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_ap06_chemistry_sg - AP Chemistry 2006 Scoring Guidelines...

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AP ® Chemistry 2006 Scoring Guidelines 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 5,000 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. © 2006 The College Board. All rights reserved. College Board, AP Central, APCD, Advanced Placement Program, AP, AP Vertical Teams, Pre-AP, SAT, and the acorn logo are registered trademarks of the College Board. Admitted Class Evaluation Service, CollegeEd, connect to college success, MyRoad, SAT Professional Development, SAT Readiness Program, and Setting the Cornerstones are trademarks owned by 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 official online home for the AP Program: apcentral.collegeboard.com.
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AP ® CHEMISTRY 2006 SCORING GUIDELINES © 2006 The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). 2 Question 1 1. Answer the following questions that relate to solubility of salts of lead and barium. (a) A saturated solution is prepared by adding excess PbI 2 ( s ) to distilled water to form 1.0 L of solution at 25 ° C. The concentration of Pb 2+ ( aq ) in the saturated solution is found to be 1.3 × 10 3 M . The chemical equation for the dissolution of PbI 2 ( s ) in water is shown below. PbI 2 ( s ) Pb 2+ ( aq ) + 2 I ( aq ) (i) Write the equilibrium-constant expression for the equation. 2 2+ [Pb ][I ] sp K = One point is earned for the correct expression. (ii) Calculate the molar concentration of I ( aq ) in the solution. By stoichiometry, [I ] = 2 × [Pb 2+ ] , thus [I ] = 2 × (1.3 × 10 3 ) = 2.6 × 10 3 M One point is earned for the correct concentration. (iii) Calculate the value of the equilibrium constant, K sp . K sp = [Pb 2+ ][I - ] 2 = (1.3 × 10 -3 )(2.6 × 10 -3 ) 2 = 8.8 × 10 -9 One point is earned for a value of K sp that is consistent with the answers in parts (a)(i) and (a)(ii).
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