SECTION II, Part A部分
Questions 1–3 are long free-response questions that require about 20 minutes each to answer and are worth 10 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.
1. A student is given the task of determining the I- content of tablets that contain potassium iodide, KI and an inert, water-soluble sugar as a filler. A tablet is dissolved in 50.0ml of distilled water, and an excess ofis 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? Justify your answer.
3. A student is given a standard galvanic cell, represented above, that has a Copper electrode and a Tin electrode. As current flows through the cell, the student determines that the Copper electrode increases in mass and the Tin 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 Tin electrode decreases, where does the mass go?
(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.
SECTION II, Part B部分