", "2" and "3". Weigh the labeled flasks empty and record the mass obtained for each. Put approximately 0.5 to 1.0 g of KHP into each flask and obtain the mass of the beaker with KHP. Record all masses accurately to the decimal position supported by the balanced used. Use KHP masses that differ by at least 50 mg. Determine the mass of KHP used for each trial as best as your balance allows. 2. Add about 25-50 mL of DI water to each Erlenmeyer flask and swirl until all the solid KHP dissolves. Add two -three drops of phenolphthalein indicator to each of the flasks. Several unstandardized solutions of NaOH, have been prepared and are stored in carboys. Each student will be assigned one of these solutions to use throughout the entire titration experiment. Record the identity of your assigned NaOH carboy on your data table. Do not change NaOH solutions. In part 1, you will standardize your NaOH and then calculate the molarity of the original base solution to three or four significant figures (depending on the balanced used for the KHP.) 4. Clean and dry a 150-mL beaker. Obtain ~100 mL of your assigned NaOH in this beaker. Remove a buret from the buret drawer, rinse with deionized water, and condition the buret with ~5 mL of your NaOH solution. Rinse solutions should be placed in your waste beaker. Fill the buret with the base solution to near the zero line. 5. In your data table, record the initial reading of your buret to the hundredths decimal place. 6. Add aliquots of the titrant base from the buret to the acid until a faint, permanent (with swirling, the color stays at least 15 seconds) pink color is obtained. Be sure to swirl and rinse the flask during this procedure. Record the final buret reading after the endpoint is Lab Experiment #10- Acid/Base Titration 7. 8. 9. Tart Flask mass reached and record in your data table. On your Data Table, note the intensity of the pink color for each finished titration (pale pink, light pink, medium pink, dark pink, etc). Determine the volume of base used, again to the hundredth decimal place. Obtain sufficient base to refill the buret and repeat steps 5 and 6 for each remaining KHP samples. For your standardization process to be useful, at least two of your three trials must have a permanent, pale pink endpoint rather than a dark pink color. Flasks containing titrated samples can be emptied into your waste beaker. Have your instructor stamp your Data Tables for part one during the lab period where your data was taken. 10. On the provided calculation page, calculate the molarity of NaOH for each of the three trials separately and average. Your instructor will check your calculations before assigning an unknown for part 2. Your instructor will let you know if you have sufficient time to move on to Part 2. 11. If there is insufficient time to continue titrating, empty the remaining NaOH from your buret into your waste beaker then your waste beaker into the class titration waste container. Rinse your buret with several aliquots of deionized water and return to the buret drawer. Rinse solutions should be also placed in the hazardous waste container. Part 2 - Analysis of a Vinegar Sample Your standardized base solution (your "secondary standard") can now be used to determine the amount of acid in other samples. All calculations using this base in Part 2 will use the average concentration that you calculated in step 11 above. In part 2 of the lab, you will determine the amount of acid (acetic acid) in a vinegar sample. Acetic acid is a monoprotic acid. You only have 3 tries to get the required pale pink color, and are still required to have data from 2 flasks. 12. Use a clean, DRY 100 mL beaker to obtain about 40 mL of your assigned unknown vinegar sample. Record the unknown number/letter in your data table. 13. Rinse and condition a 10 mL pipet and pipet 10.00 mL of the unknown vinegar sample into a pre-weighed 125 or 250-ml flask. 14. Weigh the flask with the vinegar aliquot and record the mass in your data table. Add 2-3 drops of phenoiphthalein indicator to the flask. When you have finished pipetting both unknown vinegar samples, rinse the pipet with di water and return to pipet rack, tip-up. 15. Using the same standardized base solution used in part 1 as the titrant, titrate the vinegar solution until a faint, permanent pink color is obtained in the same fashion as in part 1. Record initial and final buret readings and an assessment of the final phenolphthalein color of your titrated sample in your data table. 16. Perform a duplicate trial of your vinegar sample, recording the same data as in steps 12-15. 17. If you are done titrating at this time, refer to step 11 for proper waste disposal and buret care. Lab Experiment #10- Acid/Base Titration 183 168 280 3 beaker wit 3 C I reaclin regeling intensit e of be y of N by VI. CALCULATIONS (calculations pages provided) Part 1 - Write the balanced chemical equation for the reaction taking place in the flasks for part 1. From the mass of KHP, calculate the moles of KHP used for each of 3 trials. Using the moles of KHP in each flask, use stoichiometry to calculate the moles of NaOH that were used to reach the equivalence point in each flask. Finally, calculate the molarity of your NaOH solution using the moles of NaOH and the volume of titrant used for each trial individually and averaging molarity for your trials. The average molarity of your NaOH should fall between 0.17 and 0.25 M. If your value does not fall in this range, there is definitely something wrong and you are encouraged to take your work to your instructor to see where the problem might be. These calculations must be complete and shown to your instructor prior to obtaining your unknowns. Part 2 - These calculations can be completed at home, prior to submitting your finished report. Write the balanced chemical equation for the reaction taking place in the flasks for part 2. From the volume of titrant used in each trial and the calculated M for your base, calculate the moles of NaOH used for each trial. Using the moles of NaOH calculated, use stoichiometry to calculate the moles of acetic acid that were in each flask. From your data, calculate the mass of the vinegar sample used in each trial. Using the mass of acetic acid and the mass of the vinegar solution in each flask, calculate the percent acetic acid in each individual trial and the average percent for your trials. Although you do not know the true percent by mass of acetic acid in your vinegar, if your value falls between 2% and 10% acetic acid in the vinegar, you may not be correct but you are "in the ballpark". Remember that asking questions about this part COULD result in points lost on the lab. Lab Experiment #10- Acid/Base Titration 185 a²0 A E F (put on a header and use this page in your report) B² 1.0g 2) Calculations and Calculated Results Tables Taure Calculations: After getting all data and calculations checked by the instructor, put the results of your calculations in the following table; be sure to show all work in the space provided. If more than 3 trials were completed, attach an additional piece of paper with the same features for those calculations and results. PART 1 B C present D 644 Balanced molecular equation for the reaction occurring grams of KHP used Moles of KHP used Moles of base Volume of titrant used for that trial Molarity of base for that trial Average Molarity of Base A: Flash mass - 1 1 1 1 1 KHP+ NaOH-> H₂O + Nak P 204.23g mol = 0,0048964403 2 Lab Experiment #10- Acid/Base Titration 1.00g 10 20.00400g 2 0.00490 2 21.63 20.227 M 0.2004 Put well-organized calculations for Part I work here or on the backside of this page: Mass of beaker with lettp = grams of инр 3) 0.89 3 =0.0049 C. ratio between KHP and NaOH = 1:1 means for mole of KHP used, there will be imole of U4₂ OH. Do I did that doring the lab experiment. E: M = moles of base Volume of titration in L page number 0.808 30.003909 3006 000390 322.59 30.173 M 204.238/md every osect. = 0.00 39171522 -0.0039 187