Homework #6 Spring 2023

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University of North Carolina, Charlotte *

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1252

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Chemistry

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Feb 20, 2024

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Homework #6 Acid-Base Equilibria - Intro Directions: Answer the following questions in order on separate sheets of paper. In order to receive full credit on the homework, you must provide a written explanation as to why you worked the problem the way you did for questions that involve calculations. To guide you in this process, you should follow the below steps for each problem you are solving: a. Write the equation that applies in the context of the problem. b. Clearly show all steps involved in working the problem, including all units. c. The final answer should be boxed. d. All the points of the problem should be solved correctly for full points. Scan each page of your worked-out solutions and combine them, in order, into one file that you can upload to Canvas. The required file format to upload is pdf or Word. Multiple files of each page will not be accepted. ------------------------------------------------------------------------------------------------------------------------------ Assume a temperature of 25 °C and that all solutions are aqueous solutions. 1. The beakers below contain an aqueous solution of three different acids, HX, HY, and HZ, respectively, and represent the solutions at equilibrium. The initial concentrations of all three solutions were the same. Assume the volume of each solution is the same. Answer the following questions. Briefly explain your answers. a) The conjugate base particles are left out of each figure. Draw the appropriate number of conjugate base particles in each beaker above, matching the legend provided for the acid in each beaker. Briefly explain your answers. (HINT: Write the dissociation equation of an acid (HA) in water. ) b) Which acid is the strongest acid? Briefly explain your answer. HX = H 3 O + = HY = H 3 O + = H 3 O + = HZ =

c) What acid will have the lowest K a value? Briefly explain your answer. d) What conjugate base will be the weakest base? Briefly explain your answer. 2. Calculate the pH of a 1.0 M NaOH solution. 3. In the reaction below please indicate the role of each species involved in the reactions below (put the corresponding label of acid/base/conjugate acid of…/conjugate base of … underneath each of the compounds in the reactions): a) CH 3 COOH(aq) + H 2 O(l) ⇌ CH 3 COO - (aq) + H 3 O + (aq) b) HCN(aq) + H 2 O(l) ⇌ CN - (aq) + H 3 O + (aq) c) HCl(aq) + H 2 O(l) ⇌ Cl - (aq) + H 3 O + (aq) d) HC 2 O 4 - (aq) + H 2 O(l) ⇌ C 2 O 4 2- (aq) + H 3 O + (aq) e) NH 3 (aq) + H 2 O(l) ⇌ NH 4 + (aq) + OH - (aq) f) C 8 H 10 N 4 O 2 H + (aq) + H 2 O(l) ⇌ C 8 H 10 N 4 O 2 (aq) + H 3 O + (aq) 4. Given the following table of information: HCN K a = 4.9 x 10 -10 C 6 H 5 COOH K a = 6.5 x 10 -5 HCOOH K a = 1.8 x 10 -4 HOBr K a = 2.0 x 10 -9 Arrange the following 0.500M aqueous solution of acids in order of decreasing acid strength (i.e. from the strongest to the weakest). Briefly explain your choice. A. HCN B. C 6 H 5 COOH C. HCOOH D. HOBr 5. Determine the pH of a 0.3 M HF solution (K a = 7.2 x 10 -4 ) 6. Determine the pH of an aqueous solution of 3.2 x 10 -4 M of the strong base Ba(OH) 2 .

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