Determining the Molecular Mass of an Unknown Acid by Titration

Transcription

1 Determining the Molecular Mass of an Unknown Acid by Titration Objectives: To perform an analytical titration. To standardize a basic solution. To determine the equivalent mass of an unknown acid. Background: Titrations are used extensively in analytical chemistry. Some applications of titrations include quality control in the food, cosmetics, beverages, and pharmaceutical industries; quantitative determination of chemical concentrations in environmental samples; and the measurement of various species in biological solutions. In this experiment you will use titrations to determine the molecular mass of an unknown acid. In analytical chemistry when we use a titration or other precise means to determine the concentration of a solution it is called standardizing the solution. In the first part of this experiment you will prepare a sodium hydroxide, NaOH, solution. You will then standardize this solution by using it to titrate a known quantity of potassium hydrogen phthalate (KHP), C 8H 5KO 4, a solid monoprotic acid that is often used for standardizing basic solutions. Potassium Hydrogen Phthalate (KHP) C 8H 5KO 4 In the second part of this experiment you will use your standardized sodium hydroxide solution to titrate a sample of an unknown solid acid. From your titration data, you will determine the number of moles of acid in the sample. From this, and the mass of the acid in the sample, you will determine the molecular mass of the unknown acid. In both parts of this experiment you will perform at least three titrations to ensure the precision of your results. You may assume that your unknown acid is monoprotic. It is assumed in this experiment that you are already familiar with the basic techniques and calculations used in performing titrations. If not, you should review this material prior to starting this experiment. Determining the Molecular Mass of an Unknown Acid by Titration Page 1 of 8

2 Proper Titration Technique: Using a Buret 1. Rinsing: Always rinse a buret (including the tip) before filling it with a new solution. You should rinse the buret first with deionized water, and then twice with approximately 2-mL aliquots of the solution you will be using in the buret. Be sure to swirl the solution to rinse all surfaces. If you are using an acid or base solution be careful to avoid spilling the solution on hands or clothing. 2. Filling: Mount the buret on a buret stand. Be sure that the tip fits snuggly into the buret and is pressed all the way in. If the tip is excessively loose, exchange it for a tighter fitting one. Using a funnel rinsed in the same manner as the buret, fill the buret with the titrant to just below the 0.00 ml mark. There is no need to fill the buret to exactly 0.00 ml since you will use the difference between the ending and starting volumes to determine the amount delivered. When the buret is full, remove the funnel as drops remaining in or around the funnel can creep down and alter your measured volume. If you overfill the buret, drain a small amount into an empty beaker. Do not re-use this "extra" solution as it may have been contaminated by the beaker or diluted slightly by any water present in the beaker. Always pour fresh solution into the buret. 3. Removing Air Bubbles: Often air bubbles will be trapped in the tip of a newly filled buret. These can be difficult to see and troublesome as they alter the measured volume when they escape. To remove air bubbles, hold the buret over an open beaker and open the stopcock fully to allow solution to flow out of the buret. You may need to give the buret a quick shake to expel the bubbles. Refill the buret as necessary. 4. Reading the Buret: You should always read the volume in a buret from the bottom of the meniscus viewed at eye level (see Figure 1). A black or white card held up behind the buret helps with making this reading. Burets are accurate to ±0.02 ml and all readings should be recorded to two decimal places. Be sure to record both the starting and ending volumes when performing a titration. The difference is the volume delivered. Figure 1. Reading a Buret Good Titration Techniques Throughout your scientific careers you will probably be expected to perform titrations; it is important that you learn proper technique. In performing a titration generally an indicator that changes color is added to a solution to be titrated (although modern instruments can now perform titrations automatically by spectroscopically monitoring the absorbance). Add titrant from the buret dropwise, swirling between drops to determine if a color change has occurred. Only if you know the approximate end-point of a titration should you add titrant faster, but when you come within a few milliliters of the endpoint you should begin to slow down and add titrant dropwise. As you become proficient in performing titrations you will get a "feeling" for how much to open the stopcock to deliver just one drop of titrant. Some people become so proficient that they can titrate virtually "automatically" by allowing the titrant to drip out of the buret dropwise while keeping a hand on the stopcock, and swirling the solution with the other hand. If you do this, be sure that the rate at which drops are dispensed is slow enough that you can stop the flow before the next drop forms! Overshooting an end-point by even one drop is often cause for having to repeat an entire titration. Generally, this will cost you more time than you will gain from a slightly faster drop rate. Refill the buret between titrations so you won t go below the last mark. If a titration requires more than the full volume of the buret, you should either use a larger buret or a more concentrated titrant. Refilling the buret in the middle of a trial introduces more error than is generally acceptable for analytical work. Determining the Molecular Mass of an Unknown Acid by Titration Page 2 of 8

