Chemistry Lab

The pKa of an Unknown Acid-Base Indicator By: Josephine Hong Lab Instructor: Yue Zhang Due: April 5, 2013 Submitted: April 5, 2013 Abstract: In this lab, the pKa of the unknown indicator of expression was determined both qualitatively and quantitatively. To verify our procedures, the experiment was tested using a known indicator, bromocresol green. Qualitatively, we used the color change of the solution with indicator to obtain the pKa value solely using a pH meter. Quantitatively, we used a pH meter and the spectrophotometer with varying concentrations of the acid and base.The maximum wavelengths of absorbance used to quantify the dissociated and undissociated forms of the bromocresol green were 440 nm (yellow, undissociated) and 616 nm (blue dissociated). For the unknown indicator, the wavelengths were 505. 96 nm (red, undissociated) and 601. 66 nm (blue, dissociated). For bromocresol green, the experimentally obtained pKa value measured qualitatively was 4. 04. Quantitatively, the pKa came out to be 4. 16. The percentage error (10. 47%) obtained was within reasonable range, allowing the same procedures to be used to determine the pKa value of the unknown indicator, Indicator of Freedom.Indicator was Freedom was found to have a pKa value of 5. 32 qualitatively essay writer typer. On the other hand, the quantitative pKa was measure to be 4. 265. Introduction: This lab applies the concept of an indicator dye, which is principally a weak acid that changes colors when reacting with the amount of hydronium ion in a solution. Thus, the qualitative part of the lab assumes that at the point where the solution changes color into an intermediate shade of the two, the concentrations of [HIn] and [In-] is approximately equal. Using the Henderson-Hasselbalch equation for them: he pH equals pKa when the ratio between the indicator and its conjugate base is 1. Subsequently, the quantitative portion of the lab deals with the relationship between absorbance and concentration: A1(? )A2(? )= c1c2 Thus, a certain absorbance of a solution can provide the concentration of the indicator and its conjugate base when taking into consideration the maximum wavelength of the two colors. Experimental Section: To reduce the volumes of acid and base needed and the amount of time to complete the lab, the stock solutions of strong acid and base were diluted to at least one-fourth their starting amount.Thus, 60 mL of deionized water was added to both stock solutions of 20 mL HCl and NaOH to create 2 new 80 mL diluted solutions. Part 1: Finding pKa Qualitatively 25 mL solution of the diluted NaOH with 3 drops of indicator was titrated against the diluted HCl drop wise until a color change occurred. To determine the pKa of the indicators, the pH of the solutions were taken with a pH meter when there was a color change. Determining the color of bromocresol green to be yellow when acidic and blue when basic, the point of color change to measure pH was when the solution turne d green.Likewise, the indicator of Freedom was red when acidic and blue when basic, which established the point of color change to be purple. These steps were performed three times for both the known and unknown indicators to find an average pKa value. Part 2: Finding pKa Quantitatively For the quantitative determination of pKa, multiple solutions of varying acid/ base concentration were prepared with 3 drops of indicator. Despite the varying concentrations of each solution, the total volume stayed constant at 20 mL.Before anything, the wavelengths of maximum absorbance were determined first by using solutions of pure acid and base with three drops of indicator. For each solution, the pH was measured before placing them in cuvettes to measure their absorbance values using the spectrophotometer. These steps were performed for both the known and unknown indicator. Results: Table 1: Qualitative Data for Bromocresol Green Trial| pH| 1| 3. 82| 2| 3. 93| 3| 4. 36| Average| 4. 04| Calculat ion of Average: Table 2: Maximum Absorbance and for Bromocresol Green | HCl w/ 3 Drops Indicator| NaOH w/ 3 Drops Indicator| Concentration (M)| . 028| . 0962| (nm)| 440| 616| Solution Color| Yellow| Blue| Absorbance| . 116| . 243| Table 3: Quantitative Data for Bromocresol Green Volume of HCl (mL)| Volume of NaOH (mL)| pH| Absorbance HIn| AbsorbanceIn-| 14| 6| 3. 50| . 055| . 015| 9| 10| 4. 39| . 044| . 048| 7| 13| 5. 30| . 024| . 103| Figure 1: Absorbance vs. pH for Bromocresol Green Percent Error Calculation for Bromocresol Green: error *pKa of Bromocresol Green was found online Table 4: Qualitative Data for Indicator of Freedom Trial| pH| 1| 5. 