CH 120 Laboratory Module 2 Titrations

Transcription

1 Week 5: Determination of Acidity of Vinegars CH 120 Laboratory Module 2 Titrations Objective: In this experiment you will be given a variety of commercial vinegar solutions and your lab group will need to find the acidity of each. One common method chemists use when they need to know the concentration of a solution is titration. Titration is the reaction of a species of unknown concentration with a species of known concentration. The reaction is run until the stoichiometric amounts of the reactants have been combined (this is the equivalence point as determined by ph measurements). In this experiment you will estimate this point in acid-base titrations by using an indicator that changes color. The point where the color change takes place is called the end point. The indicator may not change exactly at the equivalence point, but the difference is quite small if a proper indicator is used. The acidity of vinegar comes primarily from acetic acid. In this lab the acetic acid (CH 3 COOH) in the vinegar will be titrated with NaOH. CH 3 COOH + NaOH NaCH 3 COO + H 2 O Before we can titrate the vinegar with the NaOH, the exact concentration of the NaOH must be determined in a process known as standardization with a known amount of acid. The most common acid used for standardization is potassium hydrogen phthalate (abbreviated KHP) whose structure is shown below: KHP is a solid monoprotic acidic salt. The hydrogen bonded to the oxygen atom is the only acidic hydrogen capable of reacting with a base. Because KHP is a solid, the amount of acid use for standardizing the base can be very accurately measured. 49

2 Today, you will be using a program called LoggerPro which is used with the LabQuest Mini and a laptop. You will be collecting data relating the volume of NaOH you have added to your solution to the ph of the solution. One way to visualize this data is to graph the volumes of NaOH in solution versus the corresponding hydronium (H 3 O + ) ion concentrations. There is a specific file that must be opened for you to be able to graph you data in this manner. It is called AcidBaseTitration.cmbl, and your instructor will inform you of its location. As more NaOH is added to the solution, the H 3 O + ion concentration will decrease due to the neutralization of the acid. When all of the acid has been neutralized, you have reached the equivalence point of the reaction. The equivalence point of a titration occurs when the number of moles of hydroxide in solution is equal to the number of hydronium ions. Locate the equivalence volume on the graph above. 50

3 Traditionally, titrations are graphed with ph rather than hydronium ion concentrations. For a graph showing ph vs. Volume NaOH added, you would click File Open Experiments Probes & Sensors ph Sensor Drop Counter-pH.cmbl. Here is an example of a titration curve of this format for another reaction: 51

4 Prelab assignment: (Answer these questions in your notebook under a section titled Prelab Questions.) 1. You will begin your experiment by making approximately 500 ml of a 1 M NaOH solution from a more concentrated 6 M stock solution. Describe how you will make up your 1 M NaOH solution. Be sure to describe amounts of solution and the glassware you will use. Show any calculations you used. Remember that this solution will later be standardized so the concentrations do not need to be exact at this point. 2. After you make up your 1 M NaOH solution, you will standardize it with KHP. Determine the grams of KHP you will need to measure out for this standardization titration. Assume your NaOH solution is 1.0 M and you want the equivalence point to occur after adding approximately 25 ml of NaOH to your KHP. 3. Before titrating the KHP with NaOH, you will dissolve the acid in water. How much water do you think you should use? How will your results change if you used a different amount of water? 4. Vinegar bottles often list % acidity. This can be defined as: If a vinegar solution claims to be 4.50% acidic, what is the molarity of the acetic acid in the vinegar? You can assume the density of the vinegar is 1.00 g/ml. It might be useful to begin your calculation by assuming you have 100 g of vinegar. You should prepare your procedure and data tables in your lab notebook before coming to class. Leave room for modifying your procedure if needed. In Class A. Preparation You will work in groups of two. Begin by making up the 1 M NaOH using the procedure you gave in the pre-lab assignment. Next, calculate the number of grams of KHP necessary to standardize your NaOH. Weigh out two samples of the proper amount of KHP and transfer them to separate beakers. If you do not exactly weigh out the amounts you determined in the pre-lab, do not be concerned, but record the masses for each sample in your lab notebook. Dissolve your KHP samples in deionized water and add 3-4 drops of phenolphthalein indicator to them. Next, fill the plastic reservoir with NaOH, place a waste beaker under its spout, and adjust its stopcocks. % 52

