SCHOOL OF CHEMISTRY & PHYSICS UNIVERSITY OF KWAZULU-NATAL, WESTVILL LE CAMPUS APCH231 : CHEMICAL ANALYSIS NOVEMBER 2013 MAIN EXAMINATIONN DURATION: 3 HOURS TOTAL MARKS: 100 Moderators Dr D. Reddy Dr B. Moodley Internal Examiners: Dr L. Pillay Dr G. Birungii Instructions: This paper consists of 13 pages (cover page, dataa sheets, and periodic table inclusive). Please check that you have them all. Answer All Questions Please affix your barcode on the front page of each submitted answer booklet. Answer each section (A and B) ) in a separate booklet Students are requested, in their ownn interests to write legibly in black or blue ink. 1
SECTION A Question 1 (15 marks) A student compared the accuracy and precision of delivering 10 ml of water from a burette and pipette. The replicate data for the burette measurements is shown below along with the mean data for the pipette. Replicate masses for burette in grams: 10.08 9.99 9.86 9.90 9.94 10.11 Mean data for the pipette in grams (4 replicates): 9.99 ± 0.012 a. Are there any outliers in the measurements from the burette? (3) b. Taking into account (a) above, calculate the mean and standard deviation of the students burette data. (2) c. Do the masses delivered from the burette and pipette differ significantly at the 95% confidence interval? (4) d. Define precision and explain which of the two methods was more precise? (2) e. Calculate the relative error in the analysis for the pipette. (2) f. The analytical balance used for the mass measurements was calibrated with a mass weighing 988 g instead of the 1000 g mass which is recommended. State what type of error would result and explain if that error would affect the precision and/or accuracy. (2) 2
Question 2 (17 marks) To calculate the concentration of Cu in an ore sample the following method was carried out. - A solution of thiosulfate was made using sodium thiosulfate pentahydrate (Na 2 S 2 O 3 5 H 2 O, MM 248.17 g mol -1 ). - This solution was standardised using pure Cu wire. A mass of 0.4567 g of copper wire was dissolved in a mixture of HCl and H 2 SO 4. Approximately 3.0 g of KI was added to the mixture. The I 2 which formed was then titrated with 23.45 ml of the S 2 O 2-3 solution. The reaction equations are shown below: 2Cu 2+ + 4I - 2CuI(s) + I 2 I 2 + 2S 2 O 2-3 2I - 2- + S 4 O 6 - To calculate the concentration of Cu in the ore sample, a 1.0245 g sample of the ore was dissolved in a mixture of HCl and H 2 SO 4. Approximately 3.0 g of KI was then added to the sample. The iodine produced from the reaction was titrated with 34.64 ml of the standardised thiosulfate solution. a. Explain why it was necessary to standardise the thiosulfate solution. (1) b. Identify the primary standard. (1) c. List two characteristics of a good primary standard. (2) d. Identify the secondary standard. (1) e. Differentiate between an endpoint and an equivalence point. (2) f. What type of titration reaction is being carried out? Give a reason for your answer. (3) 3
