5 Acid Base Reactions

Transcription

1 Aubrey High School AP Chemistry 5 Acid Base Reactions 1. Consider the formic acid, HCOOH. K a of formic acid = a. Calculate the ph of a 0.20 M solution of formic acid. Name Period Date / / 5.2 Problems Titrations and Buffers1 c. A buffer solution with 0.20 M NH 3 and 0.20 M NH 4Cl. d. 10 ml of 1.2 M HCl added to 90 ml of the solution in part c. b. Sodium formate, NaHCOO, is added to the solution (without changing the volume) so [HCOO ] = 0.10 M. Would you expect the ph to increase or decrease? Explain. c. What is the ph of the solution if [HCOO ] = 0.20 M? What do you notice about the ph of this solution? 3. Using the list of weak acids, choose an appropriate acid for the preparation of buffers with the following ph. Then calculate the relative quantities of the acid and its conjugate base required for the buffer solution. a. ph = Find the ph of the following solutions 25 C. K b of ammonia = a. 100 ml of pure water. b. ph = 9.25 b. 10 ml of 12 M HCl added to 90 ml of pure water.

2 4. Benzoic acid is a weak monoprotic acid (K a = ). Calculate the ph of the solution at the halfway and equivalence points when 25.0 ml of a M solution of benzoic acid is titrated against M sodium hydroxide. 5. If exactly 50 ml of a M solution of hydrochloric acid is added to exactly 50 ml of 0.050M ammonia, what is the ph of the resulting solution? 6. A solution contains KH 2PO 4 and K 2HPO 4 and has a ph of What is the mole ratio of K 2HPO 4 to KH 2PO 4?

3 AP Chemistry 2002B #8 The graph below shows the result of the titration of a 25 ml sample of a 0.10 M solution of a weak acid, HA, with a strong base, 0.10 M NaOH. (a) Describe two features of the graph above that identify HA as a weak acid. (b) Describe one method by which the value of the acid-dissociation constant for HA can be determined using the graph above. (c) On the graph above, sketch the titration curve that would result if 25 ml of 0.10 M HCI were used instead of 0.10 M HA. (d) A 25 ml sample of 0.10 M HA is titrated with 0.20 M NaOH. i. What volume of base must he added to reach the equivalence point? ii. The ph at the equivalence point for this titration is slightly higher than the ph at the equivalence point in the titration using 0.10 M NaOH. Explain.

4 AP Chemistry 2007B #5 Answer the following questions about laboratory situations involving acids, bases, and buffer solutions. (a) Lactic acid, HC 3H 5O 3, reacts with water to produce an acidic solution. Shown below are the complete Lewis structures of the reactants. In the space provided, complete the equation by drawing the complete Lewis structures of the reaction products. (b) Choosing from the chemicals and equipment listed below, describe how to prepare ml of a 1.00 M aqueous solution of NH 4Cl (molar mass 53.3 g mol 1 ). Include specific amounts and equipment where appropriate. NH 4Cl (s) 50 ml buret 100 ml graduated cylinder 100 ml pipet Distilled water 100 ml beaker 100 ml volumetric flask Balance (c) Two buffer solutions, each containing acetic acid and sodium acetate, are prepared. A student adds 0.10 mol of HCl to 1.0 L of each of these buffer solutions and to 1.0 L of distilled water. The table below shows the ph measurements made before and after the 0.10 mol of HCl is added. ph Before HCl Added ph After HCl Added Distilled water Buffer Buffer (i) Write the balanced net-ionic equation for the reaction that takes place when HCl is added to buffer 1 or buffer 2. (ii) Explain why the ph of buffer 1 is different from the ph of buffer 2 after 0.10 mol of HCl is added. (iii) Explain why the ph of buffer 1 is the same as the ph of buffer 2 before 0.10 mol of HCl is added.

5 AP Chemistry 2003 #1 + C 6H 5NH 2(aq) + H 2O(l) C 6H 5NH 3 (aq) + OH (aq) Aniline, a weak base, reacts with water according to the reaction represented above. (a) Write the equilibrium constant expression, K b, for the reaction represented above. (b) A sample of aniline is dissolved in water to produce 25.0 ml of a 0.10 M solution. The ph of the solution is Calculate the equilibrium constant, K b, for this reaction. (c) The solution prepared in part (b) is titrated with 0.10 M HCl. Calculate the ph of the solution when 5.0 ml of the acid has been added. (d) Calculate the ph at the equivalence point of the titration in part (c). (e) The pk a values for several indicators are given below. Which of the indicators listed is most suitable for this titration? Justify your answer. Indicator pk a Erythrosine 3 Litmus 7 Thymolphthalein 10