2002 AP Chemistry Form B Scoring Guidelines

1 AP Chemistry 2002 Scoring Guidelines Form B These materials were produced by Educational Testing Service (ETS ), which develops and administers the examinations of the advanced placement Program for the College Board. The College Board and Educational Testing Service (ETS) are dedicated to the principle of equal opportunity, and their programs, services, and employment policies are guided by that principle. The College Board is a national nonprofit membership association dedicated to preparing, inspiring, and connecting students to college and opportunity. Founded in 1900, the association is composed of more than 4,200 schools, colleges, universities, and other educational organizations. Each year, the College Board serves over three million students and their parents, 22,000 high schools, and 3,500 colleges, through major programs and services in college admission, guidance, assessment, financial aid, enrollment, and teaching and learning.

2 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 equity and excellence, and that commitment is embodied in all of its programs, services, activities, and concerns. Copyright 2002 by College Entrance Examination Board. All rights reserved. College Board, advanced placement Program, AP, SAT, and the acorn logo are registered trademarks of the College Entrance Examination Board. APIEL is a trademark owned by the College Entrance Examination Board. PSAT/NMSQT is a registered trademark jointly owned by the College Entrance Examination Board and the National Merit Scholarship Corporation. Educational Testing Service and ETS are registered trademarks of Educational Testing Service. The materials included in these files are intended for use by AP teachers for course and exam preparation in the classroom; permission for any other use must be sought from the advanced placement Program.

3 Teachers may reproduce them, in whole or in part, in limited quantities, for face-to-face teaching purposes but may not mass distribute the materials, electronically or otherwise. These materials and any copies made of them may not be resold, and the copyright notices must be retained as they appear here. This permission does not apply to any third-party copyrights contained herein. AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board. All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board. 2 Question 1 10 points HC3H5O3(aq) H+(aq) + C3H5O3 (aq) 1. Lactic acid, HC3H5O3 , is a monoprotic acid that dissociates in aqueous solution, as represented by the equation above. Lactic acid is percent dissociated in M HC3H5O3(aq) at 298 K. For parts (a) through (d) below, assume the temperature remains at 298 K.

4 (a) Write the expression for the acid-dissociation constant, Ka , for lactic acid and calculate its value. Ka =]OH[HC]OH][C[H353_353+ M = M = x HC3H5O3(aq) H+(aq) + C3H5O3 (aq) I ~0 0 C x +x +x E x +x +x Ka =]OH[HC]OH][C[H353353__+ = [ ][ ][ - ] Ka = 10 4 1 point earned for equilibrium expression 1 point earned for amount of HC3H5O3 dissociating 1 point earned for [H+] = [C3H5O3 ] set up and solution (b) Calculate the pH of M HC3H5O3. From part (a): [H+] = M pH = log [H+] = log ( ) = 1 point earned for correctly calculating pH AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board. All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board. 3 Question 1 (cont d.) (c) Calculate the pH of a solution formed by dissolving mole of solid sodium lactate, NaC3H5O3 , in 250.

5 ML of M HC3H5O3. Assume that volume change is negligible. = M C3H5O3 HC3H5O3(aq) H+(aq) + C3H5O3 (aq) I ~0 C x +x +x E x +x + x Ka = ]OH[HC]OH][C[H353353__+ = ] [] ][x[ + Assume that x << M Ka = 10 4 = ] [] ][x[ x = 10 4 M = [H+] pH = log [H+] = log ( 10 4) = OR pH = pKa + log or = 1 point earned for [C3H5O3 ] (or L mol/L = mol HC3H5O3 and mol C3H5O3 ) 1 point earned for [H+] (set up and calculation) 1 point earned for calculating the value of pH AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board. All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board. 4 Question 1 (cont d.) (d) A 100. mL sample of M HCl is added to 100. mL of M HC3H5O3. Calculate the molar concentration of lactate ion, C3H5O3 , in the resulting solution.

