Lab6. Titration Curves

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Florida SouthWestern State College, Lee *

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2046L

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Chemistry

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Feb 20, 2024

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Titration Curve of Acid Introduction This lab is a continuous of last week’s lab: Unknown Acid Problem. Here are the possible acids that you will get. In this lab, you will titrate a weak acid, obtain a titration curve and learn how to use the titration curve to identify the acid. Note that it is highly likely that the acid you get for this lab is different from the acid you get for last lab, as a new acid will be given each time the webpage is refreshed. Acid pK a Chemical formula crotonic 4.69 C 4 H 6 O 2 methylmalonic 3.07 C 4 H 6 O 4 diphenylacetic 3.94 C 14 H 12 O 2 picric ~0.4 C 6 H 3 N 3 O 7 alloxanic 6.64 C 4 H 4 N 2 O 5 methyl- m -aminobenzoic 5.10 C 8 H 9 NO 2 If you forgot how to use ChemCollective, watch the video at http://www.chemcollective.org/chem/common/vlab_walkthrouh_html5.php Now it is time for you to carry out the virtual lab at http://chemcollective.org/vlab/103 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. Withdraw 25.00 mL unknown acid into the pipette and transfer the solution into the empty Erlenmeyer flask. iii. Transfer 1 mL of phenolphthalein indicator solution into the flask. iv. 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 Na 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. ii. Pour 50.00 mL 0.1 M NaOH solution into the burette. Set the bottle of NaOH on the right side of bench. And read the burette. Remember to read the very bottom of the meniscus. Note that the volume increases as you go downwards. This reading is called the initial burette reading. The initial burette reading is ______________ mL iii. Deliver the NaOH solution from the burette into the Erlenmeyer flask containing the unknown acid and phenolphthalein solution. If the addition box didn’t show up, drag the burette up and down or left and right till you see it. Add 1.00 mL each time until the solution turns pink . Read the burette. This reading is called the final burette reading. The final burette reading is ______________ mL Calculate the volume of NaOH used to reach the end point: volume of NaOH = final burette reading- initial burette reading =_____________ A proper volume of NaOH consumed should be between 5 mL to 50 mL. If it takes more than 50 ml NaOH solution to see the color change , start over with a new titration. You can right click the glassware to remove the liquids. You can also remove the glassware or solutions and get new ones from the stockroom. For the new titration, you can either choose to titrate less (for instance, 10 mL) unknown solution, or choose to titrate 25 mL unknown Bottom of meniscus

solution with more concentrated (for instance , 1 M) NaOH solution. Don’t forget to add the phenolphthalein indicator for each titration. If it takes less than 5 mL NaOH solution to see the color change , start over with a new titration. You can right click the glassware to remove the liquids. You can also exchange glassware or solutions. For the new titration, use less concentrated (for instance, 0.01 M) NaOH solution or more unknown acid (but no more than 50 mL, like 45 mL). Don’t forget to add the phenolphthalein indicator for each titration. 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. 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. Empty the burette and refill it with 50 mL selected NaOH solution. Read the burette by the bottom of the meniscus and record this initial reading in Data Sheet, Table 1, the first blank in the “burette reading” vertical column. Click the icon of the flask containing the unknown solution and the phenolphthalein solution and see its specifications in the Information window on the right side. And record the pH in Data Sheet, Table 1, the first blank in the “pH” column. See the following screen shot to find out where to locate the pH value of the solution.

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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. Deliver 1 mL of NaOH solution into the weak acid. Record both the current burette reading and the pH value of the acid-base mixture in the Data Sheet Table 1. Leave the column of “mL of NaOH” blank for the time being. Then add another 1 mL of NaOH solution and record both burette reading and pH. Repeat. Keep in mind that you have already known the approximate amount of NaOH to reach the end point. About 5 mL ahead of the end point, add 0.5 mL of NaOH each time for a total about 4.5 mL, and then switch to 0.1 mL of addition, and then 0.02 mL, till you see a sudden pH increase and the solution turns pink. To obtain a complete titration curve, don’t stop the titration yet. Beyond the end point, reverse the order of addition, that is, add 0.02 mL for 4-5 times, then, 0.1 mL for 4-5 time, then 0.5 mL for 9-10 time, then 1 mL, until the solution reach or almost reach the 50mL mark on the burette. Step4: calculate volume of NaOH solution So far, two columns of Table 1 have been filled up. Now let’s calculate for the last column “mL of NaOH”. The volume of NaOH solution delivered from the burette equals the current burette reading minus the initial burette reading taken before the titration starts. The initial burette reading is recorded in the very first blank of the “burette reading” column . Each mL of NaOH equals the corresponding burette reading in the same horizontal row minus the initial burette reading recorded in the first blank of the “burette reading” column mL of NaOH = current burette reading – initial burette reading Step5: Plot the titration curve Plot pH on the y-axis and mL of NaOH on the x-axis. You can use some software (recommend Microsoft Excel) to plot or plot it manually on a graph paper. If you do use excel, see this link to learn how to scatter data points with Excel. During Step 6 within that link, choose use “Scatter with smooth lines”. You will obtain your titration curve after that. Different versions of Excel have different rules about which data column is plotted as x or y axis. If your titration curve looks wrong, simply switch the order of the two data column on the spread sheet and re-plot. See the Atom First textbook for an example of weak acid titration curve (Figure 14.18 within the link) Save your plot and upload it to Canvas when you take the online quiz.

Data Sheet Selected volume of unknown acid: ___________________ mL Selected molarity of NaOH solution: ____________________ mol/L Table 1 pH burette reading mL of NaOH pH burette readin g mL of NaOH pH burette reading mL of NaOH

Data Analysis Base on your titration curve, read the volume of NaOH and pH at the equivalence point (the midpoint of the vertical section of the curve) mLof NaOH at equivalence point: _______________________________ pH: ____________________________ Calculate the volume of at the half equivalence point (1/2 of the mL at equivalence point): _____________________________ Read the pH at the half equivalence point: ____________________________ For a weak acid, the value of pH at the half equivalence point equals pKa of the weak acid. pKa of the weak acid is: __________________________ Identify the unknown acid with pKa and molar mass using the following table. The unknown acid is : ____________________________ Continue to Post Lab Assignment Acid pK a Chemical formula Crotonic 4.69 C 4 H 6 O 2 Methylmalonic 3.07 C 4 H 6 O 4 Diphenylacetic 3.94 C 14 H 12 O 2 Picric ~0.4 C 6 H 3 N 3 O 7 Alloxanic 6.64 C 4 H 4 N 2 O 5 methyl- m -aminobenzoic 5.10 C 8 H 9 NO 2

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Post lab Assignment: 1. Referring to your textbook, use Henderson-Halsselbalch equation to explain why the pH value at half equivalence point of a weak acid’s titration curve equals pKa of the weak acid. 2. Based on the titration curve shown below, what’s the pKa of the acid being titrated? 3. Identify the unknown acid using the following table. 4. To a new word file, paste together these Acid pK a Chemical formula Crotonic 4.69 C 4 H 6 O 2 Methylmalonic 3.07 C 4 H 6 O 4 Diphenylacetic 3.94 C 14 H 12 O 2 Picric ~0.4 C 6 H 3 N 3 O 7 Alloxanic 6.64 C 4 H 4 N 2 O 5 methyl- m -aminobenzoic 5.10 C 8 H 9 NO 2

three items: the titration curve that you plotted, completed sections of Data Sheet and Data Analysis. Upload the file to Canvas when you take the online quiz. (5 points) 5. Take the online 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.