Experiment involving aspirin to teach acidbase titration Experiment

Experiment involving aspirin to teach acid/base titration Experiment: Acid-Base Titration of Aspirin Objective: The objective of this experiment is to determine the mass of acetylsalicylic acid (ASA) in an Aspirin tablet. Chemicals Potassium hydrogen phthalate (KHP, 204. 23 g/mol) Deionized water Sodium hydroxide, ~0. 10 M (~150 m. L per team) Phenolphthalein indicator solution Aspirin tablet ASA Apparatus Top loading balance Weigh paper Erlenmeyer flasks, 3 x 250 m. L Scoopulas or spatulas Magnetic stirrer and stir bar Buret, 25 m. L Buret stand clamp 1 CH 113 (M. Hall) Ch 4

Data & Observations: Part I. Standardization of the Sodium Hydroxide Solution Na. OH (wet & sticky) KHP (nice & dry) Solid sodium hydroxide cannot be massed accurately because it absorbs water and carbon dioxide from the air. Consequently, it is not possible to make an aqueous solution to a very specific and accurate concentration. Before you use Na. OH(aq) to titrate Aspirin, you need to “standardize“ it, that is determine its accurate concentration. The solution of Na. OH(aq) that you are given is approximately 0. 10 M. You will titrate it against a known amount of an acid to determine the concentration of Na. OH. The acid used to standardize the concentration of Na. OH is potassium hydrogen phthalate (KHP). It can be massed very accurately. KHP is actually the potassium salt of the phthalate ion: KHP phthalate ion (acid) The phthalate ion is the acid that is titrated by Na. OH in the standardization: CH 113 (M. Hall) Ch 4&15 2

Experiment: Acid-Base Titration of Aspirin 1. 2. 3. 4. Obtain an exact mass of KHP (should be about 0. 3 g) in a 250 m. L Erlenmeyer flask. Add approximately 100 m. L of distilled water, the stir bar, and 2 drops of phenolphthalein. Place the flask on the magnetic stirrer and start stirring to dissolve the acid and mix in the indicator. Place a clean buret in a stand make sure it is straight. Pour the ~0. 10 M Na. OH solution carefully into the buret, filling to past the top marking. 5. Place a waste beaker under the buret and dispense some Na. OH to remove any bubbles from around the stopcock. Make sure the meniscus is on or below the 0 mark, but do not spend time trying to get it to exactly 0. 00. 6. Record the initial volume. Check with your instructor that you have the correct sig figs. 7. Do a calculation to estimate the approximate volume of base that will be required to reach the endpoint in this first titration. Check this value with your instructor before you proceed. 8. Proceeding slowly for the first trial, titrate the KHP with the sodium hydroxide until the endpoint is reached. You should observe the appearance of a permanent pink color based on the addition of a single drop. Record the final volume of the base. 9. Calculate the concentration of the sodium hydroxide. 10. Repeat the above procedure twice, except omit the estimation of volume (why? ). You may titrate more quickly since you can approximate where the endpoint will be. Be sure to slow down when approaching the endpoint. 11. Obtain three Na. OH concentrations that are close enough such that the relative standard deviation (RSD) is less than 3%. CH 113 (M. Hall) Ch 4&15 3

Part II. Titration of Aspirin with Sodium Hydroxide 12. Place an Aspirin tablet in a clean 250 m. L Erlenmeyer flask. Add approximately 100 m. L of distilled water, the stir bar, and 2 drops of phenolphthalein. The Aspirin tablet will not dissolve completely, but you can titrate it anyway. 13. Fill the buret with the Na. OH solution (standardized at this point) and titrate as before, proceeding slowly. Calculations: 1. Show your calculations for the concentration of sodium hydroxide, the mean, the standard deviation, and the RSD. 2. Calculate the mass of ASA in the Aspirin tablet. 3. Assuming 325 mg ASA/tablet is the accepted value, determine your percent error. Results: Summarize your results here. Error and its Source: Analyze your error according to our usual method. CH 113 (M. Hall) Ch 4&15 4