3 Procedure: Materials and Equipment: You will need the following additional items from the stockroom for this experiment: One 50-mL buret Safety: GENERAL SAFETY: Students must wear safety goggles at all times. WASTE DISPOSAL: All waste containing KHP or your unknown acid must be disposed of in the hazardous-waste container in the fume hood. All other waste may be disposed of in the sink. Experimental Set-up and Procedure: General Notes This experiment is performed solo and the grade is based mainly on the precision and accuracy of your results. This is because being able to perform a precise and accurate titration is important to a variety of fields. It is strongly suggested that you check your results before disposing of your solutions in case additional trials are required. Your report is due upon completion of the experiment. All glassware must be properly rinsed before performing this experiment. Your flasks should be rinsed several times with deionized water before use. Do not use soap in your rinsings as this could effect your results in the experiment. Your burets and funnels should be rinsed several times with deionized water and then several more times with approximately 5-mL samples the solution you will fill them with. Label all solutions to avoid cross contamination. Your buret should be properly clamped into the buret stand so that it is held in a vertical position. Be certain that buret tip is firmly pressed in and that there are no air bubbles in the tip. Also, be sure to remove your funnel before beginning your titration. Finally, it is a good idea to double-check all readings of your buret with your lab partner to ensure accurate results. A proper endpoint in a titration is reached when a single drop of titrant causes the indicator to change color and the color persists while swirling the solution for at least 30 seconds. If you are unsure whether only a single drop of titrant was dispensed, then you should discard and repeat the trial you are performing. The indicator shade being light or dark is not sufficient to determine if a proper endpoint has been reached; some trials may have a darker color than others because the titration curve is very steep in the region of the endpoint. The only accurate determination that the proper endpoint has been reached is if you are certain that only a single drop was required to reach the endpoint signaled by the indicator color change. Any trials you have uncertainties about should be discarded and repeated. Determining the Molecular Mass of an Unknown Acid by Titration Page 3 of 8

