AP* Buffer Equilibrium Free Response Questions KEY. Essay Questions

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1 Page 1 AP* Buffer Equilibrium Free Response Questions KEY Essay Questions 1972 Since ammonium chloride is a salt of a weak base, the weak base is needed, ammonia, NH3. (a) When moderate amounts of a strong acid, H+, are added, the ammonia reacts with it. The concentration of the hydrogen ion remains essentially the same and therefore only a very small change in ph. NH 3 + H + <> NH 4 + (b) When moderate amounts of a strong base, OH-, are added, the ammonium ion reacts with it. The concentration of the hydrogen ion remains essentially the same and therefore only a very small change in ph. NH OH - <> NH 3 + H 2 O (c) By diluting with water the relative concentration ratio of [NH 4 + ]/[NH 3 ] does not change, therefore there should be no change in ph (a) three points A buffer solution resists changes in ph upon the addition of an acid or base. Preparation: Mix a weak acid + a salt of a weak acid. Or mix a weak base + a salt of a weak base. Or mix a weak acid with about half as many moles of strong base. Or mix a weak base with about half as many moles of strong acid. Or mix a weak acid and a weak base. (b) five points Carla has the correct procedure. She has mixed a weak base, NH3, with the salt of a weak base, NH4Cl. Archie has buffer solution but it has a ph around 5. Beula does not have a buffer solution, since her solution consists of a strong acid and a salt of a weak base. Dexter does not have a buffer solution, since his solution consists of a weak base plus a strong base Average score 1.76 a) two points The sharp vertical rise in the ph on the ph-volume curve appears at the equivalence point (about 23 ml). Because the acid is monoprotic, the number of moles of acid equals the number of moles of NaOH. That number is the product of the exact volume and the molarity of the NaOH. The molarity of the acid is the number of moles of the acid divided by L, the volume of the acid. b) two points (1) AP is a registered trademark of the College Board. The College Board was not involved in the production of and does not endorse this product. (2) Test Questions are Copyright by College Entrance Examination Board, Princeton, NJ. All rights reserved. For face-to-face teaching purposes, classroom teachers are permitted to reproduce the questions. Web or Mass distribution prohibited.

2 Page 2 At the half-equivalence point (where the volume of the base added is exactly half its volume at the equivalence point), the concentration [HX] of the weak acid equals the concentration [X ] of its anion. Thus, in the equilibrium expression, [H + ] [X ] / [HX] K a. Therefore, ph at the half-equivalence point equals pk a c) one point Cresol red is the best indicator because its pk a (about 8) appears midway in the steep equivalence region. This insures that at the equivalence point the maximum color change for the minimal change in the volume of NaOH added is observed. d) three points

3 Page 3, -,,'- r-c,..jv\ '~. I r-..., 'f " _ :-:-.e ~:.:..::::cr.s.j.:".': -:.:'r.5~'::::.s fer :~e ::sscc::.::or: ot :.'1:-:: 2;::e:-e:;: JC:cs a;""~ ;~\ e:-: :e:.;~,y. HC0 3 - ~ H CO J : 1 H:PO~- ~ H-- -;- HPO J HSO~- ~ H-- + SO~1- K a 4.2 x 10-i K a 6.1 x 10-8 K a 1.3 x 10-1 (a) (b) (c) (d) From the systems above, identify the conjugate pair that is best for pre?:j.ring a burter with a ph of i.2. Explain your choic~. ; Explain brieny how you would prepare the butte:- solution described in (a) ""ith the conjugate pair you have! chosen. 1 If the concentr:ltions of both the acid and the conjugate base you have chosen were doubled, how would the i. ph be aitected? Explain how the capacity of the butte:- is affe':ted by this change in conce:1trations of acid i and base. E.'(plain briefly how you could pre?are the butter solution in (a) if you had available the solid salt of only one member of the conjugate pair and solutions of a strong acid and a strong base ' '". -. 1,~~) :~_~_ei-~i -~~Cl S-p~--:-~ f-i. j~6; -t+ l' 0, ~ ~--~ _(1) _ :'-? ~ 1. '2 ~ ~H~. p ~o- f'\. ~ ~~ ~L8PO~~~] ~LH~fDy-J..(t) (1).. ~~ ~ ~ C61- ~o~'h,) '} ~~ fo~- ~. pt \~ POl{"-.( o-t..-..~~.~f~) v- Wcv~.J.i ~ V'w"'-. [C\.) W~. H- LO:;-) Uh,) 2.- \ rt L-V- L~) ~ ~ S~ +w ~ ~t-w. w~..~.._ (0,2.-; 'tjr(o; 't'kh )J...~~ ~,i: l' eli).~ UM-- prj ~ to ~v~ ~0--: ~b~'~~h~~ ~ ~~ ~ iv U"'-H~ b1-4-e-

