AP Chemistry Lab 18 1 Determination of the Molar Mass and pka of an Unknown Acid

Transcription

1 AP Chemistry Lab 18 1 PRELIMINARY LAB ASSIGNMENT 1. Calculate the molarity of a solution of sodium hydroxide, NaOH, if ml is needed to neutralize g of potassium hydrogen phthalate, abbreviated KHP (see the lab for the structure of KHP). 2. It is found that ml of M NaOH is needed to titrate g of an unknown acid to the phenolphthalein end point. Calculate the molar mass of the acid assuming the acid has two ionizable protons. 3. The following values were experimentally determined for the titration of g of a weak acid with M NaOH: Table 1. Titration Data ml NaOH ph ml NaOH ph a. Graph the data on the chart below. Titration of Weak Acid with NaOH ph Volume of M NaOH (ml)

2 AP Chemistry Lab 18 2 b. What is the ph at the equivalence point? c. Does the acid appear to be monoprotic or diprotic? Explain. d. Give the K a and pk a value (or values) of the acid. Explain. e. Calculate the molar mass of the acid. f. Table 2 shows some common indicator dyes. Which of them would be most appropriate for following the above titration? Explain. Table 2. Common Indicator Dyes Indicator Dye Color Change Acid Form Base Form ph Transition Bromophenol Blue Yellow Blue Bromothymol Blue Yellow Blue Phenolphthalein Colorless Pink

3 AP Chemistry Lab 18 3 INTRODUCTION In this experiment you will determine the molar mass and K a of an unknown acid. The acid, a solid crystalline substance, will be weighed and titrated with a standard solution of sodium hydroxide. You will plot a titration curve of the acid, and from this graph, you will determine the value of the equilibrium constant(s) for the dissociation of the acid. Acids are substances that contain ionizable hydrogen atoms within the molecule. Strong acids ionize totally, weak acids only to a small extent. The value of K a, the equilibrium constant for the dissociation of the acid, is an indication of the strength of the acid. We can also speak of the pk a, the -log(k a), as an indication of acid strength. A graph of ph versus ml of NaOH added can be drawn by carefully following the titration with a ph meter. There should be a significant change in ph in the vicinity of the equivalence point. Note that the equivalence point will probably NOT be at ph 7, but will be on the basic side. The value of the equilibrium constant for the dissociation of the acid can be obtained graphically. If we represent the dissociation of the acid as: the equilibrium expression is: HA + H 2O H 3O + + A - Ka = [H 3O + ] [A - ] [HA] When the acid is half neutralized, [HA] = [A - ], so these terms cancel in the above equation, and K a = [H 3O + ]. Therefore, when the acid is half-neutralized, the ph = pk a. The point where ph is equal to pk a can be found from the graph. Refer to Figure 1. Figure 1. ph During Titration of a Monoprotic Weak Acid with Sodium Hydroxide where A = volume NaOH at equivalence point B = one-half the volume of A or the volume when half -neutralized C = ph when half neutralized, or pk a If the acid contains two ionizable hydrogen atoms, the titration curve shows two separate inflections corresponding to the neutralization of each hydrogen ion. Figure 2 shows a titration curve for a diprotic acid such as H 2A. The dissociation of a diprotic acid can be written in two separate steps:

4 AP Chemistry Lab 18 4 The two equilibrium expressions are: H 2A + H 2O H 3O + + HA - HA - + H 2O H 3O + + A 2- [H 3O + ] [HA - ] [H 3O + ] [A 2- ] Ka1 = Ka2 = [H 2A] [HA - ] Both pk a values can be determined from the graph of ph versus volume of sodium hydroxide added. Refer to Figure 2 to see how this is done. Figure 2. ph During Titration of a Diprotic Acid with Sodium Hydroxide where B = volume of NaOH needed to react with one of the acid hydrogens A = the volume of NaOH needed to react with both of the acid hydrogens D = the volume of NaOH needed to neutralize half of the first acid hydrogen E = the ph when half of the first hydrogen is neutralized, or pk al C = the volume of NaOH used when all of the first and half of the second hydrogens are neutralized F = ph when half of the second hydrogen is neutralized, or pk a2