4 Part A: Standardization of a Sodium Hydroxide Solution using KHP In this part of the experiment you will prepare and standardize a sodium hydroxide (NaOH) solution using a solid potassium hydrogen phthalate (KHP) primary standard. Rinse a 500-mL Florence or Erlenmeyer flask, a 600-mL beaker, a 50-mL buret, a small funnel, a small graduated cylinder, and three 250-mL Erlenmeyer flasks several times using deionized water. There is no need to dry these items after rinsing. Using the small graduated cylinder, measure 10 ml of 6-M NaOH solution into your 500-mL Florence or Erlenmeyer flask. Using your 600-mL beaker, measure approximately 400 ml of deionized water and add this to the 10 ml of 6-M NaOH in the 500-mL flask. (These measurements do not need to be exact because you will standardize this solution later). Label this solution, dilute sodium hydroxide solution. Swirl your dilute sodium hydroxide solution well before proceeding to be certain the solution is completely mixed. Rinse your buret twice with small (approximately 5-mL) samples of your dilute sodium hydroxide solution; NOT the 6M NaOH stock solution. (Do not use more than 5-mL samples for each rinsing because you will need an excess of this solution if you have to repeat a trial or two later). Then fill the buret with your dilute sodium hydroxide solution to just below the 0.00-mL mark and clamp the buret vertically in your buret stand. Check the buret tip to make certain it is pressed firmly into the buret. Check that the tip is not dripping and that there are no air bubbles present. If you used a funnel to fill the buret, remove the funnel before proceeding so that drops from the funnel do not drip into the buret during the titration and alter its volume. We will now use a technique called weighing by difference to measure this mass of solid potassium hydrogen phthalate (KHP) used in each trial. Weighing by difference is an analytical technique for precisely determining the amount of substance added to a container. Simply placing your flask onto the balance and weighing its contents is not always precise because moisture or other substances on the outside of your flask may result in some error in your measurements. Get a vial of oven-dried solid KHP from your instructor. Handle this vial only with a clean paper towel or tongs to avoid getting moisture, hand cream, or fingerprints on this vial. Using the technique described below, measure between and grams of solid KHP into a 250-mL Erlenmeyer flask. It is critical that at least grams of KHP are used so that the molarity of your standardized NaOH solution will have four significant digits. To weigh the solid KHP by difference, carefully remove the cap from the sample vial and set it aside. Weigh your open sample vial and its contents on the analytical balance. Record this mass on your data sheet. Now transfer a small amount of the solid KHP from the vial directly into your 250-mL Erlenmeyer flask. Do not use a spatula or other device to transfer the solid as this may result in some loss due to sticking to the spatula. Now reweigh the vial containing the remaining solid KHP sample. The difference between the initial and final mass of the vial and its contents is equal to the mass of the solid KHP transferred to the flask. Be certain to record all masses to three decimal places. You need to transfer between Determining the Molecular Mass of an Unknown Acid by Titration Page 4 of 8

5 1.000 and grams of KHP to the flask. If the mass of KHP that you added is less than grams, transfer more of the KHP to the flask and then reweigh the vial and its contents, repeating as necessary. Proceed cautiously because if you transfer significantly more than grams of the KHP, you will need to dispose of it, rinse the flask again, and start your weighing by difference procedure again. You should weigh by difference for all trials in this experiment. Now add about 50 ml of deionized water (this amount does not need to be exact) and 2 to 3 drops of phenolphthalein indicator to the contents of the flask. Swirl to mix. Do not be concerned if all of your KHP does not dissolve in this step. We are now ready to begin the titration. Record the initial volume of NaOH solution in your buret on your data sheet. Use this solution to titrate the KHP in the 250-mL Erlenmeyer flask. The titration endpoint is signaled when a single drop of NaOH solution added to your flask causes a faint pink color to persist in the solution for at least 30 seconds while swirling. (See the general notes at the beginning of this procedure for more details about determining the endpoint). Be careful not to add the NaOH too quickly, because if you go past the endpoint in this step by more than a single drop you must discard the trial and perform an additional titration. Refill your buret and perform two additional titrations of the solid KHP standard in a similar manner. Calculate the exact molarity (M) of your NaOH solution using the data from each trial to the correct number of significant figures. Then calculate the average of these three trials. If each of the three molarities you calculated is within ± M of your average molarity, proceed to Part B. If not, you will need to perform additional titrations until you obtain three trials that meet this criterion. You should discard the results from any outlying trials when computing your new average. Once you have achieved these conditions, your dilute sodium hydroxide solution has been standardized. Record the average (or standardized) molarity of this solution on your data sheet to the correct number of significant figures. You will use this value for your calculations involving this solution in Part B. Part B: Determining the Molar Mass of an Unknown Acid In this part of the experiment you will use your standardized dilute sodium hydroxide solution to titrate an unknown acid and determine its molecular mass. Clean and rinse a 250-mL Erlenmeyer flask several times using deionized water. This flask does not need to be dried. Obtain a vial containing your solid acid unknown from your instructor. Handle this vial only with a clean paper towel or tongs to avoid getting moisture, hand cream, or fingerprints on this vial. Record the unknown number of your solid acid on your data sheet. Determining the Molecular Mass of an Unknown Acid by Titration Page 5 of 8

