AP CHEMISTRY 2008 SCORING GUIDELINES
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1 AP CHEMISTRY 2008 SCORING GUIDELINES Question 2 Answer the following questions relating to gravimetric analysis. In the first of two experiments, a student is assigned the task of determining the number of moles of water in one mole of MgCl 2 n H 2 O. The student collects the data shown in the following table. Mass of empty container Initial mass of sample and container Mass of sample and container after first heating Mass of sample and container after second heating Mass of sample and container after third heating g g g g g (a) Explain why the student can correctly conclude that the hydrate was heated a sufficient number of times in the experiment. No additional mass was lost during the third heating, indicating that all the water of hydration had been driven off. One point is earned for the correct explanation. (b) Use the data above to (i) calculate the total number of moles of water lost when the sample was heated, and mass of H 2 O lost = = g OR = g g H 2 O 1 mol H2O g H O = mol H 2 O 2 One point is earned for calculating the correct number of moles of water. (ii) determine the formula of the hydrated compound. mass of anhydrous MgCl 2 = = g 1 mol MgCl g MgCl g MgCl mol H2O mol MgCl formula is MgCl 2 6H 2 O 2 2 = mol MgCl 2 = mol H 2 O per mol MgCl 2 One point is earned for calculating the correct number of moles of anhydrous MgCl 2. One point is earned for writing the correct formula (with supporting calculations) The College Board. All rights reserved.
2 AP CHEMISTRY 2008 SCORING GUIDELINES Question 2 (continued) (c) A different student heats the hydrate in an uncovered crucible, and some of the solid spatters out of the crucible. This spattering will have what effect on the calculated mass of the water lost by the hydrate? Justify your answer. The calculated mass (or moles) of water lost by the hydrate will be too large because the mass of the solid that was lost will be assumed to be water when it actually included some MgCl 2 as well. One point is earned for the correct answer with justification. In the second experiment, a student is given 2.94 g of a mixture containing anhydrous MgCl 2 and KNO 3. To determine the percentage by mass of MgCl 2 in the mixture, the student uses excess AgNO 3 (aq) to precipitate the chloride ion as AgCl(s). (d) Starting with the 2.94 g sample of the mixture dissolved in water, briefly describe the steps necessary to quantitatively determine the mass of the AgCl precipitate. Add excess AgNO 3. - Separate the AgCl precipitate (by filtration). - Wash the precipitate and dry the precipitate completely. - Determine the mass of AgCl by difference. Two points are earned for all three major steps: filtering the mixture, drying the precipitate, and determining the mass by difference. One point is earned for any two steps. (e) The student determines the mass of the AgCl precipitate to be 5.48 g. On the basis of this information, calculate each of the following. (i) The number of moles of MgCl 2 in the original mixture 5.48 g AgCl mol AgCl 1 mol AgCl AgCl g = mol AgCl 1 mol Cl 1 mol AgCl 1 mol MgCl2 2 mol Cl = mol MgCl 2 One point is earned for calculating the number of moles of AgCl. One point is earned for conversion to moles of MgCl 2. (ii) The percent by mass of MgCl 2 in the original mixture g MgCl mol MgCl 2 1 mol MgCl 2 = 1.82 g MgCl g MgCl g sample 100% = 61.9% MgCl 2 by mass One point is earned for calculating the correct percentage The College Board. All rights reserved.
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10 AP CHEMISTRY 2008 SCORING COMMENTARY Question 2 Overview This question assessed student knowledge and skills relating to gravimetric analysis, which is included in several of the laboratory experiments recommended in the AP Chemistry Course Description. In parts (a) through (c) students were asked to analyze and interpret a data table. They had to explain how they correctly determined that all the water of hydration had been driven off from a sample of a hydrate; calculate an appropriate formula for the hydrate; and determine the effect of an error in laboratory procedure on the calculation of the mass of water released upon heating. Parts (d) and (e) required students to describe a quantitative laboratory procedure to determine the mass of a precipitate from a mixture and then calculate the number of moles and percent by mass of a component of the mixture. Sample: 2A Score: 10 This response earned all 10 points: 1 for part (a), 1 for part (b)(i), 2 for part (b)(ii), 1 for part (c), 2 for part (d), 2 for part (e)(i), and 1 for part (e)(ii). Sample: 2B Score: 8 In part (d) no points were earned because the necessary steps were not described. Sample: 2C Score: 6 In part (d) no points were earned because the necessary steps were not described. In part (e)(i) 1 point was earned for dividing by the molar mass of AgCl, but the other point was not earned owing to the misapplication of the mole ratio in the calculation. In part (e)(ii) no point was earned. Although the answer brought down from part (e)(i) was multiplied by the correct molar mass, that product was not subsequently divided by 2.94 g, the mass of the original sample The College Board. All rights reserved.
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