test_key_2018_ab_applications

AP Chemistry: Test Equilibrium Applications Name k(’t’f Questions 1-2: The graph below shows the titration curve that results when 100. mL of 0.0250 A acetic acid is titrated with 0.100 A/ NaOH. = pH 8.0 W 6.0 -V 4.0 YeF——— o 00 57510 15 20 25 30 35 40 45 mL of 0.100 A NaOH Added 1. (D) Which of the following indicators is the best choice for this titration? pH Range of Indicator Color Change (A) Methyl orange 32- 44 (B) Methyl red 48- 6.0 (C) Bromothymol blue 6.1- 76 (D) Phenolphthalein 82- 10.0 2. (A) What part of the curve corresponds to the optimum buffer action for the acetic acid/acetate ion pair? (A) PointV (B) Point X (C) PointZ (D) Along all of section WY Compound Kgpat298K Agel 1.6 x 10710 AgxCOs 8.1 x 10712 AgCrOy 9.0 x 10712 b0y 1.8x 10718 3. (C) Based on the values in the table above, a saturated solution of which of the following compounds has the highest [Ag+] 2 (A) AgCl (B) Ag:COs (C) Ag:CrOy4 (D) AgsPOq4 g - 12 2. ~12 N / X5 ) bxto N Yxis gdnlo Yx"s 4.0x10 2’ 1. § %00 % - -5 x:[.lfiKID.SM Xz "27Y/04M )(:/.J_/)NoyM x: /.(/XID M Unauthorized copying or reuse of any part of this page is illegal. Go ON TO THE NEXT

Matching A. A solution with a pH less than 7 that is not a buffer solution B. A buffer solution with a pH between 4 and 7 C. A buffer solution with a pH between 7 and 10 D. A solution with a pH greater than 7 that is not a buffer solution E. A solution with a pH of 7 Acid lonization constants: HC,H;0:: K = 1.8x107 H,COs: Ki= 4.0x107 K>=4.0x10" 4.(A) A solution prepared to be initially IM NaCl and 1M HCI 5.(D) A solution prepared to be initially 1M Na,COs and 1M NaC>H;0, 6. (B) A solution prepared to be initially 0.5M HC;H;0, and IM NaC;H;0; 7. (B) Mixtures that would be considered buffers include which of the following? [. 0.10 MHCI+0.10 M NaCl 1. 0.10 M HF +0.10 M NaF 1. 0.10 M HBr+ 0.10 M NaBr (A) Tonly (B) I only (C) Wlonly (D) IlandlIIl 8. (C) In which of the following solutions would CuBr have the highest solubility? (A) 0.ISMKBr commen oo & (B) 0.10MLiBr ¢t¢mmen ron & . - (C) 0.10 M AgNOs /‘19‘ veadls witt Br° —p fl;c/( vemove§ Br~ —> (D) Pure Water 9. (B) A student prepares a buffer by mixing 0.20 mol of HC;H;0 (aq) (Ka = 1.8 x 10-5) with 0.20 mol of KC:H30: (aq). What is the pH after 25 mL of 2.0M NaOH is added? HA o A (A) 4.63 (B) 4.97 ©) 5. o ;D) 5.28 b 2 2% P Ha 4,744 log (&2 AT = 10. (C) In the titration of a weak base of unknown‘concen ratlon w1th a standard solution (of known concentration) of a strong acid, a pH meter was used to follow the progress of the titration. Which of the following is true for this experiment? (A) The pH is 7 at the equivalence point. (B) The pH at the equivalence point depends on the indicator used. (C) The graph of pH versus volume of acid added falls gradually in the buffer range. (D) The pH after the equivalence point is determined by the amount of excess weak base. 11. (A) In a saturated solution of Zn(OH) at 25°C the value of [OH-] is 2.0 x10-6 M. What is the value of the solubility-product constant, Kgp, for Zn(OH)2 at 25°C? (A) 4.0x10"8 (B) 8.0x10'® (€) 1.6x10"7 (D) 4.0 x107'? ksp : [2'\”.] [‘7”-]1' (1.0 xlo"}(l"’ Wfl/l g ox10® " Unauthorized copying or reuse of any part of this page is illegal. GO ON TO THE NEXT

AP® CHEMISTRY 2013 SCORING GUIDELINES Question 1 (10 points) Answer the following questions about the solubility of some fluoride salts of alkaline earth metals. (a) A student prepares 100. mL of a saturated solution of MgF, by adding 0.50 g of solid MgF, to 100. mL of distilled water at 25°C and stirring until no more solid dissolves. (Assume that the volume of the undissolved MgF, is negligibly small.) The saturated solution is analyzed, and it is determined that [F7] in the solution is 2.4 x 1073 M. (i) Write the chemical equation for the dissolving of solid MgF, in water. MgF,(s) 2 Mg*(ag) + 2F(ag) 1 point is earned for the correct equation. (ii) Calculate the number of moles of MgF, that dissolved. 1 point is earned for the correct calculation of moles 1 mol MgF, from concentration. - 24x107mol B o 1091, x = 1.2 x 107* mol MgF, 1LOL 2mol F~ - 1 point is earned for the correct stoichiometry. (iii) Determine the value of the solubility-product constant, K. for MgF, at 25°C. [Mg*] = %[F‘] = %(2,4 %1073 M) =12x 103 M | | point is earned for the correct value of [Mg**] 1 point is earned for the correct setup for 2 E2 = -3 -3,2 X [MEIEFT" = (125 107324 % 107 determining the value of K, p 1 -9 6.9 x 10 1 point is earned for the correct value of K, . © 2013 The College Board. Visit the College Board on the Web: www.collegeboard.org.

AP® CHEMISTRY 2013 SCORING GUIDELINES Question 1 (continued) (b) A beaker contains 500. mL of a solution in which both Ca*(ag) and Ba®*(agq) arc present at a concentration of 0.10 M at 25°C. A student intends to separate the ions by adding 0.20 M NaF solution one drop at a time from a buret. At 25°C the value of K, for CaF, is 3.5 107'"; the value of K, for BaF, is 1.8 x 1076, (i) Which salt will precipitate first, CaF, or BaF, ? Justify your answer. CaF, will precipitate first. Its K, value is smaller, 1 point is earned for the correct choice thus the ion-concentration product [Ca®*][F~)? will be e with its justification. the first to exceed the K, value. For parts (b)(ii) and (b)(iii) below, assume that the addition of the NaF solution does not significantly affect the total volume of the liquid in the beaker. (i) Calculate the minimum concentration of F~(ag) necessary to initiate precipitation of the salt selected in part (b)(i). K, = 35x107" = [Ca*|[F ] = (0.10)[FJ? 3.5% 1070 = [F)? 1 point is earned for the correct value of [F]. [F] = {35x1070 = 1.9x 105 M (iii) Calculate the minimum volume of 0.20 M NaF that must be added to the beaker to initiate precipitation of the salt selected in part (b)(i). Assuming that the volume of added NaF(ag) is negligible, the total volume of the solution at the point of precipitation is 500. mL. (020 M)(V) = (1.9 X 1075 M)(0.500 L) 1 point is earned for the correct volume. V=47x10°L (or4.8x107L) 4.7x 102 mL (or4.8 x 1072 mL) I I (c) There are several ways to dissolve salts that have limited solubility. Describe one procedure to redissolve the precipitate formed in part (b). Valid procedures include adding water, adding acid (H), heating (i.c., increasing the temperature), and any valid 1 point is earned for a description statement that implies a shifting of the equilibrium toward of a valid procedure. the products side of the dissolution equation. © 2013 The College Board. Visit the College Board on the Web: www.collegeboard.org.