CHEMISTRY
Section II
7 Questions
Time—90 minutes
YOU MAY USE YOUR CALCULATOR FOR THIS SECTION.
Directions: Questions 1–3 are long free-response questions that require about 20 minutes each to answer and are worth 10 points each. Questions 4–7 are short free-response questions that require about 7 minutes each to answer and are worth 4 points each.
Write your response in the space provided following each question. Examples and equations may be included in your responses where appropriate. For calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Pay attention to significant figures.
Mass of KI tablet0.425 g
Mass of thoroughly dried filter paper  1.462 g
Mass of filter paper + precipitate after first drying  1.775 g
Mass of filter paper + precipitate after second drying  1.699 g
Mass of filter paper + precipitate after third drying  1.698 g
1. A student is given the task of determining the I−content of tablets that contain KI and an inert, water-soluble
sugar as a filler. A tablet is dissolved in 50.0 mL of distilled water, and an excess of 0.20 M Pb(NO3)2(aq)is added to the solution. A yellow precipitate forms, which is then filtered, washed, and dried. The data from the  experiment are shown in the table above.
(a) For the chemical reaction that occurs when the precipitate forms,
(i) write a balanced, net-ionic equation for the reaction, and
(ii) explain why the reaction is best represented by a net-ionic equation.
(b) Explain the purpose of drying and weighing the filter paper with the precipitate three times.
(c) In the filtrate solution, is[K+]greater than, less than, or equal to[NO3−] ? Justify your answer.
(d) Calculate the number of moles of precipitate that is produced in the experiment.
(e) Calculate the mass percent of I−in the tablet.
(f) In another trial, the student dissolves a tablet in 55.0 mL of water instead of 50.0 mL of water. Predict
whether the experimentally determined mass percent of I−will be greater than, less than, or equal to the  amount calculated in part (e). Justify your answer.
(g) A student in another lab also wants to determine the I−content of a KI tablet but does not have access to
Pb(NO3)2. However, the student does have access to 0.20 M AgNO3, which reacts with I−(aq)to produce AgI(s). The value of K sp for AgI is 8.5 × 10−17.
(i) Will the substitution of AgNO3for Pb(NO3)2result in the precipitation of the I−ion from solution?
Justify your answer.
(ii) The student only has access to one KI tablet and a balance that can measure to the nearest0.01 g.
Will the student be able to determine the mass of AgI produced to three significant figures? Justify
your answer.
CH 3CH 2COOH(aq )  +  H 2O(l )      CH 3CH 2COO −(aq )  +  H 3
O +(aq )  2. Propanoic acid, CH 3CH 2COOH, is a carboxylic acid that reacts with water according to the equation above. At 25 C the pH of a 50.0 mL sample of 0.20 M  CH 3CH 2COOH  is 2.79.
(a) Identify a Brønsted-Lowry conjugate acid-base pair in the reaction. Clearly label which is the acid and which is the base.
(b) Determine the value of  K a  for propanoic acid at 25 C.
(c) For each of the following statements, determine whether the statement is true or false. In each case, explain the reasoning that supports your answer.
(i) The  pH  of a solution prepared by mixing the 50.0 mL sample of 0.20 M  CH 3CH 2COOH  with a
50.0 mL 0.20 M  NaOH  is 7.00.
(ii) If the  pH  of a hydrochloric acid solution is the same as the pH of a propanoic acid solution, then the
molar sample of concentration of the hydrochloric acid solution must be less than the molar concentration of the propanoic acid solution.
A student is given the task of determining the concentration of a propanoic acid solution of unknown concentration. A 0.173 M  NaOH  solution is available to use as the titrant. The student uses a 25.00 mL
volumetric pipet to deliver the propanoic acid solution to a clean, dry flask. After adding an appropriate indicator to the flask, the student titrates the solution with the 0.173 M  NaOH, reaching the end point after 20.52 mL of the base solution has been added.
(d) Calculate the molarity of the propanoic acid solution.
(e) The student is asked to redesign the experiment to determine the concentration of a butanoic acid solution  instead of a propanoic acid solution. For butanoic acid the value of  p K a  is 4.83. The student claims that a different indicator will be required to determine the equivalence point of the titration accurately. Based on your response to part (b), do you agree with the student’s claim? Justify your answer.
3. A student is given a standard galvanic cell, represented above, that has a Cu electrode and a Sn electrode.
As current flows through the cell, the student determines that the Cu electrode increases in mass and the Sn electrode decreases in mass.
(a) Identify the electrode at which oxidation is occurring. Explain your reasoning based on the student’s
observations.
(b) As the mass of the Sn electrode decreases, where does the mass go?
reaction mass(c) In the expanded view of the center portion of the salt bridge shown in the diagram below, draw and label a
particle view of what occurs in the salt bridge as the cell begins to operate. Omit solvent molecules and use arrows to show the movement of particles.
(d) A nonstandard cell is made by replacing the 1.0 M solutions of Cu(NO3)2and Sn(NO3)2in the standard cell
with 0.50 M solutions of Cu(NO3)2and Sn(NO3)2. The volumes of solutions in the nonstandard cell are identical to those in the standard cell.
(i) Is the cell potential of the nonstandard cell greater than, less than, or equal to the cell potential of the
standard cell? Justify your answer.
(ii) Both the standard and nonstandard cells can be used to power an electronic device. Would the nonstandard cell power the device for the same time, a longer time, or a shorter time as compared with the standard cell? Justify your answer.
(e) In another experiment, the student places a new Sn electrode into a fresh solution of 1.0 M Cu(NO3)2.
Half-Reaction E° (V)
+→  Cu(s)0.52
Cu  +  e−
−0.34
Cu2+  +  2 e→  Cu(s)
Sn4+  +  2 e−→  Sn2+0.15
Sn2+  +  2 e−→  Sn(s)−0.14
(i) Using information from the table above, write a net-ionic equation for the reaction between the
Sn electrode and the Cu(NO3)2solution that would be thermodynamically favorable. Justify that
the reaction is thermodynamically favorable.
(ii) Calculate the value of D G°for the reaction. Include units with your answer.

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。