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5 Page 5 AP Chemistry 2000 Scoring Standards Question 8 (8 points) (a) NH 3 (aq) + H + (aq) NH + 4 (aq) or NH 3 (aq) + H 3 O + (aq) NH + 4 (aq) + H 2 O(l) Note: phase designations not required to earn point 1 pt. (b) Sketch of Titration Curve: 3 pts ph Volume of 0.20 M HCl Added 1 st pt. initial ph must be > 7 (calculated ph 11) 2 nd pt. equivalence point occurs at 15.0 ml ± 1 ml of HCl added (equivalence point must be detectable from the shape of the curve or a mark on the curve) 3 rd pt. ph at equivalence point must be < 7 (calculated ph 5). Note: a maximum of 1 point earned for any of the following: - a line without an equivalence point - a random line that goes from high ph to low ph - an upward line with increasing ph (equivalence point MUST be at 15.0 ml) Copyright 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board.

6 Page 6 AP Chemistry 2000 Scoring Standards Question 8 (continued) (c) Methyl Red would be the best choice of indicator, 1 pt. because the pk a for Methyl Red is closest to the ph at the equivalence point. 1 pt. Notes: explanation must agree with equivalence point on graph alternative explanation that titration involves strong acid and weak base (with product an acidic salt) earns the point d) The resulting solution is basic. 1 pt. K b for NH 3 ( ) and K a for NH + 4 ( ) indicate that NH 3 is a stronger base than NH + 4 is an acid or 1 pt. [OH ] K b because of the equimolar and equivolume amounts of ammonium and ammonia cancellation in the buffer ph calculation. Thus 0.05 poh 5 and ph 9 (i.e., recognition of buffer, so that log poh pk b 5 ph 14 poh 9) Copyright 2000 College Entrance Examination Board and Educational Testing Service. All rights reserved. AP is a registered trademark of the College Entrance Examination Board.

7 Page Problems (a) CH 3 COOH(aq) <> H + (aq) + CH 3 COO - (aq) K a [ H + ][ CH 3 COO ] [ CH 3 COOH ] [H + ] [CH 3 COO - ] X [CH 3 COOH] X, X << 2.0, (2.0- X) 2.0 X x X 6.0x10-3 [H + ]; ph -log [H + ] 2.22 (b) 0.1 L x 2.0 mol/l 0.20 mol CH 3 COOH 0.1 L x 1.0 mol/l 0.10 mol NaOH the 0.10 mol of hydroxide neutralizes 0.10 mol CH 3 COOH with 0.10 mol remaining with a concentration of 0.10 mol/0.20 L 0.5 M. This also produces 0.10 mol of acetate ion in 0.20 L, therefore, [CH 3 COO - ] 0.50 M [ H + ][ 0. 50] [ 0. 50] [H + ] 1.8x10-5 ph of 4.74 (c) [CH 3 COOH] o [CH 3 COO - ] o L 0.50 M x 0.14 L M 0.10 L [H + ] o 0.50 M x 0.14 L M the equilibrium will be pushed nearly totally to the left resulting in a decrease of the hydrogen ion by 0.143M. Therefore, the [H + ] eq 0.357M M 0.214M (a) HOCl + H 2 O <> H 3 O + + OCl [ H 3 O + ][ OCl ] X 2 [ HOCl ] ( X ) X [H 3 O + ] 4.0x10-5 M (b) HOCl + H 2 O <> H 3 O + + OCl - [ H 3 O + ][ X ] [ X ] ; X << X [H 3 O + ] 8.0x10-8 M (c) Cl 2 + H 2 O --> HCl + HOCl [HOCl] [HCl] M HCl as principal source of H 3 O + ph -log[h 3 O + ] 2.40

