CHM 152 updated May 2011 Lab 8: Titration curve of a Weak Acid

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1 CHM 152 updated May 2011 Lab 8: Titration curve of a Weak Acid Introduction A titration curve plots the ph of a solution as a second solution (the titrant) is slowly added, usually via a burette. Titration curves of weak acid or base solutions using a strong base or acid, respectively, are especially useful as it can allow one to experimentally determine the acid s dissociation constant, K a. A typically acid-base titration curve is shown below. The equivalence point is the ph where the acid has been completely converted to its conjugate (HA A - ). On a titration curve, it s located on the part of the graph that turns sharply upward, at the halfway point of this slope (see figure below). It shouldn t be confused with the endpoint of a titration, which usually refers to the point where the indicator changes color. When choosing an indicator for an acid-base titration, the indicator s endpoint should match the predicted equivalence point of the titration as closely as possible. The ph of the equivalence point is primarily determined on the type of salt that is formed during the reaction. For example, a weak acid and strong base usually react to produce a basic salt, so the equivalence point is predicted to have ph>7. A second point of interested is the midpoint, which is the ph where the concentrations of the acid and conjugate base are present in equal concentrations, [HA] = [A - ]. This gives an acid/conjugate ratio of 1. At this ratio, the solution ph should be equal to the pk a of the weak acid. On a titration curve, the midpoint is the halfway point between the initial ph and the point where it the slope increases sharply ph 8 6 Midpoint Equivalence point Volume of base added, ml In this lab you will determine the identity of an unknown weak acid by titrating it with a sodium hydroxide, a strong base. You will then plot a titration curve to find the acid s dissociation constant as well as its equivalence point.

2 Procedure Part I: Preparing the burette 1. Use a burette clamp attached to a ring stand to hold a 50.0mL burette 2. Add approximately 20mL of 1.00M NaOH to the burette. 3. Open the stopcock for a couple of seconds to rinse out the tip, and then slowly pour the remaining solution down the drain with running water, rotating to ensure that the entire burette is rinsed. 4. Repeat steps 2 and 3 if needed. Part II: Calibrating the ph meter 1. Fill a 50mL beaker with 20-30mL of ph=7 buffer. 2. Carefully remove the probe of the ph meter from the storage bottle and place it in the buffer solution. 3. After turning on the ph meter, it should read MEAS (for measure mode) at the top of the screen. Switch to calibration mode (CAL), using the cal/meas button. 4. The meter will automatically guess what the ph of the calibration solution is. After a second the bottom number on the meter should read Press the Enter button to accept this value. The top number should now also read Switch the meter back to MEAS mode. If the top number changes slightly ( ), this is normal. The meter should now be calibrated and ready for use. Leave the probe in the buffer solution until you re ready to begin the titration. Part III: Titration of an unknown weak acid 1. Fill the burette with 1.00M NaOH to the 0.0mL mark. 2. Use a graduated cylinder to measure 25.0mL of the weak acid and transfer it to a 250 ml beaker. 3. Place the ph meter in the acid solution and record the initial ph on your data sheet ( 0.0mL NaOH added ). 4. Add 5.0mL of NaOH to the acid solution, carefully swirl to mix, and record the new ph. 5. Continue adding NaOH to the solution in the increments outlined on the data sheet. Make sure you mix the solution before measuring the ph. If an addition is different than what s listed (e.g., 11.0mL instead of 10.0mL), note the actual volume added. 6. Repeat steps 1-5 for a second determination Part IV: Preparing a Titration Curve 1. Using Excel, prepare a graph of ph over milliliters of NaOH added for each set of data. Use an XY scatter plot, but only plot the points (Excel s trendline function isn t very accurate for this type of curve). Format the y-axis to show major tick marks (i.e., your axis labels) in increments of 1 ph unit and minor tick marks (i.e., your grid) in increments of 0.2 ph units. 2. After printing your plot, carefully draw a best-fit titration curve similar to that shown in the introduction. Label the midpoint and equivalence point on your plot. Waste Disposal Solution waste can be poured slowly down the drain with running water. Rinse the burette twice with approximately 20mL of water, similar to what was done at the start of lab. Don t forget to drain a small amount through the stopcock each time to rinse the tip.

3 Name: Section: Data Unknown code ph ph Volume NaOH added, ml (Trial #1) (Trial #2) 0.0 (initial ph) Equivalence point Average Midpoint pk a K a Average K a Include your titration curves with your lab report.

4 Name: Section: Post-lab Questions 1. Based on your results, determine the identity of your unknown from the following list. Circle your choice. K a Hydrogen sulfate ion 1.3 x 10-2 Formic acid 1.7 x 10-4 Acetic acid 1.8 x 10-5 Hydrogen sulfite ion 5.6. x 10-8 Boric acid 5.8 x Based on your results, which of the following indicators would work best for this titration? Explain. Indicator Color in acidic solution Color in basic solution ph range of color change Thymol blue Red Yellow Bromophenol blue Yellow Bluish purple Chlorophenol blue Yellow Red Bromothylmol blue Yellow Blue Cresol red Yellow Red Phenolphthalein Colorless Red Given your answer to question 1, is the salt that s formed during your titration consistent with your observed equivalence point? Explain. 4. How would each of the following errors affect your calculated value of K a (incorrectly high, low, or no effect)? In each case, explain your answer. a. The ph meter was not calibrated before beginning the titration and all ph readings were erroneously low. b. A student failed to read the burette correctly and volume measurements were erroneously high.

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6 Name: Section: Pre-lab Questions 1. Define the following. a. Equivalence point b. Midpoint c. Endpoint 2. Using the Henderson-Hasselbach equation (see textbook or lecture notes), explain why the ph of the midpoint is predicted to be the same value as the weak acid s pk a. 3. Calculate the dissociation constant of a weak acid that has a pk a of If you were analyzing a diprotic acid (H 2 A), how would the titration curve differ from the one shown in the introduction section of this lab?