Titration of Acids and Bases

Transcription

1 Exercise 3 Page 1 Illinois Central College CHEMISTRY 132 Titration of Acids and Bases Name: Equipment 1-25 ml burette 1-pH electrode 1-50 and ml beaker 1-stir plate and stir bar 1-Vernier Interface 1-pH 7 buffer for calibration Objective The objective of this experiment is to determine the concentration of an unknown acid and to illustrate the importance of titration curves and their derivatives. Background When neutralization is carried out using carefully measured amounts of acid and base, the procedure is called titration. One reagent (acid or base) is slowly added to the other until the endpoint is reached, that is, the point where the acid and base have exactly neutralized one another. The endpoint can be detected using an acid-base indicator (an organic compound that appears one color in acid and another in base) or can be determined instrumentally. The purpose of the titration is to determine the concentration of a solution of unknown strength. This solution is measured against a known quantity of the opposite reagent until the endpoint is reached. The known quantity can be a carefully weighed sample of a pure compound or a carefully measured volume of solution of precisely known concentration. The concentrations of acids and bases are often expressed in Normalities (N) rather than Molarities (M), where the Normality of a solution is defined as the number of "equivalents" of acid or base present per liter. In this respect, an equivalent is either 1 mol H +1 (for an acid) or 1 mol OH -1 (for a base). So an acid with two hydrogen ions to donate such as H 2 SO 4 has a Normality twice as great as its Molarity since each mol of H 2 SO 4 provides two "equivalents" of acid. Thus, the Normality of the solution more accurately reflects its true acidity (or basicity) in terms of the total number of H +1 (or OH -1 ) ions available. Since it would require 1 equivalent of acid (1 mol H +1 ) to neutralize 1 equivalent of base (1 mol OH -1 ) we can show that the following relationship exists at the endpoint of a titration. Normality(acid) x Volume(acid) = Normality(base) x Volume(base) In this experiment, you will be given a sample of acid of unknown concentration and titrate it against a standard solution of M NaOH. We will follow the ph of the titration through the endpoint using the LabWorks Interface. The first derivative of this "titration curve" will indicate the volume of base used to reach the endpoint allowing you to calculate the "Normality" of the unknown acid.

2 Exercise 3 Page 2 Procedure Connecting the ph probe Locate the Logger Pro icon and double-click on it, or use the Start menu. From the Menu Bar select File/Open and click on the folder Advanced Chemistry with Vernier. Open the file 07aAcid-base.cmbl. You should now see the window displayed here. Connect the Vernier ph Probe to the GoLink USB interface and connect the GoLink to the USB input on your computer. After the computer recognizes the interface, a ph reading should appear in the lower left ph display. Calibration of the ph probe 1. The ph probe used in this experiment must not be allowed to dry out, so when not in use, it should soak in distilled water or a ph 7 buffer. 2. Obtain approximately 30 ml of a ph 7 buffer and place it in a 50 ml beaker with a magnetic stir bar. Place the beaker on a stir plate and set the speed so that the solution is very gently mixed. 3. Using a ring stand and a clamp, lower the ph probe into the solution so that the glass sphere at the end of the probe is completely submerged. Be careful that the stir bar does not strike the tip of the ph probe as the glass is very fragile. 4. From the Menu Bar select Experiment/Calibrate/Go-Link 1:pH. In the Calibration dialogue box, check the "one point calibration" box. 5. Click on "Calibrate now" and under Reading 1: enter a value of "7" for the ph of your buffer solution. 6. Click the Keep button, then click Done. You should now see ph 7.00 in the lower left ph display. You need calibrate only once for this experiment.