6 M HC3H5O3 100 mL200 mL = M HC3H5O3 M HCl 100 mL200 mL = M H+ HC3H5O3(aq) H+(aq) + C3H5O3 (aq) I 0 C x +x + x E x + x + x Ka = ]OH[HC]OH][C[H353353__+ = ] []x][ [ + Assume x << M Ka = 10 4 = [ ][x][ ] x = 10 4 M = [C3H5O3 ] 1 point earned for initial [H+] and [HC3H5O3] OR (10 mmol H+; 50 mmol HC3H5O3) 1 point earned for showing dilution or moles of each 1 point earned for [C3H5O3 ] setup and calculation AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board. All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board. 5 Question 2 10 points 2. A rigid L flask contains a mixture of moles of H2, mole of O2, and sufficient Ar so that the partial pressure of Ar in the flask is atm. The temperature is 127 C. (a) Calculate the total pressure in the flask.

7 PH2 = nH2 RTV = )400)(mol KL )( (K = atm PO2 = nO2 RTV = )K400)(mol KL )( ( = atm PAr = atm PT = PH2 + PO2 + PAr = atm + atm + atm = atm 1 point earned for the partial pressure of H2 1 point earned for the partial pressure of O2 1 point earned for the total pressure (b) Calculate the mole fraction of H2 in the flask. Mol fractionH2 = molH2 molH2 + molO2 + molAr molH2 = mol molO2 = mol molAr = PVRT = )K400)(mol KL () )( ( = mol Ar molH2 + molO2 + molAr = mol + mol + mol = mol total Mol fractionH2 = molH2 molH2 + molO2 + molAr = 1 point earned for mol Ar 1 point earned for mol fraction of H2 AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board. All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board.

8 6 Question 2 (cont d.) (c) Calculate the density (in g L 1) of the mixture in the flask mol H2 g H21 mol H2 = g H2 mol O2 g O21 mol O2 = g O2 mol Ar g Ar1 mol Ar = g Ar total mass = g + g + g = g density = volumemass total= g L = g L 1 1 point earned for mass of all species 1 point earned for density The mixture in the flask is ignited by a spark, and the reaction represented below occurs until one of the reactants is entirely consumed. 2 H2(g) + O2(g) 2H2O(g) (d) Give the mole fraction of all species present in the flask at the end of the reaction. 2 H2(g) + O2(g) 2 H2O(g) I 0 C 2(+ ) E 0 total moles after reaction = molH2 + molH2O + molAr = mol + mol + mol = mol total mol fractionH2 = mol fractionO2 = 0 (not necessary) mol fractionAr = mol fractionH2O = 1 point earned for mol H2O 1 point earned for total moles 1 point earned for any two mol fractions, excluding O2 AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board.

9 All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board. 7 Question 3 10 points 3. Nitrogen monoxide, NO(g), and carbon monoxide, CO(g), are air pollutants generated by automobiles. It has been proposed that under suitable conditions these two gases could react to form N2(g) and CO2(g), which are components of unpolluted air. (a) Write a balanced equation for the reaction described above. Indicate whether the carbon in CO is oxidized or whether it is reduced in the reaction. Justify your answer. 2 NO(g) + 2 CO(g) N2(g) + 2 CO2(g) CO is oxidized. Carbon in CO has an oxidation number of +2 and in CO2 carbon has an oxidation number of +4. The oxidation number increases OR The number of oxygen atoms around the carbon atom increases; this is characteristic of oxidation. 1 point earned for the balanced reaction 1 point earned for the prediction and the explanation.

10 (b) Write the expression for the equilibrium constant, Kp , for the reaction. Kp = 2CO2NO22CO2N)()()()(PPPP = 22222[CO][NO]]][CO[N(RT) 1 1 point earned for the equilibrium expression according to part (a) (c) Consider the following thermodynamic data. NO CO CO2 Gfo (kJ mol 1) + (i) Calculate the value of G for the reaction at 298 K. G rxn = Gf (products) + Gf (reactants) = 2 Gf (CO2) [2 Gf (NO) + 2 Gf (CO)] = 2( kJ) [2( kJ) + 2( kJ)] = kJ [+ kJ + ( kJ)] = kJ [ kJ] = kJ (units not required) 1 point earned for the correct coefficients 1 point earned for the answer, sign, and noting that it is products minus reactants AP Chemistry 2002 Scoring Guidelines (Form B) Copyright 2002 by College Entrance Examination Board. All rights reserved. advanced placement Program and AP are registered trademarks of the College Entrance Examination Board.