5 AP Chemistry Lab 18 5 DEFINITIONS: Standard solution, equivalence point, K a, pk a. MATERIALS ~0.12 M sodium hydroxide 0.1 M phenolphthalein unknown weak acid potassium hydrogen phthalate ph 7.00 buffer solution buret and stand ph meter magnetically-coupled stirrer PROCEDURE 1. Standardization of ~0.12 M NaOH Since solid NaOH rapidly absorbs both water and carbon dioxide, a solution of exact molarity cannot be prepared by weighing the solid and diluting to volume. To "standardize" the NaOH, that is to find its exact molarity, the NaOH is titrated against a solid acid, potassium hydrogen phthalate, abbreviated KHP. KHP is chosen because it is analytically pure, easily dried and weighed, and has a relatively high equivalent mass. The formula of KHP is: COOH COOK or KHC 8H 4O 4 It contains one ionizable H +. The titration can be followed using phenolphthalein as an indicator. a. Use the analytical balance to accurately mass ~0.6 g of KHP. b. Transfer the KHP to a 250 ml Erlenmeyer flask (rinse the weighing boat with water to transfer all the KHP), add ~40 ml of distilled water and swirl to dissolve. c. Clean a buret, rinse it with distilled water and then twice with about 5 ml of the NaOH solution. Fill it with the NaOH solution to between 0 and 1 ml. Open the stopcock briefly to remove air bubbles from the tip. Read the initial volume to 0.01 ml. d. Add 2 drops phenolphthalein solution to the acid in the flask and then titrate with the NaOH until the first trace of pink color persists for 15 seconds. Remember to constantly swirl the flask, and to rinse the walls of the flask with distilled water before you reach the end point. Record the volume of NaOH used, estimating the nearest 0.01 ml. e. Repeat one more time. If you use slightly more acid the second time, the titration can be done faster because you will know how much NaOH you can safely add before you get close to the end point. 2. Determination of the pka and Equivalent Mass of the Unknown Acid a. Use the analytical balance to accurately mass ~0.2 g of your unknown acid on a weighing boat. b. Transfer the acid completely to a beaker, and dissolve the sample in approximately 100 ml distilled water. c. Set up a ph meter and electrode. Calibrate the ph meter as your teacher directs using a buffer solution of ph Rinse the electrode well with distilled water. d. Set the beaker with the unknown acid on a magnetic stirrer. Submerged the ph electrode in the acid solution. Be sure the magnet does not hit the electrode. Titrate with standard base, recording the volume of base and ph of the solution during the titration. You should record the volume and ph every ml during the initial part of the titration, but as you get closer to the inflection points, you must use smaller increments of NaOH solution. Continue the titration curve at least three ml beyond the second inflection point.

6 AP Chemistry Lab 18 6 ANALYSIS Standardization of ~0.12 M NaOH 1. For each titration, use the mass of KHP, the molar mass of KHP, and the net volume of NaOH solution needed to reach the equivalence point to calculate the molarity of the NaOH solution. 2. Calculate the average molarity of the NaOH solution. Determination of the pka and Molar Mass of the Unknown Acid 3. Graph your data, with ph on the vertical axis and volume NaOH on the horizontal axis. Make your graph large enough to reflect the care you took with the measurements of ph and volume NaOH. From the graph, determine pk a1 and pk a2. If you use Excel for the graph, make sure your graph has enough grid lines to read pk a with precision. Show this analysis on your graph. 4. Calculate K a1 and K a2 of your acid. 5. Use the mass of unknown acid, the molarity of NaOH determined from step 2, and net volume of NaOH needed to reach the second inflection point (equivalence point) to determine the molar mass of your acid. 6. The unknown acid is either maleic acid, malonic acid, or tartaric acid. All three are diprotic acids. Use the Chemical Handbook to look up the molar mass of each. Use the calculated molar mass to determine which acid is the unknown. Write the chemical formula and structure of the acid. Additional Analysis Questions 7. When the ph increases by one unit, how does the hydrogen ion concentration change? 8. Why is the equivalence point of a weak acid not at ph 7? 9. How precisely can you read a buret? Why is it better to titrate with a volume of base that fills most of your buret? EXPERIMENTAL ERROR Calculate the percent error in K a1 and K a2 ( book values for this acid are: 1.5E-3 and is 2.0E-6 respectively). Determine the percent error in the molar mass. What would be the effect on the concentration of sodium hydroxide you determined if the KHP were not dried, i.e. if KHP contained a small amount of moisture?