5 Skill Box: How to Adjust Double Stopcocks Turn both nozzles vertically. Solution should be flowing freely through the spout. Adjust the top nozzle so that drops are released from the spout at a rate of about one drop per second. The bottom nozzle should serve only as an on/off switch. The vertical position will allow drops to flow at the speed you set with the top nozzle; the horizontal position will stop them. The ph sensor should be connected to CH1 and the drop counter to DIG1. To calibrate the drop counter, click Experiment, Set Up Sensors, and Show All Interfaces. Click the Drop Counter and select Calibrate. Here, enter a drops/ml ratio given by your instructor. Place your first KHP sample on top of the stir plate and underneath the titration apparatus, ensuring that the ph meter is submerged in solution and that the mouth of the beaker is positioned in a way that will catch falling drops. Drop in a stir bar and turn on the plate so that the stir bar is spinning rather quickly. B. Standardizing NaOH Once the apparatus has been fully prepped, you will titrate your KHP. Your first trial should be graphed with the AcidBaseTitration.cmbl file open. Once you have located and opened the correct file, press the green play button and then turn the bottom nozzle vertically, allowing drops of NaOH to flow. Allow the solution to titrate until well past its equivalence point; you can observe this on the screen of your instrument and within the beaker itself, as the solution will change colors. What would you expect to see at the equivalence point? When you have finished, press the stop button and turn the bottom nozzle horizontally. Save your trial by going to the File menu and clicking Save As. Make a new folder that is appropriately named for today s experiments, as you will be saving all of today s work in it. Once your graph is named and saved, open the Drop Counter-pH.cmbl file (refer to the introduction for its location). Repeat the experiment with this graphing method. In your notebook, record the approximate volume at which the solution changes color. Don t stop collecting data until about 30 ml of NaOH have been added. Find the volume that corresponds with the equivalence point on the first graph. Find this equivalence volume on the second graph. What is the graph doing at this point? What are the moles of NaOH at the equivalence point of the reaction? Using this value and your estimated equivalence point volume, determine the concentration (in moles/l) of the NaOH solution. C. Titrating Acetic Acid in Vinegar Each group will collect a sample of vinegar. You and your partner will titrate the acid in this vinegar with the NaOH you standardized. Measure a precise volume of vinegar into a beaker, add phenolphthalein, and begin collecting data. You may use either graphing method (ph or hydronium ion concentration) for these two trials. Make sure you save your graphs after each run, and name them appropriately. Find the equivalence point volume of NaOH, and use it to 53

6 calculate the molarity of acetic acid in the vinegar solutions. Also, calculate the % acidity of your vinegar and compare it to the value given on the bottle. If you have LoggerPro on your personal laptop, you can simply insert your jump drive and save your graph to it (under the File menu similarly to most programs). If you do not have LoggerPro, you can click File Export As Text. You can then name your file and save it to your jump drive. This will save a text file of the data points you collected, and you will later have to copy and paste these points into Excel to recreate the graph you observed earlier. Outline of the Experiment I. Standardize NaOH solution A. Titrate KHP using indicator; two trials, two different graphs B. Calculate molarity of NaOH solution to 4 sig. figs. II. Determine acidity of vinegar solutions A. Titrate vinegar samples two times B. Calculate molarity of acetic acid in vinegar C. Calculate % acidity of vinegar. III. Compare values for different vinegars using class data Laboratory notebook outline: Heading: Title, Date, Lab partners Prelab questions: Answer the questions in complete sentences. Show all calculations used to answer these questions. Procedure: Write out a detailed stepwise procedure for your measurements. Be sure to include the specific glassware and measuring devices you will use and the amount of reagents. Data: Make a data table for your KHP titrations. You also will need data tables for your vinegar titrations. Be sure that you always record the final and initial buret readings for your volume measurements. Calculations and Results: Show your calculations. Be sure to give any formulas you used and clearly define the variables. All calculations should include proper units and final values circled. For this lab you will need to calculate the molarity of the NaOH, the molarity of acetic acid in vinegar, and the % acid in vinegar. Be sure to calculate the standard deviation of the class values if multiple trials were performed on a single sample. Include your values in a table of the class results for the vinegar titrations. 54

7 Conclusions: a. Which method of graphing do you prefer for finding the equivalence volume? Explain your choice. b. How do the acidity values for vinegar vary within a given type of vinegar and between different types of vinegars? Quantify you comparisons. c. Why do labels use % acidity instead of molarity to report the concentration of acid in vinegar? 55