8 Page 8 AP* Acid-Base Buffer Free Response Questions KEY 1982 a) 3 points HCOOH + H 2 O <> H 3 O + + HCOO K a ([H3O + ] [HCOO ]) / [HCOOH] [HCOOH] [H 3 O + ] [HCOO ] Initial: Equil: y y y K a 1.8 x 10 4 (y ( y)) ( y) neglect the +y and the -y [H 3 O + ] y 1.2 x 10 4 ph - log (1.2 x 10 4) 3.92 Alternative Approach: ph pk a + log [A ] / [HA] - log (1.8 x 10 4) + log (0.40 / 0.60) 3.92 b) 1 point The ph remains unchanged because the ratio of the formate and formic acid stays the same c) 3 points Initial amounts of concentrations: 1.00 M HCl x L mole HCl 0.40 M HCOOH x L mole HCOOH 0.60 M HCOO x mole HCOO or 1.00 M HCl x (5.00 ml / 105 ml) M HCl 0.40 M HCOOH x (100 ml / 105 ml) 0.38 M HCOOH 0.60 M HCOO x (100 ml / 105 ml) 0.57 M HCOO Amounts or concentrations after H + reacts with HCOO : mole HCOOH mole HCOO or 0.43 M HCOOH and 0.52 M HCOO [H 3 O + ] 1.8 x 10 4 x ((0.045/vol.) / (0.055/vol.)) 1.5 x 10 4 M d) 2 points liter x 2.00 M HCOOH 1.60 moles HCOOH liter x 4.80 M NaOH 0.96 mole OH 1.60 moles HCOOH mole OH yields 0.64 mole HCOOH and 0.96 mole HCOO [H 3 O + ] 1.8 x 10 4 x (0.64 mole/liter) / (0.96 mole/liter) 1.2 x 10 4 The alternative approach in part (a) can also be used in part (d)

9 Page Average score 4.36 a) two points ph 8.60 [H + ] 1 x M 2.5 x 10 9 M [OH ] (1 x ) / (2.5 x 10 9) M 4.0 x 10 6 M b) three points C 6 H 5 CO 2 + HOH ---> C 6 H 5 CO 2 H + OH K ([C 6 H 5 CO 2 H][OH ]) [C 6 H 5 CO 2 ] ((4.0 x 10 8) (4.0 x 10 6)) ( x 10 6) 1.6 x c) one point C 6 H 5 CO 2 H <> H + + C 6 H 5 CO 2 K a ([H + ][C 6 H 5 CO 2 ]) [C 6 H 5 CO 2 H] ((2.5 x 10 9) (0.10)) (4.0 x 10 6) 6.3 x 10 5 d) two points C 6 H 5 CO 2 H <> C 6 H 5 CO 2 + H ph 2.88 [H + ] 1 x M 1.3 x 10 3 K a ([H + ][C 6 H 5 CO 2 ]) [C 6 H 5 CO 2 H] 6.3 x 10 8 ((1.3 x 10 3) (1.3 x 10 3)) x x 2.8 x 10 2 M Total C 6 H 5 CO 2 H in solution (2.8 x x 10 3) M 2.9 x 10 2 M 1991 a) three points K a ( [H + ] [C 3 H 5 O 2 ] ) [HC 3 H 5 O 2 ] 1.3 x 10 5 x x [H + ] 1.6 x 10 3 b) one point % dissoc. [H + ] [HC 3 H 5 O 2 ] 1.6 x %

10 Page 10 c) two points [H + ] antilog (- 5.20) 6.3 x x 10 5 (6.3 x 10 6) x ([C 3 H 5 O 2 ] [HC 3 H 5 O 2 ] [C 3 H 5 O 2 ] [HC 3 H 5 O 2 ] 1.3 x x An alternate solution for (c) based on the Henderson-Hasselbalch equation. ph pk a + log ([base] [acid]) log ([C 3 H 5 O 2 ] [HC 3 H 5 O 2 ]) log ([C 3 H 5 O 2 ] [HC 3 H 5 O 2 ]) 0.31 [C 3 H 5 O 2 ] [HC 3 H 5 O 2 ] 2.0 d) six points 0.10 L x 0.35 mol/l mol HC 3 H 5 O L x 0.50 mol/l mol C 3 H 5 O mol mol mol HC 3 H 5 O mol mol mol C 3 H 5 O x 10 5 [H + ] x [(0.054 mol/0.1 L) (0.031 mol/0.1 L)] Can use 0.54 and 0.31 instead. [H + ] 7.5 x 10 6 ph 5.13 An alternate solution for (d) based on the Henderson-Hasselbalch equation. use [ ]s or moles of HC 3 H 5 O 2 and C 3 H 5 O 2 ph pk a + log (0.054 / 0.031)