g. Starch is the most suitable indicator for this reaction, what type of indicator is starch? (1) h. Calculate the accurate concentration of the standardised thiosulfate solution. (3) i. Calculate the %Cu in the ore. (3) Question 3 (18 marks) The concentration of Fe in the blood of 8 patients was measured using two different methods, one titration and the other spectrophotometric. The results are recorded below. Patient Fe concentration in μg dl -1 Spectrophotometric Titration A 65 60 B 72 69 C 88 84 D 96 89 E 146 139 F 144 150 G 137 129 H 98 93 Average 106 102 Standard deviation 32.4 33.4 a. Calculate if there is a significant difference between the two methods at the 90% confidence interval. (4) 4
b. The average value of Fe in blood for a healthy patient is 100 μg dl -1. Using the spectrophotometric method, comment on the health of the patients as a group (use the 99% confidence interval). (3) c. The spectrophotometric method required standards to be made. A 250 ml stock solution of 300 ppm Fe was made using ammonium iron(ii) sulfate hexahydrate, (NH 4 ) 2 Fe(SO 4 ) 2 6H 2 O. Calculate the mass of the salt required to make this solution. (3) d. Two attempts were made at calibration. The calibration curves obtained are shown below. Which calibration curve would you prefer to use. Discuss the reasons for your choice in detail. (3) 600 Calibration A 600 Calibration B intensity 400 200 0 y = 2.7357x + 7.4286 R² = 0.9983 0 50 100 150 200 concentration intensity 400 200 0 y = 2.4158x + 61.526 R² = 0.988 0 50 100 150 200 concentration e. The titration method was tested using an Fe sample with the following reaction: 5Fe 2+ + MnO 4 - + 8H + 5Fe 3+ + Mn 2+ + 4H 2 O A 5.00 ± 0.04 ml aliquot of the sample was be titrated with a 0.0564 ± 0.045 M KMnO 4 solution. The initial burette reading was 0.06 ± 0.02 ml and the final reading 32.21 ± 0.02 ml. Calculate the molarity and the associated error in the unknown Fe sample. (5) 5
SECTION B Question 4 (20 marks) a. In the titration of 25.0 ml of 0.125 M NH 3 (K b = 1.75 10 5 ) with 0.0625 M HCl, what is the equivalence volume? (1) Calculate the ph after addition of the following amounts of HCL i. 0.00 ml (2) ii. 10.00 ml (2) iii. 50.00 ml (2) iv. 70.00 ml (2) b. The figure below shows the titration curve for the mixture of weak acids HA (pk a = 3) and HB (pk a = 7) with NaOH. Use it to answer the questions that follow. i. State what the points A and B represent. (1) ii. Points A is at 38.0 ml and point B is at 50.0 ml. Which of the acids has a higher concentration? Give an explanation for your answer. (2) 6
iii. What do regions X and Y represent? Write down the species which exist in these regions. (2) c. Consider the speciation diagram of aqueous phthalic acid, H 2 P, below: 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Fractional composition of phthalate solution as a function of ph 0 1 2 3 4 5 6 7 8 9 10 ph i. What is the fractional composition of a phthalate solution at ph = 4.1 (2) ii. Estimate the value of K a2 for phthalic acid from the diagram. Give an explanation for your answer. (2) iii. Using your data sheet choose a suitable indicator for the titration of H 2 P with NaOH. Explain your choice in detail. (2) 7
