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Unknown Acid Problem
Simulation provided by ChemCollective
Background
Your company has just purchased a warehouse containing an old chemistry laboratory, and the first order of business is to take inventory of the chemicals in the stockroom so that you can properly dispose of them according to current EPA regulations. Unfortunately, the previous chemist did not label the bottles very thoroughly and left two bottles marked simply " acid " with no further information. Fortunately you find an old notebook in the lab with a table of the acids used by the previous chemist. (See table to the right). You decide that you can collect some experimental data which will enable you to both (1) identify the unknown acids by comparison with the lab notebook information, and (2) determine their concentrations. Since the disposal of unidentified chemicals is more expensive than that of known compounds, a few hours of lab work can translate into considerable savings for your company. Acid pKa crotonic 4. methylmalonic 3. diphenylacetic 3. picric ~0. alloxanic 6. methyl- m - aminobenzoic 5. Before you try the virtual activity to determine the unknown acid, watch the following video to learn how to use the simulation. http://www.chemcollective.org/chem/common/vlab_walkthrouh_html5.php Now it is time for you to identify the unknown acid with a virtual lab at http://chemcollective.org/vlab/ Copy the address to your web browser to do the virtual lab.
Procedure
Step1: Preparation of the unknown acid solution. i. Place the unknown acid solution, an empty 250 mL Erlenmeyer flask, a 25- mL pipette, and a bottle of phenolphthalein indicator solution on the bench. ii. Click the icon of the unknown solution to see its specifications in the Information window on the right side. And record the molarity of H+^ in the unknown acid solution on the Data Sheet. iii. Withdraw 25.00 mL unknown acid into the pipette and transfer the solution into the empty Erlenmeyer flask. iv. Transfer 1 mL of phenolphthalein indicator solution into the flask. v. Set the unknown acid bottle, pipette, and indicator solution bottle on the very right side of the "bench" to save space for titration. See the following screen shot. Step2: Selection of the proper amount of unknown weak acid and the best concentration of NaOH. In this step, you will determine the appropriate volume of the unknown acid and the best concentration of NaOH to use. i. Place a bottle of 0.1M NaOH solution and a burette (if you can’t find it, it could be in “Stockroom” → “solutions” → “strong bases”) on the bench.
After you decide the best amount of unknown acid solution, and the best concentration of NaOH solution. Record the selected volume of unknown acid and concentration of NaOH on the Data Sheet. Step3: Titration of the unknown acid with NaOH. Start a new titration using the selected amount of unknown weak acid and the best NaOH solution. Frist, make sure you have enough NaOH solution within the burette. If NaOH runs low, refill the burette. To start a new titration, you can right click the titration flask and select “Remove Liquid”, or you can simply remove the titration flask and get a new one from the stockroom. Then add selected amount of unknown weak acid and 1 mL of phenolphthalein solution into the flask. Activate the new titration by dragging the burette down into the flask containing the unknown acid and the phenolphthalein solution, until an addition box appears. Read the burette by the bottom of the meniscus and record this current reading as initial burette in Data Sheet, Table 1, Trial #1. Keep in mind that you have already known the approximate amount of NaOH to reach the end point. So add smaller amount of NaOH before you approach the end point, for instance, 2 mL before you reach the predicted end point, you should decrease the volume of each addition to 0.1 mL for several time, and then 0.02 mL for several times, and eventually 0.01 mL, till you see the solution turns slightly pink. Record the current burette reading as final burette reading in Data Sheet, Table 1, Trial #1. The following shows the possible colors you could see. The less pink the better. The first one (or even less pink) is the best. As soon as you see the hue of pink appears, you can stop the titration. The second and third are still acceptable, but the fourth and beyond are too pink where you must start a new titration.
Data Sheet
Molarity of H+^ in the original unknown acid solution: 0.000511635 mol/L Selected volume of unknown acid: _____ 25 ______________ mL Selected molarity of NaOH solution: ________0.1____________ mol/L Table 1 Trial #1 Trial #2 Trial # Initial Burette 13ml 23. 6 ml 34.1 ml Final Burette 23 .6 ml 34.1 ml 44.6 ml mL of NaOH added = Final - Initial
- 6 - 13 = 10. 6 ml 34.1 - 23.6 = 10.5 ml 44.6 – 34.1 = 10.5 ml Concentration of weak acid [HA]
- 6 mL NaOH x 0 .1M NaOH 25 mL of acid = 0.
- 5 ml NaOH x 0. 1 M NaOH 25 mL of acid = 0.
- 5 ml NaOH x 0. 1 M NaOH 25 mL of acid = 0. Calculate the average concentration of weak acid: [̅𝐻𝐴̅̅̅̅̅] = [𝐻𝐴] 1 +[𝐻𝐴] 2 +[𝐻𝐴] 3 3
= ___0.04213 3333 __ (1)
Data Analysis
Calculate the Ka and pKa of the unknown acid:
The molarity of H+^ in the unknown acid solution, [H+^ ], from Step 1 is: [H+^ ]eq = ______0.000511635 ___________ mol /L (2) Then calculate Ka and pKa based on the following reaction equation: HA⇌ H+^ + A-^ Ka = [𝐻+]eq [𝐴−]eq [HA]eq
Post lab Assignment
- The pH of an unknown monoprotic acid HA is 3.58. Its concentration is tested to be 0.475 M. Calculate the Ka and pKa of this weak acid. Show your work. pH = - log10 [H+] → [H+] = 10 −𝑝𝐻= 10 −^3.^58 [H+] = 10 −^3.^58 = 0.000263027 = 2.63 027 X 10 −^4 Ka = [𝐻+]eq [𝐴−]eq [HA]eq Ka = [ 10 −^3.^58 ] [ 10 −^3.^58 ] [ 0. 475 ] Ka = [ 2. 63027 X 10 −^4 ] [ 2. 63027 X 10 −^4 ] [ 0. 475 − 2. 63027 X 10 −^4 ] = 0.000000146 = 1.46 x 10 −^7 pKa = - log Ka = - log (1.46 x 10 −^7 ) = 6.835647 = 6.
- A 21.35 mL solution of 0.124 M NaOH is used to titrate 25.00 mL of an unknown monoprotic acid to the end point. Calculate the molarity of this weak acid. Mol NaOH = Volume NaOH (L) x Molarity NaOH (M) Mol NaOH = 21.35 mL x 0.124 M Mol NaOH = 2.6474 mol/mL → 0.0026474 mol/L M= mol acid Volume of acid solution L
- 0026474
- 025
- A 15.00 mL of an unknown monoprotic acid solution was titrated by 25.89 mL of 0.214 M of Ba(OH) 2 to the end point. Calculate the molarity of this weak acid. Hint: think about how many OH-^ anion each Ba(OH) 2 formula unit has. Mol Ba(OH) 2 = Volume Ba(OH) 2 (L) x Molarity Ba(OH) 2 (M) = 0. Mol Ba(OH) 2 = 25.89 mL x 0.214 M Mol Ba(OH) 2 = 5.54046 mol/mL → 0.00554046 mol/L M= mol acid Volume of acid solution L
- 00554046
- 015
4 .To a new word document, paste together the following two items: completed Data Sheet and Data Analysis. Upload the file to Canvas when you take the online quiz. (5 points) 5 .Take the online Lab 5 quiz (10 points). The quiz is based on Post Lab Assignments. Make sure you know how to answer the post lab questions before you take the quiz. The post lab questions themselves won’t be graded, instead, they are used for you to study for the quiz.