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14 Page 14 AP CHEMISTRY 2001 SCORING GUIDELINES Question 3 (10 points) (a) (0.325 g/2.00 g) 100% 16.2% 1 point (b) g H 2 O mol H 2 O g/mol ( mol H 2 O) (2 mol H /mol H 2 O) mol H ( mol H)(1.008 g/mol H) g H 1 point n CO 2 PV (750 / 760) atm 3.72 L RT -1-1 (0.0821L atm mol K ) (298 K) mol CO 2 1 point (0.150 mol CO 2 )(1 mol C /mol CO 2 ) mol C (0.150 mol C) (12.0 g/mol) 1.80 g C 1 point grams of oxygen 3.00 g (1.80 g g) 1.07 g O 1 point Note: The first point is earned for getting the correct mass of H : the second point is earned for using the Ideal Gas Law and substituting consistent values of P, V, R, and T. The third point is earned for converting moles of CO 2 to moles of C and then grams of C. If the number of moles of CO 2 is calculated incorrectly, but that incorrect value is used correctly, the third point is earned. The fourth point is earned for using the values of H and C to get the mass of oxygen by difference. If one (or both) of the previously determined values is incorrect, but the student uses those incorrect values correctly, the fourth point is still earned. (c) moles OH ( L) (0.102 mol/l) mol OH, 1 point therefore, mol H + molar mass g mol neutralized, therefore mole acid 180. g/mol 1 point Note: The first point is earned for setting up the calculation to determine the number of moles of OH used in the titration; the second point is earned for using the number of moles of OH correctly to get the molar mass. If the number of moles of OH is incorrectly calculated, credit can be earned for this step if the student uses the incorrect value correctly. Copyright 2001 by College Entrance Examination Board. All rights reserved. Advanced Placement Program and AP are registered trademarks of the College Entrance Examination Board. 6

15 Page 15 AP CHEMISTRY 2001 SCORING GUIDELINES Question 3 (cont.) (d) (i) The pk a is equal to the ph halfway to the equivalence point. 1 point At ml of added NaOH, ph 3.44, therefore pk a 3.44 K a Other paths to the correct answer include using the initial data point and the acid equilibrium value, or using the Henderson-Hasselbalch equation (ii) Beyond the end point, there is excess OH, and the [OH ] determines the ph. 1 point Moles of excess OH ( L) (0.100 mol/l) mol OH [OH ] L mol OH M OH poh 1.90 ph point Note: The first point is earned for recognizing that the ph past the end point is determined by the amount of excess OH ions; the second point is earned for the calculations and the final answer. Copyright 2001 by College Entrance Examination Board. All rights reserved. Advanced Placement Program and AP are registered trademarks of the College Entrance Examination Board. 7

16 Page 16 AP CHEMISTRY 2002 SCORING GUIDELINES (Form B) Question 1 10 points H+(aq) + C H O (aq) HC3H5O3(aq) Lactic acid, HC3H5O3, is a monoprotic acid that dissociates in aqueous solution, as represented by the equation above. Lactic acid is 1.66 percent dissociated in 0.50 M HC3H5O3(aq) at 298 K. For parts (a) through (d) below, assume the temperature remains at 298 K. (a) Write the expression for the acid-dissociation constant, Ka, for lactic acid and calculate its value. [H + ][C 3 H 5 O 3 ] [HC 3 H 5 O 3 ] _ Ka 1 point earned for equilibrium expression 1 point earned for amount of HC3H5O3 dissociating 0.50 M M x HC3H5O3(aq) H+(aq) + C3H5O3 (aq) I 0.50 ~0 0 C x +x +x E 0.50 x +x +x [H + ][C3 H 5 O 3 ] [0.0083][ ] Ka [ ] [HC 3 H 5 O 3 ] 1 point earned for [H+] [C3H5O3 ] set up and solution Ka (b) Calculate the ph of 0.50 M HC3H5O3. From part (a): 1 point earned for correctly calculating ph [H+] M ph log [H+] log (0.0083) 2.08 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

