Characterization of a Weak Acid Jiatong Chen University of Illinois Urbana Champaign Chem 205 Section AB3 TA Tiana Huynh April 4 2024 Abstract The purpose of this lab was to characterize an unknown weak acid by determining its molar mass and pKa value The assigned number of the unknown weak acid was 29 A standardized NaOH solution which was found to have a concentration of 0 09945 M was used to titrate three samples of the unkown acid solution By using a calibrated DropCounter and pH probe three trials of titration curves were obtained The unknown acid was found to be a monoprotic acid From the average of the three trials its pKa value was calculated to be 5 25 with a standard deviation of 0 254 its molar mass was calculated to be 201 6 g mol with a standard deviation of 21 34 g mol These values characterize unknown weak acid 29 to be potassium hydrogen phosphate KHP The percent errors for pKa and molar mass were 2 78 and 1 27 respectively Introduction In this lab we aim to identify a weak acid by determining its molar mass and pKa value To find these values we titrate the weak acid with a strong base NaOH in this experiment and analyze the titration curve which is a plot of pH as a function of the volume of NaOH solution added The titration curve is very useful in gathering information about the acid At the equivalence point all of the acid is converted to its conjugate base therefore the amount of NaOH consumed is equal to the initial amount of the acid At the halfway point half amount of the acid is converted and by the Henderson Hasselbalch equation pH pKa log the pH of the solution is equal to the pKa of the acid For this experiment the pH at the equivalence point of the titration is expected to be greater than 7 as we titrate a weak acid with a strong base In 2014 R R Khoury et al conducted a research to study the effect of types and positions of amino acid residues on copper II complexes with 27 tripeptides formed from the amino acids glutamic acid glycine and histidine1 The formation equilibrium constants of the complexes were determined through the titration of 10 mL solutions of copper II ion and ligand solutions with same molarity The titration results allowed them to develop a model to predict pKa values of tripeptides and formation equilibrium constants of complexes unknown In a study done in 2007 A V Bogachev et al investigated the sodium binding properties of Na translocating NADH quinone oxidoreductase Na NQR using a redox titration2 Redox titration is a titration technique based on a redox reaction between the sample and titrant They found that The dissociation constant of Na for the oxidized enzyme is significantly higher than that of the reduced enzyme and that the reduced enzyme has one or more sodium binding site s with lower ion affinity2 In our experiment a LabQuest device with calibrated DropCounter and pH probe were used to record titration data By recording the volume of each individual drop prior to the titration and counting the number of drops added during the titration the DropCounter could accurately determine the volume of the NaOH solution added The pH probe was calibrated with two buffer solutions of known pH and was able to record the pH of the solution continuously during the titration Experimental The experiment was completed following the procedure found in the lab manual An Introduction to Chemical Systems in the Laboratory3 The NaOH solution used to titrate was provided in the lab and was standardized by titrating a KHP solution of known molarity Then the unknown weak acid was titrated using a automatic DropCounter which was connected to a LabQuest A pH probe was used to record the pH change of the solution during titration The DropCounter and pH probe were both calibrated before titration Three trials of titration were completed and the data were collected from the LabQuest Results Discussion In order to determine the exact amount of the weak acid in the sample solutions the NaOH solution used as the titrant needed to be standardized Table 1 NaOH Standardization Data Trial 1 Mass of KHP g 0 3059 Volume of NaOH mL 15 0 2 0 3035 14 9 3 0 3065 15 2 Molarity of NaOH M 0 09986 0 09974 0 09874 The molarity of the NaOH solution was calculated to be 0 09945 M from the average of the three trials as shown in Table 1 Figure 1 Titration curve of Trial 1 Figure 2 Derivative of the titration curve of Trial 1 The titration curve indicates that the weak acid is monoprotic as it has only one equivalence point For each of the three trials a titration curve and its derivative curve were generated and they were observed to have similar trends respectively Table 2 Data at equivalence points of the titration Trial 1 Mass of the unknown acid g 0 3054 Volume of NaOH added at 16 71 2 0 3001 13 03 3 0 3022 16 04 equivalence point mL pH at equivalence point 8 45 8 75 8 54 The equivalence point occurs at the point on the titration curve where the rate of change in pH is the highest Thus the maximum point on the derivative curve corresponds to the equivalence point Table 3 Molar mass calculations based on titration results Trial Average Molar mass g mol Standard deviation 1 183 8 201 6 21 34 2 231 6 3 189 4 The molar mass of the unknown acid is calculated from the mass of the sample and the amount of NaOH added at the equivalence point which is equal to the initial amount of the acid in the sample Table 4 pKa calculations based on titration results Trial pKa Average Standard deviation 1 5 04 5 25 0 254 2 5 61 3 5 11 The pKa of the weak acid was calculated by the volume of NaOH solution added at the halfway of the titration This is the point when half amount of the acid has been consumed by NaOH so that the concentration of the acid remaining is equal to the concentration of the conjugate base By the Henderson Hasselbalch equation the pKa of the acid is equal to the pH at this point With a molar mass of 201 6 g mol and a pKa value of 5 25 calculated from the experiment the weak acid 29 was determined to be potassium hydrogen phosphate KHP as the results are closest to its actual molar mass of 204 2 g mol and pKa of 5 4 The percent errors of pKa and molar mass were determined to be 2 78 and 1 27 respectively The automated titration was successful in measurement of pKa value and the stoichiometical amount of the acid as the results have low percent errors References 1 Khoury R R Sutton G J Ebrahimi D and Hibbert D B Formation Constants of Copper II Complexes with