Lab 5_ What is the Weak Acid

Lab 5: What is the Weak Acid? Chemistry 116 Liliana Hurtado, Kate Klavon, Mia Peterson, Pam Heiken Lucas Bierscheid Wednesday, 10:00 - 11:50 am

Introduction ➔ Were important background concepts described? – titration, equivalence vs endpoint, etc. ➔ Were all relevant chemical formulas/equations or mathematical equations/relationships introduced and their relevance explained? –weak acid dissociation equation, NaOH + KHP rxn, NaOH + Weak Acid rxn, etc. ➔ Were the main goals and approach for the experiment appropriately introduced? – three main goals, 0.1 each. ➔ Were the techniques, equipment and their purpose in the investigation briefly described? – where/how standardization was used, equivalence point vs endpoint (which was detected in this case?), how was it detected, etc. In this experiment, there were several main goals that the group worked to accomplish. In this lab, the goals were to successfully complete two trials of titration, plot a titration curve and determine the equivalence and endpoint values, and to successfully identify the identity of the unknown acid the group was working with. For this lab, there are numerous significant background concepts to introduce. To begin, an important technique used in this lab is titration. This process can be described as the slow, consistent addition of one solution into another known concentration of a different solution, until the reaction is neutralized. In this instance, neutralization can be noted when the color changes permanently. Furthermore, there are several important concepts to note, including equivalence and endpoint. The equivalence point can be described as the point when the entire chemical reaction ends, or in other words, neutralizes. Whereas, the endpoint in titration is noted when the indicator changes color. Additionally, there are several chemical formulas that are relevant and were used throughout this lab. This lab revolves around the dissociation of acids, more specifically weak acids. As mentioned in the lab handout, weaker acids typically dissociate less and therefore, their reactions move forward until it reaches equilibrium. The acid dissociation constant, K a , represents the equilibrium constant for the dissociation of a weak acid. This can be found using

the endpoint was reached and the buret reading was determined. Two trials of this procedure was performed. During part two, Titration was performed by using a drop counter and a pH meter in order to create a graph that helped determine an equivalence point. A pH probe with a magnetic microstirrer, a drop counter, a 100mL beaker, 60mL of an unknown acid, a hot plate, and a reservoir/buret were gathered. 60mL of an unknown acid was placed into a 100mL beaker. The reservoir/buret was primed using the unknown acid. From here, a calibrated pH probe was attached to the drop counter and placed directly under the tip of the reservoir/buret. The beaker containing 60mL of the unknown acid was placed under the drop counter, but on top of the hot plate. The end of the pH probe with the magnetic stirrer was submerged into the solution. Starting the stirrer on the hotplate, 30mL of the titrant was then added into the plastic reservoir. The valves were slowly opened allowing the solution to drop through the drop counter and into the unknown acid solution, allowing the drop counter and the pH probe to create a graph. The data recorded was pH v.s volume. Once the endpoint was reached, which was determined by a reference graph, the procedure was concluded. Results

➔ Were all appropriate tables or figures included for Part 1, and Part 2? ➔ Did the weak acid titration curve graph have an equivalence pt and 1/2 equivalence pt marked? ➔ Were the initial pH, volumes at eq/pt, and volumes/pH at 1/2 eq pt indicated somewhere? ➔ Was there a summary table or statement for calculated Kas with average? Was there a presentation of the identity of the acid? ➔ Did each Table/Figure have a title and number, and were appropriate significant figures and units used? (Buret measurements should be to 2 decimal places.) ➔ Did each Table/Figure include a brief text description to introduce and/or describe the contents? ➔ For Part 1: Were example calculations for each type of calculation performed using the actual experimental numbers shown? -Included but not limited to mol KHP, M NaOH, class avg NaOH. ➔ For Part 2: Were example calculations for each type of calculation performed using the actual experimental numbers shown? - M of weak acid, Ka from initial point, Ka from 1/2 eq pt, avg Ka. ➔ Was only one of each example calculation shown, including which table and trial the numbers came from? Were units shown on all numbers? Were appropriate significant figures used? Was appropriate use of the equation editor and/or electronically generated tables/figures demonstrated? KHP (g) Mol KHP V i V f ? M NaOH (mL) Total Vol ?M NaOH (mL) M NaOH Avg M Trial 1 0.252 0.00123 9.7 59.7 0.1268 M 0.13125 M Trial 2 0.269 0.00132 8.5 58.5 0.1357 7 M Calculation 1: Moles of KHP Equation: ___ g KHP ___ mol NaOH × 1 ?𝑜? ?𝐻? 204.23 ? ?𝐻? × 1 ?𝑜? ?𝑎?𝐻 1 ?𝑜? ?𝐻? = Trial 1: 0.252 g KHP 0.00123 mol NaOH × 1 ?𝑜? ?𝐻? 204.23 ? ?𝐻? × 1 ?𝑜? ?𝑎?𝐻 1 ?𝑜? ?𝐻? = Calculation 2: Molarity of NaOH