17 Page 17 AP CHEMISTRY 2002 SCORING GUIDELINES (Form B) Question 1 (cont d.) (c) Calculate the ph of a solution formed by dissolving mole of solid sodium lactate, NaC 3 H 5 O 3, in 250. ml of 0.50 M HC 3 H 5 O 3. Assume that volume change is negligible mol NaC3H5O L 0.18 M C 3 H 5 O 3 HC 3 H 5 O 3 (aq) H+ (aq) + C 3 H 5 O 3 (aq) I 0.50 ~ C x +x +x E 0.50 x +x x 1 point earned for [C 3 H 5 O 3 ] (or L 0.50 mol/l mol HC 3 H 5 O 3 and mol C 3 H 5 O 3 ) K a [H + ][C [HC 3 3 H H 5 5 O O 3 3 ] ] [x][ x] [0.50 x] Assume that x << 0.18 M K a [x][0.18] [0.50] x M [H + ] 1 point earned for [H + ] (set up and calculation) 1 point earned for calculating the value of ph ph log [H + ] log ( ) 3.41 OR ph pk a + log 0.18 or 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

18 Page 18 AP CHEMISTRY 2002 SCORING GUIDELINES (Form B) Question 1 (cont d.) (d) A 100. ml sample of 0.10 M HCl is added to 100. ml of 0.50 M HC 3 H 5 O 3. Calculate the molar concentration of lactate ion, C 3 H 5 O 3, in the resulting solution M HC 3 H 5 O ml 200 ml 0.25 M HC 3 H 5 O M HCl 100 ml 200 ml M H+ HC 3 H 5 O 3 (aq) H+ (aq) + C 3 H 5 O 3 (aq) I C x +x + x E 0.25 x x + x + [H ][C3H5O3 ] [ x][x] K a [HC3H5O3] [0.25 x] Assume x << M K a [0.050][x] [0.25] x M [C 3 H 5 O 3 ] 1 point earned for initial [H + ] and [HC 3 H 5 O 3 ] OR (10 mmol H + ; 50 mmol HC 3 H 5 O 3 ) 1 point earned for showing dilution or moles of each 1 point earned for [C 3 H 5 O 3 ] setup and calculation 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

19 Page 19 AP CHEMISTRY 2003 SCORING GUIDELINES Question 1 C 6 H 5 NH 2 (aq) + H 2 O(l) C 6 H 5 NH 3 + (aq) + OH (aq) 1. Aniline, a weak base, reacts with water according to the reaction represented above. (a) Write the equilibrium expression, K b, for the reaction represented above. K b [C 6 H 5 NH 3 + ][OH ] [C 6 H 5 NH 2 ] 1 point for correct expression (b) A sample of aniline is dissolved in water to produce 25.0 ml of a 0.10 M solution. The ph of the solution is Calculate the equilibrium constant, K b, for this reaction. ph 8.82 poh [OH ] M [C 6 H 5 NH 3 + ] [OH ] M 1 point for calculation of [OH ] 1 point for [C 6 H 5 NH 3 + ] [OH ] K b [C 6 H 5 NH 3 + ][OH ] [C 6 H 5 NH 2 ] ( ) K b point for calculation of K b Note: Following this point, any value of K b obtained must be carried through. Copyright 2003 by College Entrance Examination Board. All rights reserved. Available at apcentral.collegeboard.com. 2

20 Page 20 AP CHEMISTRY 2003 SCORING GUIDELINES Question 1 (cont d.) (c) The solution prepared in part (b) is titrated with 0.10 M HCl. Calculate the ph of the solution when 5.0 ml of the acid has been added. n C6 H 5 NH L 0.10 mol 1 L mol C 6 H 5 NH 2 n HCl L 0.10 mol 1 L mol HCl (or H + ) C 6 H 5 NH 2 (aq) + H + (aq) C 6 H 5 NH + 3 (aq) I mol mol 0 mol C E C 6 H 5 NH 2 (aq) + H 2 O(l) C 6 H 5 NH 3 + (aq) + OH (aq) mol L M mol L M I ~0 C x +x +x E x x x 1 point for initial number of moles or molarities of C 6 H 5 NH 2 and HCl/H + 1 point for final number of moles or molarities of C 6 H 5 NH 2 and C 6 H 5 NH 3 + after mixing K b [C 6 H 5 NH 3 + ][OH ] [C 6 H 5 NH 2 ] ( x)(x) ( x) assume that x << M : (0.0167)(x) x [OH ] M poh log ( ) 8.76 ph OR poh pk b + log [C 6 H 5 NH 3 + ] [C 6 H 5 NH 2 ] poh log ( ) + log poh log point for ph poh ( 0.60) 8.76 ph Copyright 2003 by College Entrance Examination Board. All rights reserved. Available at apcentral.collegeboard.com. 3

