Idea Transcript
Experiment 5: Identification of an Unknown Weak Acid In this experiment, an unknown weak acid will be identified by titration with standardized base. The progress of the titration will be measured using a pH meter. The titration data will then be used to construct a titration curve from which the following information may be obtained: 1. the number of protons removed from the acid during titration. 2. the pKa value(s) of the acid. 3. the molecular weight of the unknown acid. The unknown acid can be identified by matching these experimentally determined values with the provided list of possible acids. Procedure: NOTE: It doesn’t matter which you do first, the standardization of your base or the pH titration. If a pH meter is available, do that part of this experiment first. If all pH meters are in use, use that time to perform the standardization. 1. Receive an unknown acid and record the number in your lab book. •
You should provide a dry, labeled weighing bottle to the TA for your unknown one week prior to this experiment.
2. If necessary, prepare a 0.1 M NaOH solution. If you have a sufficient amount of base solution remaining from previous experiments, use it. However, in either case, you must standardize your solution. The successful generation of a titration curve depends on the accuracy of your base concentration. 3. Standardize your NaOH with standard KHP as described in Experiment 2. 4. To determine the sample weight required, assume your acid is monoprotic and that its molecular weight is 100 g/mol. Into a 250 mL beaker, accurately weigh enough of your sample to be equimolar with about 15 mL of a 0.1 M NaOH solution. 5. Add 50 mL deionized water and fully dissolve your acid sample before starting the titration. Stirring with a magnetic stir bar or gentle heating may facilitate dissolution. 6. Calibrate the pH meter as described by the TA. 7. Set up your beaker, pH probe and burette with NaOH as described by the TA. CAUTION: do not allow the stir bar to hit against the pH probe. 8. In your notebook, record the initial burette level and the initial pH reading. 9. Begin titrating. Record the burette level and the pH reading at regular intervals. For best results, stop and record data whenever the pH changes 0.25-0.50 units and/or after adding 1 mL NaOH. Do not waste time by plotting the data, just simply record the data in tabular form. CAUTION: the pH will change very rapidly near the equivalence point; titrate slowly through this point. 10. Once the solution reaches pH 11, you have fully titrated all of your acid and may stop. 1
Report: You will submit both an unknown card and a formal written report for this experiment. The following information will be useful in helping you interpret your data: 1. Construct a titration curve by plotting pH vs. mL NaOH (Fig. 1). Point D is the equivalence point, and Points A-C are 1/4, 2/4 and 3/4 of Point D (mL). 2. Determine the number of protons/mole of unknown acid by calculating the change in pH from point A to point C. If ∆pH < 1, this indicates a monoprotic acid; if ∆pH > 1, this indicates a di- or triprotic acid. 3. Use the following equation to determine the pKa (s) of your unknown: pH = pKa + log ([AG]/[HA]) Notice that this equation does not require knowledge of the absolute concentrations. Only the concentration ratio [AG]/[HA] is needed. For example, let=s say that at 1/4 of the volume required to reach the equivalence point the pH is 2.34 (found by reading the graph at point A in Fig. 1). The pKa can be determined by using the equation as follows: 2.34 = pKa + log [(1/4)/(3/4)] This relation is true because at this point, 1/4 of your acid has been converted to its conjugate base and 3/4 of your acid remains in its initial protonated state. It is recommended that you calculate the pKa values from a number of points and average the data to find a good approximation of the true pKa value. For a diprotic acid, your set of data should reveal two pKa values. 4. To determine the molecular weight of your unknown, use the data at the equivalence point.
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pH
8
6
4
2
0 0
A
5
B
10
C
15
D
Volume of Base Added/mL
Figure 1 Titration Curve 2
20
25
For your unknown card, include the information used to identify the unknown, your unknown number, the identity of your unknown, and the chemical structure of your unknown. For your report, include: 1. A title page, which includes the title of the experiment, your name, date and unknown number. 2. An introduction, which discusses some relevant principles of acid/base chemistry and the purpose of this experiment. 3. An experimental section, which describes the methods used to perform the experiment. 4. A results and discussion section, which includes the relevant data, calculations, and your titration curve. Fully label your titration curve and indicate the species present at the various points of the titration. 5. A conclusion section, which indicates the identification of the unknown. Justify your selection by discussing other possible choices. Discuss sources of errors and answer the following questions: a) What is the difference between the endpoint and the equivalence point? b) What does a pH meter measure? c) Describe a back titration for the titration of a monoprotic acid with NaOH. When would this be necessary? 6. References, as necessary.
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Molec. Wt.
pKa
Adipic HOOC-(CH2)-COOH
146.1
4.4, 5.4
Benzoic C6H5-COOH
122.1
4.20
Boric
61.8
9.14 12.74 13.80
Citric-monohydrate HOOC-CH2-C(OH)(COOH)-CH2-COOH*H20
210.1
3.13, 4.76, 5.40
372.4
6.16
Fumaric HOOC-CH=CH-COOH(trans)
116.1
3.02, 4.39
Maleic HOOC-CH=CH-COOH(cis)
116.1
1.83, 6.59
Oxalic-dihydrate HOOC-COOH*2(H2O)
126.1
1.21, 4.22
Phthalic C6H4(COOH)2
166
2.89, 5.41
204.2
2.95, 5.41
Salicylic C6H4(COOH)(OH)
138.1
2.98
Succinic HOOC-(CH2)2-COOH
118.1
4.1, 5.4
Sulfanilic Hydrate NH2-C6H4(SO3H)*H2O
191.2
3.2
Tartaric HOOC-(CHOH)2-COOH
150.9
3.03, 4.54
Acid
Ethylenediaminetetra Acetic (disodium) (EDTA)
Potassium Hydrogen Phthalate (KHP)
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