Experiment 6: Acids, Bases, Salts, and Buffers Formal Lab Report CHEM 132L GENERAL REQUIRMENTS The entire report must be typed and follow the format outlined below. Please include the main section headings as titled below. This ENTIRE report must be your own work. You may NOT turn in tables, graphs or text that are identical to another student’s work. Include the following information on the title page of your report: Name: Lab Partner(s): Lab Section Number: Experiment Title: Due Date: DATA AND OBSERVATIONS Please keep a neat and organized lab notebook as you will turn in all notebook copy pages for this experiment. For this section you can write the following: “See attached notebook copy pages.” CALCULATIONS AND GRAPHS Complete the following calculations for this experiment. Be sure to show all work as described below. Graphs must be completed using Excel. Part A Calculations – Calculate the actual [H+] concentration using the measured pH for all hydrochloric and acetic acid solutions measured in part A. Show one sample calculation for the determination of the [H+]. Data Table: Create a single typed table including the following information for both the hydrochloric and acetic acid solutions: Nominal (on bottle) concentration (M), Measured pH, and Calculated [H+] concentration. Graph – A graph of [H+](calculated) vs. Nominal Concentration. Create a single graph that includes both the HCl and the CH3COOH data sets. Also two trendlines should be included, one for HCl and one for CH3COOH. Part B Graphs (2) – Both graphs should plot pH vs. Volume (mL). 1. The first graph should be of ALL data collected for the titration in part B (see Expt. 6, Figure 1). 2. The second graph should zoom-in to the equivalence point region (see Expt. 6, Figure 2). This graph should also include hand-drawn lines to locate the pH and volume at the equivalence point (see Expt. 6, Figure 3). Calculations – 1. Report the pH and volume at the equivalence point as determined from your second graph. 2. Calculate the molarity (M) of HCl using the volume of NaOH added at the equivalence point. Show your calculations. Part C Calculations – Calculate the actual [H+] concentration from the measured pH for all salt solutions measured in part C. Show a sample calculation based on the KNO3 solution. Data Table: Create a single typed table including the following information for all the salt solutions: Salt Solution, Measured pH, and Calculated [H+] concentration. Part D Graphs (2) – Both graphs should plot pH vs. Volume (mL). 1. The first graph should zoom-in to the equivalence point region (see Expt. 6, Figure 2). This graph should also include hand-drawn lines to locate the pH and volume at the equivalence point. 2. The second graph should zoom-in to the half-neutralization point region (see Expt. 6, Figure 5). This graph should also include a hand-drawn line to locate the pH and volume at the half-neutralization point. Calculations – 1. Report the pH and volume at the equivalence point from your first graph. 2. Report the pH and volume at the half-neutralization point from your second graph. 3. Find the pKa and Ka of the weak acid. 4. Look at the Ka tables in the textbook and identify the unknown acid based on the Ka value you obtained. Part E Graph – pH vs. Volume (mL) making sure to indicate whether the volume refers to HCl or NaOH. This graph should include both the buffer and the non-buffer data sets on the same graph (clearly indicate which set of data is which). Make sure the points for each set of data are connected by a line, so that a smooth curve is observed for each set of data on the graph. NO trendlines are required! Calculations – 1. Report the measured, initial pH for the buffer solution. 2. Calculate the number of moles of sodium hydroxide and acetic acid used to prepare the buffer. 3. The buffer prepared in part C is a mixture of the acetate anions (weak base) and acetic acid (weak acid). See section 17.2 of textbook for more information about this buffer. Calculate the theoretical initial pH for your prepared buffer solution using the Henderson-Hasselbach equation. Show all work for this calculation, which should include an ICE table and the balanced chemical equation showing the reaction of NaOH with acetic acid. Acetic acid has a Ka = 1.8 x 10-5 𝑝𝑝𝑝𝑝 (𝑡𝑡ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒) = 𝑝𝑝𝐾𝐾𝑎𝑎 + 𝑙𝑙𝑙𝑙𝑙𝑙 [𝐶𝐶𝐻𝐻3 𝐶𝐶𝐶𝐶𝑂𝑂− ] [𝐶𝐶𝐻𝐻3 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶] RESULTS AND DISCUSSION For each part below, you should write a paragraph that answers all of the questions for that section. Do NOT just make a numbered list of answers. Part A: 1. What is the chemical significance of the slope of the lines found in part A? 2. Discuss why the slopes are different for each acid. Part B: 1. What was the pH at the equivalence point in part B, and was it acidic, basic, or neutral? 2. Write the balanced chemical equation for the titration reaction performed in part B. Based on this reaction, explain what chemical species are in solution that give rise to the pH at the equivalence point. Part C: 1. Predict the pH range (7) expected for each salt solution. 2. Explain how you reached your predictions and discuss how your results compare with the predictions. Part D: 1. What was the pH at the equivalence point in part D, and was it acidic, basic, or neutral? 2. What chemical reaction is taking place at the equivalence point of the titration in part D that gives rise to the pH at the equivalence point? Using the generic formula HA to represent the unknown weak acid, write a chemical equation in net ionic form to help explain your answer. 3. In what pH range would you expect the equivalence point to lie for the titration of a weak base (such as ammonia) with a strong acid (such as HCl)? Write a chemical equation in net ionic form to help explain your answer. Part E: 1. Compare the two curves on the graph you made (buffered vs non-buffered) in part E. Why are they so different? 2. Do the results support the definition of buffer given in the introduction? Include net ionic chemical equations to support your answer.
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