Question 5 (16 Marks) a. For a titration of 100.0 ml of 0.0025 M Pb 2+ with 0.0100 M EDTA at ph 10, calculate ppb at the equivalence point given that log K f = 18.04. Show your working. (6) b. Sketch two titration curves, on a single set of labelled axes, to show the effect of ph on the titration of Pb 2+ with EDTA. (2) c. Explain briefly how a metallochromic indicator works in a titration of a metal ion with EDTA. (2) d. Calcium and magnesium were extracted from 2.3175 g of soil and the extract was made up to a final volume of 250.0 ml. A 25.00 ml portion of the diluted extract was adjusted to ph 10 with a suitable buffer, an indicator was added and the mixture was titrated with 0.0468 M EDTA solution. The titration required 9.72 ml. A second 25.00 ml portion of the diluted extract was made strongly basic to precipitate out the calcium. The remaining solution required 2.47 ml of the 0.0468 M EDTA solution to reach the end point. Calculate the percentage of calcium and magnesium in the soil sample. (6) Question 6 (14 marks) a. In the gravimetric determination of chloride by precipitation of silver chloride, after the precipitate is first formed it is heated for several minutes and then allowed to stand. The precipitate is then filtered and washed with very dilute nitric acid. 8
i. What happens to the silver chloride precipitate when it is heated? Explain clearly. (2) ii. Why is the precipitate washed with dilute nitric acid rather than with pure water? Explain briefly. (2) b. In a titration of 50.0 ml of a mixture of 0.0500 M iodide ion and 0.0800M chloride ion with 0.1000 M silver nitrate. Given that the solubility products are 8.3 10-17 for AgI and 1.8 10-10 for AgCl, calculate: i. The pag after addition of 10.00 ml of silver nitrate (2) ii. The pag after addition of 25.00 ml of silver nitrate (2) iii. Sketch a curve for this titration and on it label the endpoint due to the iodide (2) iv. Calculate the percentage of iodide unprecipitated after addition of 25.00 ml of silver chloride. (4) 9
Data s pooled 2 a a b 2 b s ( Na 1) s ( Nb 1)... N N... Nsets of data x μ t calculated s / N Critical values of F at the 5% Probability Level (95% confidence) Degrees of Freedom (Denominator) Degrees of Freedom (Numerator) 2 3 4 5 6 7 8 9 10 20 2 19.00 19.16 19.25 19.30 19.33 19.35 19.37 19.38 19.40 19.4 3 9.55 9.28 9.12 9.01 8.94 8.89 8.85 8.81 8.79 8.66 4 6.94 6.59 6.39 6.26 6.16 6.09 6.04 6.00 5.96 5.80 5 5.79 5.41 5.19 5.05 4.95 4.88 4.82 4.77 4.74 4.56 6 5.14 4.76 4.53 4.39 4.28 4.21 4.15 4.10 4.06 3.87 7 4.74 4.35 4.12 3.97 3.87 3.79 3.73 3.68 3.64 3.44 8 4.46 4.07 3.84 3.69 3.58 3.50 3.44 3.39 3.35 3.15 9 4.26 3.86 3.63 3.48 3.37 3.29 3.23 3.18 3.14 2.94 10 4.10 3.71 3.48 3.33 3.22 3.14 3.07 3.02 2.98 2.77 15 3.68 3.29 3.06 2.90 2.79 2.71 2.64 2.59 2.54 2.33 20 3.49 3.10 2.87 2.71 2.60 2.51 2.45 2.39 2.35 2.12 25 3.39 2.99 2.76 2.60 2.49 2.40 2.34 2.28 2.24 2.08 26 3.37 2.98 2.74 2.59 2.47 2.39 2.32 2.27 2.22 2.05 27 3.35 2.96 2.73 2.57 2.46 2.37 2.31 2.25 2.20 2.01 28 3.34 2.95 2.71 2.56 2.45 2.36 2.29 2.24 2.19 1.98 29 3.33 2.93 2.70 2.55 2.43 2.35 2.28 2.22 2.18 1.96 30 3.32 2.92 2.69 2.53 2.42 2.33 2.27 2.21 2.16 1.93 10
