17B. An Acid Base Titration Curve

Transcription

1 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 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

2 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

3 198 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 g/mol g KHP 1 mol KHP Mass KHP 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 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) L mol NaOH/L Molarity HC 2 H 3 O 2 soln L M 1 mol HC 2 H 3 O 2 1 mol NaOH

4 An Acid Base Titration Curve Date:... Student name:... Course:... Team members:... Section:... Instructor:... Results Sample 1 2 Mass of KHP and paper (g) Mass of paper (g) Mass of KHP (g)

5 200 Experiment 17B Sample 1 Initial ph: Initial buret reading (ml): Buret Volume Buret Volume Reading (ml) Added (ml) ph Reading (ml) Added (ml) ph

6 An Acid Base Titration Curve 201 Sample 2 Initial ph: Initial buret reading (ml): Buret Volume Buret Volume Reading (ml) Added (ml) ph Reading (ml) Added (ml) ph

7 202 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) ph at equivalence point b. Calculate the molarity of the NaOH solution from each result, and calculate the mean. Sample 1: 1 mol KHP g KHP g KHP 1 L ml 10 3 ml 1 mol NaOH 1 mol KHP M Sample 2: 1 mol KHP g KHP g KHP ml 1 L 10 3 ml 1 mol NaOH 1 mol KHP M Mean value 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) ph at halfway point pk a Mean pk a

8 An Acid Base Titration Curve 203 d. Use the mean pk a to calculate K a for KHP. K a antilog( 5.10) 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 mol x [H 3 O ] M ph 3.22 x mol/0.050 L Sample 2: obs. ph 3.50 Mol KHP mol x [H 3 O ] M ph 3.22 x 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 /

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 x [OH ] M poh 5.20 ph 8.80 x mol/0.074 L Sample 2: obs. ph x [OH ] M poh 5.20 ph 8.80 x mol/0.074 L