ap08_chemistry_form_b_sgs - AP Chemistry 2008 Scoring...

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AP ® Chemistry 2008 Scoring Guidelines Form B The College Board: Connecting Students to College Success The College Board is a not-for-profit membership association whose mission is to connect students to college success and opportunity. Founded in 1900, the association is composed of more than 5,400 schools, colleges, universities, and other educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools, and 3,500 colleges through major programs and services in college admissions, guidance, assessment, financial aid, enrollment, and teaching and learning. Among its best-known programs are the SAT ® , the PSAT/NMSQT ® , and the Advanced Placement Program ® (AP ® ). The College Board is committed to the principles of excellence and equity, and that commitment is embodied in all of its programs, services, activities, and concerns. © 2008 The College Board. All rights reserved. College Board, AP Central, Advanced Placement Program, AP, SAT, and the acorn logo are registered trademarks of the College Board. PSAT/NMSQT is a registered trademark of the College Board and National Merit Scholarship Corporation. All other products and services may be trademarks of their respective owners. Permission to use copyrighted College Board materials may be requested online at: www.collegeboard.com/inquiry/cbpermit.html. Visit the College Board on the Web: www.collegeboard.com. AP Central is the online home for AP teachers: apcentral.collegeboard.com.
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AP ® CHEMISTRY 2008 SCORING GUIDELINES (Form B) © 2008 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. Question 1 Answer the following questions regarding the decomposition of arsenic pentafluoride, AsF 5 ( g ) . (a) A 55.8 g sample of AsF 5 ( g ) is introduced into an evacuated 10.5 L container at 105 ° C. (i) What is the initial molar concentration of AsF 5 ( g ) in the container? mol AsF 5 = 55.8 g AsF 5 × 5 5 1 mol AsF 169.9 g AsF = 0.328 mol [AsF 5 ] i = 5 0.328 mol AsF 10.5 L = 0.0313 M One point is earned for the correct molar mass. One point is earned for the correct concentration. (ii) What is the initial pressure, in atmospheres, of the AsF 5 ( g ) in the container? PV = nRT P = 1 1 0.328 mol 0.0821 L atm mol K 378 K 10.5 L × × = 0.969 atm One point is earned for the correct substitution. One point is earned for the correct pressure. At 105 ° C, AsF 5 ( g ) decomposes into AsF 3 ( g ) and F 2 ( g ) according to the following chemical equation. AsF 5 ( g ) AsF 3 ( g ) + F 2 ( g ) (b) In terms of molar concentrations, write the equilibrium-constant expression for the decomposition of AsF 5 ( g ). K = 3 2 5 [AsF ] [F ] [AsF ] One point is earned for the correct equation. (c) When equilibrium is established, 27.7 percent of the original number of moles of AsF 5 ( g ) has decomposed. (i) Calculate the molar concentration of AsF 5 ( g ) at equilibrium.
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