21 Page 21 AP CHEMISTRY 2003 SCORING GUIDELINES Question 1 (cont d.) (d) Calculate the ph at the equivalence point of the titration in part (c). At the equivalence point, moles of C 6 H 5 NH 2 moles of H + C 6 H 5 NH 2 (aq) + H + (aq) C 6 H 5 NH 3 + (aq) I mol mol 0 mol C E Need 25 ml of 0.1 M HCl to reach the equivalence point of this titration. The total volume of the solution is 50.0 ml [C 6 H 5 NH mol 3 ] L M 1 point for number of moles or molarity of C 6 H 5 NH 3 + C 6 H 5 NH 3 + (aq) C 6 H 5 NH 2 (aq) + H + (aq) I M 0 0 C x + x + x E x x x K a K w K b [C 6 H 5 NH 2 ][H + ] [C 6 H 5 NH ] 5 (x)(x) (0.050 x) assume that x << M : (x)(x) x [H + ] M ph point for ph (e) The pk a values for several indicators are given below. Which of the indicators listed is most suitable for this titration? Justify your answer. Indicator pk a Erythrosine 3 Litmus 7 Thymolphthalein 10 The ph at the equivalence point is acidic. The best indicator is erythrosine, for which the value of pk a is closest to the ph at the equivalence point. Copyright 2003 by College Entrance Examination Board. All rights reserved. Available at apcentral.collegeboard.com. 4 1 point for correct indicator and justification

22 Page 22 AP CHEMISTRY 2005 SCORING GUIDELINES Question 1 HC 3 H 5 O 2 (aq) C 3 H 5 O 2 (aq) + H + (aq) K a Propanoic acid, HC 3 H 5 O 2, ionizes in water according to the equation above. (a) Write the equilibrium-constant expression for the reaction. K a + [H ][C 3H5O 2 ] [HC H O ] Notes: Correct expression without K a earns 1 point. Entering the value of K a is acceptable. Charges must be correct to earn 1 point. One point is earned for the correct equilibrium expression. (b) Calculate the ph of a M solution of propanoic acid. HC 3 H 5 O 2 (aq) C 3 H 5 O 2 (aq) + H + (aq) I ~0 C x +x +x E x +x +x K a + [H ][C3H5O 2 ] [HC H O ] ( x)( x) (0.265 x) Assume that x 0.265, then x ( )(0.265) x x 2 x [H + ] M ph log [H + ] log ( ) ph One point is earned for recognizing that [H + ] and [C 3 H 5 O 2 ] have the same value in the equilibrium expression. One point is earned for calculating [H + ]. One point is earned for calculating the correct ph. Copyright 2005 by College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 2

23 Page 23 AP CHEMISTRY 2005 SCORING GUIDELINES Question 1 (continued) (c) A g sample of sodium propanoate, NaC 3 H 5 O 2, is added to a 50.0 ml sample of a M solution of propanoic acid. Assuming that no change in the volume of the solution occurs, calculate each of the following. (i) The concentration of the propanoate ion, C 3 H 5 O 2 (aq) in the solution 1molNaC3H5O2 mol NaC 3 H 5 O g NaC 3 H 5 O g NaC H O mol NaC 3 H 5 O mol NaC 3 H 5 O 2 mol C 3 H 5 O 2 One point is earned for calculating the number of moles of NaC 3 H 5 O 2. [C 3 H 5 O mol C3H5O2 2 ] volume of solution mol C H O L M One point is earned for the molarity of the solution. (ii) The concentration of the H + (aq) ion in the solution HC 3 H 5 O 2 (aq) C 3 H 5 O 2 (aq) + H + (aq) I ~0 C x +x +x E x x +x K a + [H ][C3H5O 2 ] [HC3H5O 2] ( x)( x) (0.265 x) Assume that x and x K a ( x )(0.103) x [H + ] ( ) M One point is earned for calculating the value of [H + ]. The methanoate ion, HCO 2 (aq), reacts with water to form methanoic acid and hydroxide ion, as shown in the following equation. HCO 2 (aq) + H 2 O(l) HCO 2 H(aq) + OH (aq) (d) Given that [OH ] is M in a M solution of sodium methanoate, calculate each of the following. Copyright 2005 by College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 3