50| 2| 5. 33| 3| 5. 12| Average| 5. 32| Table 5: Maximum Absorbance and for Bromocresol Green HCl w/ 3 Drops Indicator| NaOH w/ 3 DropsIndicator| Concentration (M)| . 1028| . 0962| (nm)| 505. 96| 601. 66| Solution Color| Red| Blue| Absorbance| . 703| . 945| Table 6: Quantitative Data for Bromocresol Green Volume of HCl (mL)| Volume of NaOH (mL)| pH| Absorbance HIn| AbsorbanceIn-| 18| 2| 2. 07| . 360| . 046| 14| 6| 2. 36| . 374| . 048| 10| 10| 3. 88| . 347| . 087| 10| 10| 3. 04| . 312| . 041| 9| 11| 9. 35| . 148| 1. 127| 9. 5| 10. 5| 5. 95| . 171| . 686| 9. 75| 10. 25| 5. 45| . 230| . 424| 9. 25| 10. 75| 9. 13| . 146| . 913| Figure 2: Graph of Absorbance vs. pH for Indicator of Freedom Discussion:The qualitative part of the lab had room for a lot of errors. The unreliability of visual observation of the slight color change in solution could have dramatically affected the results. Because the experimentally determined pKa was less than the actual pKa for both cases, strong enough color change were probably not observed. Since the titration was performed roughly using drops, the drops could have added more acid than needed, resulting in a smaller pKa.. Quantitatively, the pKa can be determined by plotting the data, with absorbance as a function of pH. There will be two separate lines, one for each wavelength of the two colors.The intersection of these two lines will indicate the point at which the pH should be equal to the pKa. This works because at the intersection point: pH=pKa+logIn-HIn It is known that A1A2= c1c2. Since A1= A2 at the intersection point of the graph, then 1= c1c2= In-HIn. Thus,pH=pKa+log1 pH=pKa at the intersection of both curves The procedure for determination of bromocresol green pKa appeared to have worked, because the percent error was roughly around 10%. Although slightly high, these errors are unpreventable. For example, cuvettes with smudges on the sides would have increased the absorbance readings of the solutions.Moreover, the maximum wavelength is crucial because it is used as a standard to know where to record the absorbance levels of the other solutions. This is because it is where the maximum absorbance occurs for the particular color produced by the solution is. It is important to measure the absorbance levels at these standardized wavelengths to keep the data con sistent and to have the ability to compare the absorbance levels of two solutions without the need to calibrate or adjust the readings. Finally, a limited number of data points when determining the pH graphically could have also added to the error in this experiment. Chemistry Lab The pKa of an Unknown Acid-Base Indicator By: Josephine Hong Lab Instructor: Yue Zhang Due: April 5, 2013 Submitted: April 5, 2013 Abstract: In this lab, the pKa of the unknown indicator of expression was determined both qualitatively and quantitatively. To verify our procedures, the experiment was tested using a known indicator, bromocresol green. Qualitatively, we used the color change of the solution with indicator to obtain the pKa value solely using a pH meter. Quantitatively, we used a pH meter and the spectrophotometer with varying concentrations of the acid and base.The maximum wavelengths of absorbance used to quantify the dissociated and undissociated forms of the bromocresol green were 440 nm (yellow, undissociated) and 616 nm (blue dissociated). For the unknown indicator, the wavelengths were 505. 96 nm (red, undissociated) and 601. 66 nm (blue, dissociated). For bromocresol green, the experimentally obtained pKa value measured qualitatively was 4. 04. Quantitatively, the pKa came out to be 4. 16. The percentage error (10. 47%) obtained was within reasonable range, allowing the same procedures to be used to determine the pKa value of the unknown indicator, Indicator of Freedom.Indicator was Freedom was found to have a pKa value of 5. 32 qualitatively essay writer typer. On the other hand, the quantitative pKa was measure to be 4. 