6 The mass of solid acid you will require for each trial depends on the particular unknown acid sample used and is given on the label of each sample. This should be a value between 0.1 and 0.6 grams. Record this value on your data sheet. The unknown solid acid should also be carefully measured out using the weighing by difference technique detailed in Part A. Using this technique, measure out the mass of solid acid printed on the label into a clean 250-mL Erlenmeyer flask. The mass that you measure should be no less than that printed on the label, and no more than 0.1 grams over this amount. (For example, if the label says to measure 0.3 grams, then you should measure between 0.3 and 0.4 grams of the solid acid into your flask). Remember to record the mass of the vial before and after you add the solid acid to the 250-mL flask and then calculate the mass added by taking the difference of these two values. Just as before, if you transfer too much of the solid you will need to dispose of it, re-rinse the flask, and start your weighing by difference procedure again. You should weigh by difference for all trials in this experiment. Now add about 50 ml of deionized water (this amount does not need to be exact) and 2 to 3 drops of phenolphthalein indicator to the contents of this flask. Swirl to mix. Your unknown acid may be relatively insoluble, so don t worry if it does not all dissolve in this step. Refill the buret containing your standardized dilute NaOH solution and remove the funnel as before. Use your standardized dilute NaOH solution to titrate your unknown solid acid sample. Once again, the endpoint occurs when a single drop of NaOH solution added to your flask causes a faint pink color to persist in the solution for at least 30 seconds while swirling. If you overshoot the endpoint in this part of the experiment, you will need to repeat the trial. Refill your buret and perform two additional titrations in a similar manner. Using the average molarity you determined in Part A for your standardized dilute NaOH solution, calculate the experimentally determined molecular mass of your unknown solid acid for each trial and determine the average of these values. Your three molecular masses should be within 2 percent of their average value. For example, if your average molecular mass is 100. grams per mole, then each trial should be within ± 2 grams per mole. If not, you should perform additional titrations until you obtain three trials that meet this criterion. Once again, discard the results from any outlying trials when computing your new average. Clean up: Only the titrated solutions containing KHP and your unknown solid acid need to be poured into the chemical waste container. All other solutions, including any excess NaOH solution, may be disposed of in the sink. Rinse your buret well and then return it to the stockroom. Return any unused KHP and unknown solid acid to your instructor, making certain that you have recorded the unknown number on your data sheet. Determining the Molecular Mass of an Unknown Acid by Titration Page 6 of 8

7 Name: Team Name: Date: Lab Section: Determining the Molecular Mass of an Unknown Acid by Titration Part A Standardization of a Sodium Hydroxide Solution using KHP Data Trial 1 Trial 2 Trial 3 (Trial 4) * Initial mass of vial: Final mass of vial: Mass of KHP used: Initial NaOH buret reading: Final NaOH buret reading: Volume of NaOH used: Calculations Trial 1 Trial 2 Trial 3 (Trial 4) * Moles of KHP used: Moles of NaOH titrated: Molarity of NaOH: *Only three trials are required, but space for a fourth is given if needed. Average molarity of NaOH solution: M (Any trials falling outside ± M of your average molarity should be discarded and repeated). Cross out any trials that were not used in determining your average in the table above. In the space below, clearly show all calculations for your Trial 1 data only: Determining the Molecular Mass of an Unknown Acid by Titration Page 7 of 8

8 Part B Determining the Molecular Mass of an Unknown Acid Unknown Number of Solid Acid: Mass per trial indicated on label: Data Trial 1 Trial 2 Trial 3 (Trial 4) * Initial mass of vial: Final mass of vial: Mass of acid used: Initial buret reading: Final buret reading: Volume of NaOH used: Calculations Trial 1 Trial 2 Trial 3 (Trial 4) * Moles of NaOH used: Moles of acid titrated: Molecular mass of acid: *Only three trials are required, but space for a fourth is given if needed. Average molecular mass of unknown acid: g mol 1 (Any trials falling outside ± 2 percent of your average molecular mass should be discarded and repeated). Cross out any trials that were not used in determining your average in the table above. In the space below, clearly show all calculations for your Trial 1 data only: Determining the Molecular Mass of an Unknown Acid by Titration Page 8 of 8