Critical Values for the t-statistic Confidence level degrees 50% 90% 95% 99% Freedom 1 1.000 6.31 12.71 63.66 2 0.816 2.920 4.303 9.925 3 0.765 2.353 3.182 5.841 4 0.741 2.132 2.776 4.604 5 0.727 2.015 2.571 4.032 6 0.718 1.943 2.447 3.707 7 0.711 1.895 2.365 3.499 8 0.706 1.860 2.306 3.355 9 0.703 1.833 2.262 3.250 10 0.700 1.812 2.228 3.169 11 0.697 1.796 2.201 3.106 12 0.695 1.782 2.179 3.055 13 0.694 1.771 2.160 3.012 14 0.692 1.761 2.145 2.977 15 0.691 1.753 2.131 2.947 16 0.690 1.746 2.120 2.921 17 0.689 1.740 2.110 2.898 18 0.688 1.734 2.101 2.878 19 0.688 1.729 2.093 2.861 20 0.687 1.725 2.086 2.845 21 0.686 1.721 2.080 2.831 22 0.686 1.717 2.074 2.819 23 0.685 1.714 2.069 2.807 24 0.685 1.711 2.064 2.797 25 0.684 1.708 2.060 2.787 30 0.683 1.697 2.042 2.750 35 0.682 1.690 2.030 2.724 40 0.681 1.684 2.021 2.704 45 0.680 1.679 2.014 2.690 50 0.679 1.676 2.009 2.678 55 0.679 1.673 2.004 2.668 60 0.679 1.671 2.000 2.660 65 0.678 1.669 1.997 2.654 70 0.678 1.667 1.994 2.648 0.674 1.645 1.960 2.576 Critical Values for the Q-statistic N 90% Confidence Rejection Quotient 95% Confidence 99% Confidence 3 0.941 0.970 0.994 4 0.765 0.829 0.926 5 0.642 0.710 0.821 6 0.560 0.625 0.740 7 0.507 0.568 0.680 8 0.468 0.526 0.634 9 0.437 0.493 0.598 10 0.412 0.466 0.568 N = number of observations Indicator Name pka Quinaldine Red 2.75 Methyl Orange 3.46 Bromophenol Blue 4.10 Bromocresol Green 4.66 Methyl Red 5.00 Bromothymol Blue 7.10 Phenol Red 7.81 m-nitrophenol 8.15 o-cresolphthalein 8.82 Phenolphthalein 9.7 11
Values of α 4 for EDTA at selected ph values. ph α 4 2.0 3.7 10 14 3.0 2.5 10 11 4.0 3.6 10 9 5.0 3.5 10 7 6.0 2.2 10 5 7.0 4.8 10 4 8.0 5.4 10 3 9.0 5.2 10 2 10.0 3.5 10 1 11.0 8.5 10 1 12.0 9.8 10 1 K a of Weak Acids Name Formula K a acetic HC 2 H 3 O 2 1.8 x 10-5 carbonic (I) H 2 CO 3 4.5 x 10-7 carbonic (II) - HCO 3 4.7 x 10-11 citric (I) H 3 C 6 H 5 O 7 3.2 x 10-7 citric (II) - H 2 C 6 H 5 O 7 1.7 x 10 5 citric (III) 2- HC 6 H 5 O 7 4.1 x 10-7 formic HCHO 2 1.8 x 10-4 hydrazidic HN 3 1.9 x 10-5 hydrocyanic HCN 6.2 x 10-10 hydrofluoric HF 6.3 x 10-4 lactic HC 3 H 5 O 3 8.3 x 10-4 nitrous HNO 2 4.0 x 10-4 oxalic (I) H 2 C 2 O 4 5.8 x 10-2 oxalic (II) - HC 2 O 4 6.5 x 10-5 phenol HOC 6 H 5 1.6 x 10-10 propanic HC 3 H 5 O 2 1.3 x 10-5 sulfurous (I) H 2 SO 3 1.4 x 10-2 sulfurous (II) - HSO 3 6.3 x 10-8 uric HC 5 H 3 N 4 O 3 1.3 x 10-4 For a weak acid H n A the denominator will be... 12
13 1 18 1 H 1.008 2 Periodic Table 13 14 15 16 17 2 He 4.003 3 Li 6.941 4 Be 9.012 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 3 4 5 6 7 8 9 10 11 12 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 40.08 21 Sc 44.96 22 Ti 47.88 23 V 50.94 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 101.1 45 Rh 102.9 46 Pd 106.4 47 Ag 107.9 48 Cd 112.4 49 In 114.8 50 Sn 118.7 51 Sb 121.8 52 Te 127.6 53 I 126.9 54 Xe 131.3 55 Cs 132.9 56 Ba 137.3 57* La 138.9 72 Hf 178.5 73 Ta 180.9 74 W 183.8 75 Re 186.2 76 Os 190.2 77 Ir 192.2 78 Pt 195.1 79 Au 197.0 80 Hg 200.6 81 Tl 204.4 82 Pb 207.2 83 Bi 209.0 84 Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra (226) 89** Ac (227) 104 Db (261) 105 Jl (262) 106 Rf (263) 107 Bh (262) 108 Hn (?) 109 Mt (?) * Lanthanide Series 58 Ce 140.1 59 Pr 140.9 60 Nd 144.2 61 Pm (147) 62 Sm 150.4 63 Eu 152.0 64 Gd 157.2 65 Tb 158.9 66 Dy 162.5 67 Ho 164.9 68 Er 167.3 69 Tm 168.9 70 Yb 173.0 71 Lu 175.0 ** Actinide Series 90 Th (232) 91 Pa (231) 92 U (238) 93 Np (237) 94 Pu (239) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (252) 99 Es (252) 100 Fm (257) 101 Md (256) 102 No (259) 103 Lr (260)