265. Introduction: This lab applies the concept of an indicator dye, which is principally a weak acid that changes colors when reacting with the amount of hydronium ion in a solution. Thus, the qualitative part of the lab assumes that at the point where the solution changes color into an intermediate shade of the two, the concentrations of [HIn] and [In-] is approximately equal. Using the Henderson-Hasselbalch equation for them: he pH equals pKa when the ratio between the indicator and its conjugate base is 1. Subsequently, the quantitative portion of the lab deals with the relationship between absorbance and concentration: A1(? )A2(? )= c1c2 Thus, a certain absorbance of a solution can provide the concentration of the indicator and its conjugate base when taking into consideration the maximum wavelength of the two colors. Experimental Section: To reduce the volumes of acid and base needed and the amount of time to complete the lab, the stock solutions of strong acid and base were diluted to at least one-fourth their starting amount.Thus, 60 mL of deionized water was added to both stock solutions of 20 mL HCl and NaOH to create 2 new 80 mL diluted solutions. Part 1: Finding pKa Qualitatively 25 mL solution of the diluted NaOH with 3 drops of indicator was titrated against the diluted HCl drop wise until a color change occurred. To determine the pKa of the indicators, the pH of the solutions were taken with a pH meter when there was a color change. Determining the color of bromocresol green to be yellow when acidic and blue when basic, the point of color change to measure pH was when the solution turne d green.Likewise, the indicator of Freedom was red when acidic and blue when basic, which established the point of color change to be purple. These steps were performed three times for both the known and unknown indicators to find an average pKa value. Part 2: Finding pKa Quantitatively For the quantitative determination of pKa, multiple solutions of varying acid/ base concentration were prepared with 3 drops of indicator. Despite the varying concentrations of each solution, the total volume stayed constant at 20 mL.Before anything, the wavelengths of maximum absorbance were determined first by using solutions of pure acid and base with three drops of indicator. For each solution, the pH was measured before placing them in cuvettes to measure their absorbance values using the spectrophotometer. These steps were performed for both the known and unknown indicator. Results: Table 1: Qualitative Data for Bromocresol Green Trial| pH| 1| 3. 82| 2| 3. 93| 3| 4. 36| Average| 4. 04| Calculat ion of Average: Table 2: Maximum Absorbance and for Bromocresol Green | HCl w/ 3 Drops Indicator| NaOH w/ 3 Drops Indicator| Concentration (M)| . 028| . 0962| (nm)| 440| 616| Solution Color| Yellow| Blue| Absorbance| . 116| . 243| Table 3: Quantitative Data for Bromocresol Green Volume of HCl (mL)| Volume of NaOH (mL)| pH| Absorbance HIn| AbsorbanceIn-| 14| 6| 3. 50| . 055| . 015| 9| 10| 4. 39| . 044| . 048| 7| 13| 5. 30| . 024| . 103| Figure 1: Absorbance vs. pH for Bromocresol Green Percent Error Calculation for Bromocresol Green: error *pKa of Bromocresol Green was found online Table 4: Qualitative Data for Indicator of Freedom Trial| pH| 1| 5. 50| 2| 5. 33| 3| 5. 12| Average| 5. 32| Table 5: Maximum Absorbance and for Bromocresol Green HCl w/ 3 Drops Indicator| NaOH w/ 3 DropsIndicator| Concentration (M)| . 1028| . 0962| (nm)| 505. 96| 601. 66| Solution Color| Red| Blue| Absorbance| . 703| . 945| Table 6: Quantitative Data for Bromocresol Green Volume of HCl (mL)| Volume of NaOH (mL)| pH| Absorbance HIn| AbsorbanceIn-| 18| 2| 2. 07| . 360| . 046| 14| 6| 2. 36| . 374| . 048| 10| 10| 3. 88| . 347| . 087| 10| 10| 3. 04| . 312| . 041| 9| 11| 9. 35| . 148| 1. 127| 9. 5| 10. 5| 5. 95| . 171| . 686| 9. 75| 10. 25| 5. 45| . 230| . 424| 9. 25| 10. 75| 9. 13| . 146| . 913| Figure 2: Graph of Absorbance vs. pH for Indicator of Freedom Discussion:The qualitative part of the lab had room for a lot of errors. The unreliability of visual observation of the slight color change in solution could have dramatically affected the results. Because the experimentally determined pKa was less than the actual pKa for both cases, strong enough color change were probably not observed. Since the titration was performed roughly using drops, the drops could have added more acid than needed, resulting in a smaller pKa.. Quantitatively, the pKa can be determined by plotting the data, with absorbance as a function of pH. There will be two separate lines, one for each wavelength of the two colors.The intersection of these two lines will indicate the point at which the pH should be equal to the pKa. This works because at the intersection point: pH=pKa+logIn-HIn It is known that A1A2= c1c2. Since A1= A2 at the intersection point of the graph, then 1= c1c2= In-HIn. Thus,pH=pKa+log1 pH=pKa at the intersection of both curves The procedure for determination of bromocresol green