3 Exercise 3 Page 3 Titration 1. Fill a ml burette with M NaOH, making certain that the tip of the burette is filled with solution and free of air bubbles. Zero the burette, clamp it to a ring stand and set it aside for the moment. 2. Obtain a ml sample of one of the unknown acids (A, B, C, or D) in a 250 ml beaker. Add approximately 50.0 ml distilled water, place the magnetic stir bar in the beaker, and place it on the stir plate (stirring gently). 3. Position the ring stand holding the ph probe such that the probe tip is submerged in your acid solution toward one side of the beaker. Position the other ring stand holding the burette such that the NaOH can be dispensed into the acid solution. A ph in the 1-3 range should now be on your ph display. 4. On the Menu Bar, click the Collect button. When the ph display stabilizes, click the button on the Menu Bar. A dialogue box should appear requesting that you input the volume of NaOH you have added so far. Enter "0.00" ml and click OK. 5. Add 0.50 ml of the NaOH and allow it to stir for seconds. Click the Keep ( ) button, and in the dialogue box that appears, type in "0.50" ml. Click OK. 6. Continue this procedure in 0.50 ml increments, always inputting the total volume of NaOH added to that point (that is, the current reading on your burette). 7. As we approach the equivalence point, the ph will begin to rise. Add a single drop of your NaOH solution, wait seconds and click the Keep button on the Menu Bar. At this point, read the volume on your burette as accurately as possible and input this current reading in the dialogue box. Click OK. 8. Continue this drop-by-drop addition through the "equivalence point" until the ph begins to level off at a value approaching ph = 11. At this point you can resume 0.50 ml increments of NaOH solution until an upper plateau is well established. To autoscale your graph, hit Ctrl-J. Your screen should now look like the one shown here. 9. On the Menu Bar, click the Stop button. 10.Carefully place your ph probe back into the ph 7 buffer, discard your first sample and obtain another ml sample of the same unknown acid. Add approximately 50 ml distilled water and, as before, place it on the stir plate with the stir bar, ph probe and your re-filled burette.

4 Exercise 3 Page Click on the Collect button to begin Trial 2. When prompted, choose Store Latest Run. 12. Before you begin your second titration, right-click your mouse anywhere on the graph and select Graph Options. On the Graph Options tab, check the "Connect Points" box under Appearance. Next select the Axes Options tab and under the Y-Axis Columns, de-select the ph data from your previous run (leave the ph checked for Latest Run). Click Done, and go back to Step Repeat steps 5-12 for a third trial. Data Analysis 1. We can now select each of the three trials and their corresponding first derivatives by right-clicking the mouse anywhere on your graph and choosing Graph Options. 2. On the Axes Options tab check the "Right Y-Axis" box and choose "d1" from your latest run. (Also select "autoscale" for this axis. Click Done. Your screen should now look like the one shown here. 3. The "spike" in the first derivative plot indicates the ml of base added at the endpoint (where the slope of the titration curve is at its maximum). Refer to the spreadsheet and locate the maximum value in the "d1" column. and note the volume of NaOH corresponding to this maximum. Record this value on your Report Sheet under Volume of Base at Equivalence Point. 4. Repeat steps 1-3 for your data from Run 1 and Run Calculate the Normality of the Unknown Acid for all three trials. Report the average value, and the %R.A.D for the three trials. 6. Print the Graph and Data Table for only one of the three trials and attach it to your report sheet. Exit the program.

5 Exercise 3 Page 5 Illinois Central College CHEMISTRY 132 Name: REPORT SHEET Titration of Acids and Bases Acid Sample Used (A, B, C,or D) Normality of standard NaOH N Titration Data Trial 1 Trial 2 Trial 3 Volume of acid used Volume of base at equivalence point Normality of acid Average Normality %R.A.D. Calculations: Trial 1: Trial 2: Trial 3: Average Normality: %R.A.D.:

6 Exercise 3 Page 6 Questions 1. What mass of NaOH pellets would be needed to prepare 300 ml of a N NaOH solution? 2. How many milliliters of M NaOH would be needed to neutralize 45.0 ml of M H 2 SO 4? (Note that the concentrations are given in Molarities and not Normalities) 3. Concentrated HCl is 37% HCl by mass. Its density is 1.19 g/ml. What is the Molarity of concentrated HCl? 4. Why did your titration curves flatten out at the top?

7 Exercise 3 Page 7 Illinois Central College CHEMISTRY 132 Name: PRELAB: Exp. 3 Titration of Acids and Bases 1. A sample of ml of an acid of unknown concentration requires exactly ml of N NaOH for complete neutralization. What is the Normality of the acid? 2. Given a bottle of NaOH pellets, how would you prepare one liter of M NaOH, that is, how many grams of NaOH would be required? 3. What is the Normality of M NaOH? 4. What is the Normality of M H 3 PO 4?

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