17B. An Acid Base Titration Curve Time: 2 hours Required chemicals and solutions: Reagent Requirement/5 Pairs Preparation of 1 L Potassium hydrogen phthalate 5.1 g NaOH, 0.1 M 250 ml 4.0 g of NaOH Other required materials: Balance, precise (at least 0.001 g), 1 8 students ph meter with electrode and ph 5.00 buffer, 1 2 pairs Buret, 50 ml, 1 pair Kimwipes Special note: 1. The time requirement is valid only if each student works with a partner and if there is at least one ph meter for every two pairs of students. Connections: Sodium hydroxide is also used to manufacture soap. 196
An Acid Base Titration Curve Date:... Student name:... Course:... Team members:... Section:... Instructor:... Prelaboratory assignment 1. Provide definitions for the following terms: a. Acid base titration curve A plot of the ph of a solution of acid (or base) against the volume of added base (or acid) b. Equivalence point The point at which a stoichiometric amount of a reactant has been added during a titration c. Standardization The procedure by which we obtain the molarity of a solution of one substance from an accurately known amount of another substance d. ph ph log[h 3 O ] e. ph meter A meter that accurately measures the ph of a solution 197
198 Experiment 17B 2. Estimate the mass of KHP that will require 25 ml of 0.10 M NaOH to reach the equivalence point in a titration. Give the chemical equation for the reaction. K HC 8 H 4 O 4 (aq) NaOH(aq) l Na K C 8 H 4 O 2 4 (aq) H 2 O(l) Molar mass of KHP 204.2244 g/mol 204.2244 g KHP 1 mol KHP Mass KHP 0.025 L 1 mol KHP 1 mol NaOH 0.10 mol NaOH/L 0.51 g 3. Calculate the molarity of the acetic acid solution in Figure 17B.1 if 25.0 ml of that solution has been titrated with the 0.101 M solution of NaOH. HC 2 H 3 O 2 (aq) NaOH(aq) l NaC 2 H 3 O 2 (aq) H 2 O(l) 0.02702 L 0.101 mol NaOH/L Molarity HC 2 H 3 O 2 soln. 0.0250 L 0.109 M 1 mol HC 2 H 3 O 2 1 mol NaOH
An Acid Base Titration Curve Date:... Student name:... Course:... Team members:... Section:... Instructor:... Results Sample 1 2 Mass of KHP and paper (g) 0.8626 0.8991 Mass of paper (g) 0.4036 0.4311 Mass of KHP (g) 0.4590 0.4680 199
200 Experiment 17B Sample 1 Initial ph: 3.42... Initial buret reading (ml): 1.24... Buret Volume Buret Volume Reading (ml) Added (ml) ph Reading (ml) Added (ml) ph 2.28 1.04 3.76 22.34 21.10 5.92 3.36 2.12 4.10 23.30 22.06 6.10 4.38 3.14 4.31 24.26 23.02 6.33 5.20 3.96 4.37 25.26 24.02 6.91 6.52 5.28 4.57 25.44 24.20 7.89 7.27 6.03 4.61 25.66 24.42 8.68 8.30 7.06 4.68 25.84 24.60 10.51 9.35 8.11 4.80 26.05 24.81 10.79 10.32 9.08 4.90 26.26 25.02 11.19 11.37 10.13 4.96 27.36 26.12 11.32 12.26 11.02 5.02 28.25 27.01 11.49 13.22 11.98 5.09 29.37 28.13 11.68 14.34 13.10 5.15 30.74 29.50 11.74 15.26 14.02 5.22 31.72 30.98 11.84 16.46 15.22 5.32 17.27 16.03 5.36 18.35 17.11 5.46 19.22 17.98 5.51 20.44 19.20 5.68 21.28 20.04 5.80
An Acid Base Titration Curve 201 Sample 2 Initial ph: 3.50... Initial buret reading (ml): 0.02... Buret Volume Buret Volume Reading (ml) Added (ml) ph Reading (ml) Added (ml) ph 1.24 1.22 3.82 23.08 23.06 6.22 2.36 2.34 4.14 24.24 24.22 6.72 3.65 3.63 4.40 24.42 24.40 6.90 4.73 4.71 4.48 24.63 24.61 7.17 5.85 5.83 4.58 24.88 24.86 9.32 7.00 6.98 4.73 25.06 25.04 10.72 8.48 8.46 4.85 25.30 25.28 10.91 9.69 9.67 4.90 26.34 26.32 11.29 10.80 10.78 4.99 27.41 27.39 11.48 12.00 11.98 5.07 28.63 28.61 11.66 13.00 12.98 5.14 29.73 29.71 11.76 14.34 14.32 5.25 30.62 30.60 11.84 15.69 15.67 5.31 31.64 31.62 11.90 16.85 16.83 5.40 17.90 17.88 5.50 19.33 19.31 5.68 20.40 20.38 5.74 21.54 21.52 5.96
202 Experiment 17B Questions 1. a. Use the graph paper that is available to plot the titration curves. Locate the equivalence point on each graph. Complete the following table. Sample 1 2 Volume at equivalence point (ml) 24.40 24.80 ph at equivalence point 8.60 8.70 b. Calculate the molarity of the NaOH solution from each result, and calculate the mean. Sample 1: 1 mol KHP 0.4590 g KHP 204.2244 g KHP 1 L 24.40 ml 10 3 ml 1 mol NaOH 1 mol KHP 0.09211 M Sample 2: 1 mol KHP 0.4680 g KHP 204.2244 g KHP 24.80 ml 1 L 10 3 ml 1 mol NaOH 1 mol KHP 0.09240 M Mean value 0.0923 M The precision justifies only three significant figures. c. Use your graphs to obtain the data required in the following table. Sample 1 2 Volume at halfway point (ml) 12.20 12.40 ph at halfway point 5.10 5.10 pk a 5.10 5.10 Mean pk a 5.10...
An Acid Base Titration Curve 203 d. Use the mean pk a to calculate K a for KHP. K a antilog( 5.10) 7.9 10 6 2. Use your value of K a to calculate the initial ph for each of your samples (Ebbing/Gammon, Section 17.1), and compare the calculated and experimental results. Sample 1: obs. ph 3.42 Mol KHP 2.248 10 3 mol 7.9 10 6 x [H 3 O ] 6.0 10 4 M ph 3.22 x 2 2.248 10 3 mol/0.050 L Sample 2: obs. ph 3.50 Mol KHP 2.292 10 3 mol 7.9 10 6 x [H 3 O ] 6.0 10 4 M ph 3.22 x 2 2.292 10 3 mol/0.050 L 3. a. Calculate K b for the C 8 H 4 O 2 4 anion from the value of K a that you obtained for KHP. K b K w /K a 1.00 10 14 /7.9 10 6 1.3 10 9
204 Experiment 17B b. Use K b, the number of moles of C 8 H 4 O 4 2 at the equivalence point, and the total volume at that point to calculate the ph for each sample at the equivalence point. Compare these calculated results with the experimental results. Sample 1: obs. ph 8.60 1.3 10 9 x [OH ] 6.3 10 6 M poh 5.20 ph 8.80 x 2 2.248 10 3 mol/0.074 L Sample 2: obs. ph 8.70 1.3 10 9 x [OH ] 6.3 10 6 M poh 5.20 ph 8.80 x 2 2.292 10 3 mol/0.074 L