pKa appeared to have worked, because the percent error was roughly around 10%. Although slightly high, these errors are unpreventable. For example, cuvettes with smudges on the sides would have increased the absorbance readings of the solutions.Moreover, the maximum wavelength is crucial because it is used as a standard to know where to record the absorbance levels of the other solutions. This is because it is where the maximum absorbance occurs for the particular color produced by the solution is. It is important to measure the absorbance levels at these standardized wavelengths to keep the data con sistent and to have the ability to compare the absorbance levels of two solutions without the need to calibrate or adjust the readings. Finally, a limited number of data points when determining the pH graphically could have also added to the error in this experiment. Chemistry Lab The purpose of this lab was to see which solutions are soluble and which are not. We were able to see this by mixing certain solutions together and observing changes that occurred. The procedure for this experiment included a few different steps. The first steps were to add the nitrate solutions into the lettered parts of the 96-well plate.Once you were done with that, you were supposed to add the sodium solutions to the numbered parts of the 96-well plate, so that the solutions were added together. You were supposed to observe the different reactions occurring. Once you observed each reaction take place, you needed to fill out your data table. Data Table: Solubility Rules Table|Negative Ion (Anions)| Positive Ions (Cations)| Solubility ofCompounds| All negative ions are with| Alkali ions (Na)| Soluble| All negative ions are with| Hydrogen (H+)| Soluble| All negative ions are with| All positive ions| Soluble| Nitrate NO3- ions are with| All positive ions are| Soluble| Acetate CH COO- ions are 3with| All positive ions are| Soluble| Chloride, Cl-Bromide Br-Iodide I-| CuAll other positive ions| Low SolubilitySolubleSoluble| Sulfate SO 2-4| BaAll other positive ions| Low SolubilitySoluble| Sulfide S -2| All positive ions| Soluble| Hydroxide, OH-| Ba| Soluble|Phosphate PO 3-4Carbonate CO 2-3Sulfite, SO 2-3| H| Soluble| Once you recorded all of the data, there were a few questions that needed to be answered. A. Compare your results with the solubility rules and/or solubility table in your chemistry text. I would say that my results turned out pretty close to the rules in the text book. I observed many different reactions occurring. The colors of the solutions changed from clear to purple, from clear to blue and yellow, and from clear to a milky white color. There were also changes from a light yellow to a dark almost orange color. B.Do your results agree with your expectations from the solubility rules/table? My results do agree with my expectations, however, I wasnà ¢â‚¬â„¢t expecting all of the solutions to be soluble. C. Which anions generally form precipitates? What are exceptions? Silver salts, Phosphates, Sulfides, Carbonates, Hydroxides. Exceptions include alkali metals. D. Which anions generally do not form precipitates? What are the exceptions? Nitrates, Alkali metals, Ammonium salts, Halides, and Acetates. The exceptions include those mentioned above that would form precipitates. E. Which cations generally do not form precipitates?Na+ generally does not form precipitates. F. Select 10 reactions that produce a precipitate, color change, or gas and write balanced chemical equation and a net ionic equation for each. Remember, a reaction may be indicated by the formation of a precipitate, color change, or the formation of gas. Record the well numbers of the precipitates you chose for your equations. (Co(No3)2+6H2O)+(Na3PO4+12H2O)Well A1 (Cu(NO3)2+3H2O )+(Na3PO4+12H2O)Well B1 (Fe(NO3)3+9H2)+(Na3PO4+12H2O)Well C1 (Ba(NO3)2) )+(Na3PO4+12H2O)W ell D1 (Ni(NO3)2+6H2O)+(Na3PO4+12H2O)Well E1 (Co(No3)2+6H2O)+(NaHCO3)Well A5 Cu(NO3)2+3H2O)+(NaHCO3)Well B5 (Fe(NO3)3+9H2)+(NaHCO3)Well C5 (Ba(NO3)2) )+(NaHCO3)Well D5 (Ni(NO3)2+6H2O)+(NaHCO3)Well E5 To wrap things up, I found this lab to be easy but confusing at the same time. I am not sure my reactions were all correct because I thought there were supposed to be some that were insoluble, however, I observed reactions occur every time. I would suggest maybe providing better rules for the solubility table because I was a little bit confused with that at first as well. I enjoyed doing this lab, as I do every one, but there were parts of it that just confused me.