CHEM 112 University of South Carolina NaOH Titration Lab Report

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CHEM 112

University of South Carolina Aiken

CHEM

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Answer the following assignment using the information included in the file. Please include your work.

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Demo Lab Weak Acid Titration with Strong Base – NaOH(aq) This Demo lab is written for those who will not be able to come on Tuesday, July 21 to do the actual titration in person in the lab. In this report there are two main tasks. One task is to determine the concentration of one of the reagents in the titration. We have the compound in the flask and the compound in the buret and for this technique to work, you must know the concentration of one of those reagents. That information is used to determine the other’s concentration experimentally. The second task for a weak acid titration with strong base is to, if possible, track the pH changes through the entire process. We have done this earlier and you will be provided with that trace. To accomplish the first task, we need the balanced reaction for the titration. Because a strong base is being used, its participation is in a strong reaction manner. Strong bases will first attack any excess acid present in the solution as a strong reaction with no equilibrium. In a typical titration, the compound added from the buret (in this lab it is the sodium hydroxide) is the Limiting Reagent, the LR, and it gets used up after each addition from the buret. This LR strong reaction continues with each addition until the number of moles of the titrant added from the buret perfectly matches the amount needed to exactly react with reagent in the flask. At that perfect point, called the Equivalence Point, the reaction can be analyzed for concentrations. In our acid/base titration, this Equivalence Point is signaled by either an indictor color change (phenolphthalein turns from colorless in acid to very pale pink near the equivalence point) or by the volume location of the pH trace that has undergone the most rapid pH change. The volume of added titrant needed to reach the Equivalence Point is used in stoichiometry calculations to determine the unknown concentration of the reagent in the flask usually, in our case that is acetic acid. The final volumes listed below were measured when the indication of Equivalence Point occurred in each experiment. Determine the [HOAc]o value for each experiment and then average the three experimental acetic acid concentrations to arrive at the best estimate of the acetic acid concentration. Balanced Strong Titration Rxn: NaOH(aq) + HOAc(aq) → H2O + Na+(aq) + OAc–(aq) Starting Volume of Acetic Acid in Flask = 20.00 mL (0.02000 L) [NaOH]o = 0.109 M (Determined by lab staff in an earlier experiment) Expt #1 Expt #2 Expt #3 Final Volume of NaOH in Buret 33.82 mL 32.47 mL 32.28 mL Initial Volume of NaOH in Buret 1.13 mL 0.45 mL NaOH Volume Added = Final Volume – Initial Volume 0.17 mL Recall: # mol HOAc = NaOH Volume Added (in L) x 0.109 M x (1 mol HOAc/1 mol NaOH) [HOAc]o = #mol HOAc present in flask / 0.02000 L The second task, to follow the titration addition using a pH probe, is shown in the attached graph. One value to be determined from this graph for a weak acid is explored in our analysis of this data. As mentioned earlier, the Equivalence Point on this trace is the Volume Point where the pH change is occurring most rapidly. For our experiments, that volume was at the average 32.27 mL added NaOH. On the trace, take a straight-edge or ruler and visually drop a perpendicular line down to the ‘x’ axis (the volume axis) that runs through this most vertical, rapid pH change part of the trace. Where that perpendicular crosses the Volume axis is 32.27 mL. Now, follow the ‘x’ axis back to exactly, or as close as you can, to one half of that value; this would be at 16.135 mL. Now determine, from the graph or the data table, what the pH value is at that half-way point. That pH is an important milestone value in a weak acid/strong base titration. Looking back at the titration reaction above, the half-way point leaves one half of the original HOAc unreacted while forming an equal number of moles of OAc – ion. Therefore, at the half-way mark [HOAc] = [OAc–]. The equilibrium expression for our weak acid is shown below. 𝐾𝑐 = [𝐻 + ][𝑂𝐴𝑐 − ] [𝐻𝑂𝐴𝑐] Because the unreacted acetic acid concentration equals the newly formed acetate ion concentration, they both cancel out of the Kc expression and so Ka = [H+]. Now, if I take the negative log of both sides of the equation I get pKa = pH at Half-Equivalence Volume. This means I have an experimental way to determine the pKa of my weak acid. We know we are using acetic acid with a pKa = 4.75, so you need to compare the result of the above analysis to the accepted known value. They should be close. The conclusions then are an average concentration of the acetic acid present in the flask during the three titrations and the pKa value experimentally determined from the pH versus Titrant Volume graph.
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Explanation & Answer

Hello!I marked this as the final answer. but you can send me the pH data through this chat when you received it. you no need to put another bid or anything. just send the data set through this chat, then I will do that and send the answer to you.Thanking you.

1.

pH Curve of acid and NaOH titration
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pH reading

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time/s

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Because when weak acid not fully dissociated in the water it only dissociates pa...


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