Faculty of Natural and Agricultural Sciences Chemistry Department Semester Test 1 MEMO Analytical Chemistry CMY 283 Date: 5 September 2016 Lecturers : Prof P Forbes, Dr Laurens, Mr SA Nsibande Time: 90 min Marks: 100 Pages: 5 Instructions: Answer all the questions. All answers (calculations, sketches, and diagrams) must be given in black or blue ink. All calculations must be shown in full. A periodic table is provided on page 5. 1
QUESTION 1 5 MARKS The thermodynamic solubility product Ksp for AgCl(s) is 1 10-10 at 25 C. Would you expect the concentration solubility product, K sp, at an ionic strength µ = 0.5 (obtained by an addition of 0.5 mol/l NaNO3) and at 25 C, to be the same as the thermodynamic solubility product? Explain your answer and show the relationship between the two constants. QUESTION 2 12 MARKS Calculate the solubility of Ag2C2O4 in a solution that has been buffered to ph 9. QUESTION 3 3 MARKS Diluted NaOH is introduced into a solution that is 0.050 M in Cu 2+ and 0.040 M in Mn 2+. What is the concentration of the cation forming the less soluble hydroxide when the more soluble hydroxide begins to form? QUESTION 4 3 MARKS What is the [Y 4- ] of a 0.0345 M EDTA solution buffered to a ph of 6.00? QUESTION 5 4 MARKS A redox titration is performed using 0.0500 M Sn 2+ titrant, for a 50.00 ml sample containing 0.100 M Fe 3+.What is the potential of the system after addition of 60.00 ml of titrant? QUESTION 6 8 MARKS 50.00 ml of 0.0500 M NaCN is titrated with 0.100 M HCl. a) What is the relevant chemical reaction equation? (1) b) What is the initial ph? (4) c) What is the ph after 12.5 ml of titrant has been added? (3) 2
QUESTION 7 5 MARKS The waste water from an industrial process contains cyanide and chloride ions, which need to be quantified by law. Could argentometric titrations be used for this purpose? Discuss and explain your answer and include a sketch showing the relevant titration curve to support your answer QUESTION 8 Ex 18-3 p 466 5 MARKS Calculate the electrode potential of a silver electrode immersed in a 0.0300 M solution of NaCl. E o Ag + /Ag=0.799 V, Ksp AgCl=1.82X10-10 This question is a copy of Ex 18-3 in the textbook. E = 0.312 V QUESTION 9 Ex 9-3 p 208 14 MARKS What mass (in grams) of Barium iodate can be dissolved in 350 ml of water at 25 o C? Ksp of barium iodate = 1.57X10-9 This question is a copy of Ex 9-3 in the textbook. Mass Ba(IO 3) 2 = 0.125 g QUESTION 10 Ex 9-7 p215 9 MARKS Calculate the hydronium ion concentration in 0.250 M nitrous acid. (K a for nitrous acid = 7.1X10-4 ) This question is a copy of Ex 9-7 in the textbook. [H 3O] + =0.0133 M 3
QUESTION 11 Ex 9-11 p211 14 MARKS What is the ph of a solution that is 0.250 M in formic acid and 1.15 M in sodium formate? (K a for formic acid = 1.80X10-4 ) This question is a copy of Ex 9-11 in the textbook. ph = 4.57 QUESTION 12 7 MARKS a) Name three sources of systematic errors? (3) Instrumental errors, Method errors, Personal errors b) Name four methods of detecting a systematic error? (4) Analysis of standard samples, Independent analysis, Blank determinations, Variation in sample size. QUESTION 13 Ex 6-8 13 MARKS A 3.579364-g sample of a solid mixture containing benzoic acid (C6H5COOH) was dissolved and titrated with base to a phenolphthalein endpoint. The acid consumed 44.25 ml of 0.2328 M NaOH. Calculate: a) The percent benzoic acid in the sample? b) The standard deviation of the result if: The position of the burette can be estimated to +/-0.03 ml, A four-decimal analytical balance used that can be read at +/- 0.0002 g The standard deviation of the NaOH reagent solution is +/- 0.0001 M The standard deviation in the molecular mass of benzoic acid is negligible. This question is a copy of Ex 6-8 in the textbook. %Benzoic Acid = (33.15 ± 0.03) % 4