Half equivalence point = 6.23 pH Equivalence point = 12.35 pH pH Volume (mL) Half-Equivalence Point 6.23 23.452 mL Equivalence Point 12.35 33.750 mL Calculation 3: Molarity of Weak Acid Moles of Weak Acid = 33.750 mL = 0.0415125 mol × 1? 1000 ?? × 0.00123 ?𝑜? ?𝑎?𝐻 1 ? Molarity of Weak Acid = 0.695355 M Weak Acid 0.0415125 ?𝑜? ??𝑎? 𝑎?𝑖? 0.0597 ? = Calculation 4: K a of Weak Acid

K a of Half-Equivalence Point: K a = 10 -pH of half equivalence point K a = 10 -6.23 = 5.88 x 10 -7 K a of Equivalence Point: ICE Table HA (aq) H 2 O (l) H 3 O + (aq) A - (aq) I 0.695355 M NA 0 M 0 M C -x NA + x + x E 0.695355 - x NA x x Plug in values to the K a expression: K a = [? 2 ] 0.695355 ? Find x using: x = [H 3 O + ] = 10 -3.872 = 0.001342 Plug in x to find K a → K a = ( 0.001342 ) 2 / 0.695355 M = 2.58 x 10 -6 Closest K a value to acetic acid = 1.8 x 10 -5 Discussion ➔ Was there sufficient discussion of each of the results presented in the Results section? Did the discussion address how the main goals of the lab were related to the results obtained? ➔ For Part 1, was there sufficient analysis of the data presented in the Results section? -- the purpose/results of the standardization (class data and/or individual data)? / molarity of NaOH from standardization seems reasonable? Why/Why not? Etc. ➔ For Part 2, was there sufficient discussion of the titration curve? -- whether the molarity of weak acid calculated seems reasonable? Why/Why not? / what the pH at eq pt is and its significance / based on Ka calculated, what is identity and what is rationale for choosing it? Etc. ➔ Were the specific sources of error discussed in detail, including how they may have affected the result? Were specific sources of error used (avoiding vague excuses like “human error”)? ➔ Was there a discussion on how to reduce or eliminate these sources of error? If applicable (perhaps referring to statistical analysis/% error/ etc. from Results), was there a discussion of what might be the cause of any errors and what could be done differently to improve the results? In lab 5, our group utilized titration in order to determine the unknown weak acid. The experiment had two parts, where the first part utilized phenolphethalein to indicate the equivalence point of NaOH through stoichiomentric calculations. The second part consisted of

The goal of this experiment in part 1 was to find the standardization of NaOH. The end results were 0.252 g of KHP used in trial 1 and 0.269g in trial 2. This is 0.00123 and 0.00132 moles. From there we used the amount of liters to get the standardization of NaOH, 0.13125 M. For part 2, the goal was to determine what the unknown acid was. We did this using titration. We had to figure out the pH and the half equivalence point of the acid, and calculate Ka in order to determine the unknown acid. The half equivalence point was 23.452 mL with a pH of 6.23. The equivalence point was 33.75 mL, with a pH of 12.35. We used this information to find the moles of the weak acid, 0.0415125 mol. Then we found the molarity, 0.695355 M Weak Acid. We used this to calculate Ka, 2.58 x 10^-6 at the equivalence point. This means our acid was most likely acetic acid. Acetic acid has a Ka of 1.8 x 10^-5, which is close to our results, validating our experiment. Titration is used outside of a lab for things like product development and quality control. It can be used to determine the acidity of a fruit, or the amount of vitamin C in orange juice, (Smith). It is also used in the water industry to test the chemical content and pH of water, (Bell-Young). References ➔ Were the handout and other appropriate resources listed? ➔ Was at least one extra source for the useful real-life situation included? ➔ Were references in ACS format? If needed, was in-text citation used appropriately? Bell-Young, Lucy. “What Is Titration Used for in Real Life? - The Chemistry Blog.” ReAgent Chemical Services , 20 Oct. 2021, www.chemicals.co.uk/blog/what-is-titration-used-for-in-real-life. Smith, Rachael. “What Is Titration: Food Testing Equipment.” Calibre Control , Calibre Control, 23 May 2022, www.calibrecontrol.com/news-blog/2022/5/23/what-is-titration#:~:text=It%20is%20used%20exten sively%20in,content%20in%20different%20food%20products.