24 Page 24 AP CHEMISTRY 2005 SCORING GUIDELINES Question 1 (continued) (i) The value of K b for the methanoate ion, HCO 2 (aq) HCO 2 (aq) + H 2 O(l) HCO 2 H + OH (aq) I ~0 C x - +x +x E x - +x +x x [OH ] M [OH ][HCO2H] K b [HCO ] 2 ( x)( x) (0.309 x) 6 2 ( ) (0.309 x) x is very small ( M), therefore x K b 6 2 ( ) One point is earned for substituting for both [OH ] and [HCO 2 H], and for calculating the value of K b. (ii) The value of K a for methanoic acid, HCO 2 H K w K a K b K a 14 Kw K b K a One point is earned for calculating a value of K a from the value of K b determined in part (d)(i). (e) Which acid is stronger, propanoic acid or methanoic acid? Justify your answer. K a for propanoic acid is , and K a for methanoic acid is For acids, the larger the value of K a, the greater the strength; therefore methanoic acid is the stronger acid because > One point is earned for the correct choice and explanation based on the K a calculated for methanoic acid in part (d)(ii). Copyright 2005 by College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 4

25 Page 25 AP CHEMISTRY 2006 SCORING GUIDELINES (Form B) Question 1 C 6 H 5 COOH(s) C 6 H 5 COO (aq) + H + (aq) K a Benzoic acid, C 6 H 5 COOH, dissociates in water as shown in the equation above. A 25.0 ml sample of an aqueous solution of pure benzoic acid is titrated using standardized M NaOH. (a) After addition of 15.0 ml of the M NaOH, the ph of the resulting solution is Calculate each of the following. (i) [H + ] in the solution [H + ] M M One point is earned for the correct answer. (ii) [OH ] in the solution [OH K ] w [H + ] M M M One point is earned for the correct answer. (iii) The number of moles of NaOH added mol OH L mol L mol One point is earned for the correct answer. (iv) The number of moles of C 6 H 5 COO (aq) in the solution mol OH added mol C 6 H 5 COO (aq) generated, thus mol C 6 H 5 COO (aq) in solution mol One point is earned for the correct answer. (v) The number of moles of C 6 H 5 COOH in the solution + - [H ][C6H5COO ] K a [C H COOH] 6 5 [C 6 H 5 COOH] + - [H ][C6H5COO ] [C 6 H 5 COOH] mol ( M) L K a M thus, mol C 6 H 5 COOH (0.040 L)( M) mol One point is earned for the correct molarity. One point is earned for the correct answer The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for students and parents). 2

26 Page 26 Alternative solution for part (a)(v): ph pk a + log AP CHEMISTRY 2006 SCORING GUIDELINES (Form B) [C6H5COO ] [C H COOH] 6 5 Question 1 (continued) ph pk a log [C 6 H 5 COO ] log [C 6 H 5 COOH] log [C 6 H 5 COOH] log [C 6 H 5 COO ] ( ph pk a ) log ( mol L ) ( ) [C 6 H 5 COOH] M thus, mol C 6 H 5 COOH ( L )( M ) mol (b) State whether the solution at the equivalence point of the titration is acidic, basic, or neutral. Explain your reasoning. At the equivalence point the solution is basic due to the presence of C 6 H 5 COO (the conjugate base of the weak acid) that hydrolyzes to produce a basic solution as represented below. C 6 H 5 COO + H 2 O C 6 H 5 COOH + OH One point is earned for the prediction and the explanation. In a different titration, a g sample of a mixture of solid C 6 H 5 COOH and solid NaCl is dissolved in water and titrated with M NaOH. The equivalence point is reached when ml of the base solution is added. (c) Calculate each of the following. (i) The mass, in grams, of benzoic acid in the solid sample mol C 6 H 5 COOH ( L) (0.150 mol OH L 1 1 mol C6H5COOH ) 1 mol OH mol C 6 H 5 COOH mass C 6 H 5 COOH g C6H5COOH mol C 6 H 5 COOH 1 mol C6H5COOH g C 6 H 5 COOH One point is earned for the correct answer The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for students and parents). 3

27 Page 27 AP CHEMISTRY 2006 SCORING GUIDELINES (Form B) Question 1 (continued) (ii) The mass percentage of benzoic acid in the solid sample mass % C 6 H 5 COOH g C6H5COOH g 60.2% One point is earned for the correct answer The College Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for students and parents). 4