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Data Ksp Cu(OH)2 = 4.8 10-20 Ksp Mn(OH)2 = 2 10-13 Ka1 H2C2O4 = 5.60 10-2 Ka2 H2C2O4 = 5.42 10-5 Ksp Ag2C2O4 = 3.5 10-11 E E o 0.0592 log n c d C D A a B b 6
STANDARD ELECTRODE POTENTIALS E 0 (Sn 4+ /Sn 2+ ) = +0.154 V E 0 (Fe(CN) 3-6 /Fe(CN) 4-6 ) = +0.360 V E 0 (Fe 3+ /Fe 2+ ) = +0.771 V E 0 (Tl 3+ /Tl + ) = +1.250 V E 0 (Tl + /Tl) = -0.336 V TABLE OF DISSOCIATION CONSTANTS Acid Ka (25C) Acid Ka (25C) Base Kb (25C) H 3PO 4 7.1 x 10 3 H 2S 9.5 10 8 NH 2CONH 2 1.3 10 14 HNO 2 7.1 x 10 4 HC 3H 5O 3 1.4 10 4 C 5H 5N 1.7 10 9 H 2CO 3 4.5 x 10 7 HS 1.0 10 19 NH 2OH 1.1 10 8 CH 3COOH 1.8 x 10-5 Co(H 2O) 3+ 6 1.0 10 5 NH 3 1.8 10 5 HClO 3.0 x 10 8 Fe(H 2O) 3+ 6 2.0 10 3 CH 3NH 2 4.4 10 4 C 6H 5COOH 6.3 x 10 5 Al(H 2O) 3+ 6 1.4 10 5 2 CO 3 1.8 10 4 HCN 6.2 10 10 (CH 3) 2NH 5.8 10 4 HF 7.2 10 4 C 6H 5NH 2 6.4 10 4 TABLE OF SOLUBILITY PRODUCT CONSTANTS Compound Ksp (25C) Compound Ksp (25C) AgBr 5.0 x 10 13 AgIO 3 3.1 x 10 8 Ag 2CO 3 8.1 x 10 12 AgI 8.3 x 10 17 AgCl 1.82 x 10 10 Ag 2S 8 10 51 AgCN 2.2 x 10-16 AgSCN 1.1 10 12 STANDARD AND FORMAL ELECTRODE POTENTIALS E 0 (V) 2H + + 2e - H2 (g) 0.000 Ce 4+ + e - Ce 3+ +1.44 Fe 2+ + 2e - Fe (s) -0.440 Fe 3+ + e - Fe 2+ +0.771 (0.700 in 1 M HCl) Sn 2+ + 2e - Sn (s) -0.136 (-0.16 in 1 M HClO4) Sn 4+ + 2e - Sn 2+ +0.154 (0.14 in 1 M HCl) UO2 2+ + 4H + + 2e - U 4+ + 2H2O +0.334 V 3+ + e - V 2+ -0.255 VO 2+ + 2H + + e - V 3+ + H2O +0.359 V(OH4) + + 2H + + e - VO 2+ + 3H2O 4 E E o +1.00 (1.02 in 1 M HCl, HClO4) c 0.0592 [ C] [ D] log a n [ A] [ B] K K 1 2 3 4 4 3 2 [ H ] K1[ H ] K1K2[ H ] K1K2K3[ H ] K1K2K3K4 K K d b WHERE K1 = 1.02 X 10-2 ; K2 = 2.14 X 10-3 ; K3 = 6.92 X 10-7 ; K4 = 5.50 X 10-11 NOTE: 4 = 2.25 X 10-5 FOR [H + ] = 1.00 X 10-6 M 7
8 THE PERIODIC TABLE OF THE ELEMENTS 1 H 1.01 2 He 4.00 3 Li 6.94 4 Be 9.01 5 B 10.81 6 C 12.01 7 N 14.01 8 O 16.00 9 F 19.00 10 Ne 20.18 11 Na 22.99 12 Mg 24.31 13 Al 26.98 14 Si 28.09 15 P 30.97 16 S 32.07 17 Cl 35.45 18 Ar 39.95 19 K 39.10 20 Ca 40.01 21 Sc 44.96 22 Ti 47.87 23 V 50.95 24 Cr 52.00 25 Mn 54.94 26 Fe 55.85 27 Co 58.93 28 Ni 58.69 29 Cu 63.55 30 Zn 65.39 31 Ga 69.72 32 Ge 72.61 33 As 74.92 34 Se 78.96 35 Br 79.90 36 Kr 83.80 37 Rb 85.47 38 Sr 87.62 39 Y 88.91 40 Zr 91.22 41 Nb 92.91 42 Mo 95.94 43 Tc 98.91 44 Ru 101.07 45 Rh 102.91 46 Pd 106.42 47 Ag 107.87 48 Cd 112.41 49 In 114.82 50 Sn 118.71 51 Sb 121.76 52 Te 127.60 53 I 126.90 54 Xe 131.29 55 Cs 132.91 56 Ba 137.33 57 La 138.91 72 Hf 178.49 73 Ta 180.95 74 W 183.84 75 Re 186.21 76 Os 190.23 77 Ir 192.22 78 Pt 195.08 79 Au 196.97 80 Hg 200.59 81 Tl 204.38 82 Pb 207.20 83 Bi 208.98 84 Po 208.98 85 At 209.99 86 Rn 222.01 87 Fr 223.02 88 Ra 226.03 89 Ac 227.03 104 Rf 261.11 105 Db 262.11 106 Sg 263.12 107 Bh 262.12 108 Hs 265 109 Mt 266 58 Ce 140.12 59 Pr 140.91 60 Nd 144.24 61 Pm 144.91 62 Sm 150.36 63 Eu 151.97 64 Gd 157.25 65 Tb 158.93 66 Dy 162.50 67 Ho 164.93 68 Er 167.26 69 Tm 168.93 70 Yb 173.94 71 Lu 174.97 90 Th 232.04 91 Pa 231.04 92 U 238.03 93 Np 237.05 94 Pu 244.06 95 Am 243.06 96 Cm 247.07 97 Bk 247.07 98 Cf 251.08 99 Es 252.08 100 Fm 257.10 101 Md 258.10 102 No 259.10 103 Lr 262.11