CHE 134 Final Project Group 6

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Jan 9, 2024

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Make sure to stir so error won’t occur. Aya Sleem, Alyssa Maiero and Yaqin Rahman CHE 134 Spring 2023 Final Project To be presented on Thursday, April 27th

Introduction In this experiment, we conducted two titrations of an unknown acid with NaOH to first calculate the molar mass and titer and then the pka of the unknown acid. In the first titration with phenolphthalein indicator, we were able to measure the mmoles of NaOH that were used to reach the equivalence point, and from there, calculate the molar mass of our acid, depending on how many milligrams of the acid we titrated. Also, using the mg of acid and mL of NaOH it took to reach the equivalence point, we calculated our titer which helped us determine how much acid was needed to be titrated with 25 ± 3 mL of NaOH.

Introduction Continued For the second titration, we weighed out approximately the amount of mg of acid needed and then using the titer we calculated, determined exactly how many mL of NaOH should be used to reach the equivalence point. In this second titration, instead of using an indicator, we used a pH electrode to take accurate measurements of the pH after each addition of a NaOH. We utilized our data to construct our titration curve, which was then used to find the pH of our solution at half equivalence, which allowed us to determine the pKa of our acid. These results allowed us to determine the identity of the unknown acid by comparing it to the molar masses and pkas of multiple acids.

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Introduction Continued ● The goal of our redesign is to obtain more accurate pH readings during our titration so that our data is more accurate and precise. By increasing the precision and accuracy of our data, our calculations will be more accurate for identifying our unknown acid. ● We redesigned this experiment by utilizing a stir plate and stir bar to thoroughly mix the solution as we were titrating. This promoted the moles of the NaOH solution to be evenly distributed among the weak acid, before taking pH measurements.

Materials and Methods Materials utilized in the initial and redesigned experiment include… ● Volumetric pipettes ● Burets ● Erlenmeyer flasks ● Beakers ● pH electrode ● Squeeze bottles ● Droppers ● Scale Materials only used in our redesign experiment include… ● Stir plate ● Stir bar

Materials and Methods Continued ● The method we used in this experiment to acquire our data and results is titration . Using a buret, we slowly titrated our weak acid solution with the strong acid, NaOH, until we reached and passed the equivalence point(moles NaOH=moles unknown weak acid). ● Throughout the entire titration, the stir bar and stir plate kept the solution thoroughly mixed so that the moles of NaOH were being distributed evenly. ● After each interval of titration, we recorded the pH of the solution from the reading on the pH electrode.

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Results and Discussion Part I: Unknown Acid: # 58 Unknown Identity: Sodium Dihydrogenphosphate Hydrate - Part A Mass of acid used: 0.1700 ± 0.0001g = 170.0 ± 0.1mg Net Volume NaOH at equivalence point: 21.90 ± 0.03mL Mmol NaOH = (Molarity) * (mL) = (0.05762M) * (21.90 ± 0.03mL) = 1.26 ± 0.03Mmol NaOH = 1.262 ± 0.03Mmol acid Molar mass of unknown acid = mg/Mmol = (170.0 ± 0.1mg)/(1.262 ± 0.03Mmol) = 134.71 ± 0.02mg/mmol Titer NaOH: mg acid/mL NaOH = (170.0 ± 0.1mg)/(21.90 ± 0.03mL) = 7.763 ± 0.001mg/mL -Part B Mass of acid used: 270.5 ± 0.1mg Net Volume NaOH at equivalence point: 24.00 ± 0.03mL Mmol NaOH = (Molarity) * (mL) = 1.383 ± 0.03Mmol NaOH = 1.383 ± 0.03Mmol acid Molar mass of unknown acid = mg/Mmol = 195.6 ± 0.02mg/mmol Percent Error pKa (%) = ((6.94 - 7.2)/7.2) * 100= 3.61% Percent Error Molar Mass (%) = ((131.5 - 138.0) / 138.0) * 100 = 4.71%

Table 1. Titration curve of original experiment Table 2. Titration curve of modified experiment Comparing the titration curves of the original experiment to the redesigned experiment:

Results and Discussion Part II: Unknown acid: #34 and Unknown identity: Potassium Hydrogen Phthalate - Part A Mass of acid used: 219.6 ± 0.1mg Net Volume NaOH at equivalence point: 20.52 ± 0.03mL Mmol NaOH = (Molarity) * (mL) = 1.182 ± 0.03Mmol acid Molar mass of unknown acid = mg/Mmol = 185.8 ± 0.02mg/mmol Titer NaOH: mg acid/mL NaOH = (170.0 ± 0.1mg)/(21.90 ± 0.03mL) = 10.70 ± 0.001mg/mL - Part B Mass of acid used: 0.1830 ± 0.0001g = 183.0 ± 0.1mg Net Volume NaOH at equivalence point: 24.16 ± 0.03mL Mmol NaOH = (Molarity) * (mL) = (0.05762M) * (24.16 ± 0.03mL) = 1.392 ± 0.03Mmol NaOH = 1.392 ± 0.03Mmol acid Molar mass of unknown acid = mg/Mmol = (183.0 ± 0.1mg)/(1.392 ± 0.03Mmol) = 131.5 ± 0.02mg/mmol Percent Error pKa (%) = ((5.35 - 5.5)/5.5) * 100= 2.72% Percent Error Molar Mass (%) = ((195.6 - 204.0) / 204.0) * 100 = 4.12%

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Conclusion ● The accuracy and precision of our experiment was reflected in the data of our modified experiment. ● Using a stir bar and stir plate helped reduce percent error for both molar mass of acid and pKa measurements. ● An improvement can be made in taking more data points in order to create a better quality titration curve