SECTION A. a) The following Gaussian distribution plots depict the analysis of lead in milk carried out by four different laboratories.

Transcription

1 Page Question 1 SECTION A a) The following Gaussian distribution plots depict the analysis of lead in milk carried out by four different laboratories. 7 Laboratory A True Value 7 Laboratory B True Value 5 5 Frequency 3 Frequency [Pb] mg/l [Pb] mg/l 7 Laboratory C True Value 8 7 Laboratory D True Value 5 Frequency 3 Frequency [Pb] mg/l [Pb] mg/l Compare the performance of the four laboratories in terms of precision and accuracy. () b) A.8 ±.1 g sample of a solid mixture containing potassium hydrogen phthalate (KHP) (. g/mol) was dissolved and titrated with NaOH.55 ±.5 mol/l to a phenolphthalein endpoint. The amount of base required to reach the endpoint was 8.3 ±.3 ml. Calculate the percent KHP in the sample and the absolute uncertainty. () (Assume no uncertainty in the molar mass of KHP).

2 Page 3 Question 1 (Continued) c) A stainless steel alloy that contained a certified 5.38% chromium was analysed to check if there was a bias in an analytical method for the determination of chromium in steel. The following data were obtained: 5., 5.3, 5., 5.38 and 5.3% Cr. i) What is the: - average - standard deviation - relative standard deviation of the replicate analysis? (3) ii) What is the absolute and the % relative error? () iii) Calculate the 95% confidence interval. () iv) Is there a bias in this analytical procedure? Justify your answer. () v) Will any bias affect the precision of these measurements? Briefly justify your answer. (1) vi) How many measurements would be required to obtain a mean that is four times more reliable? () [] Question a) An analytical protocol exhibits a 95% confidence interval of ±.. If a 9% limit of ±. is required by the government laboratory, could this protocol still be used? Briefly justify your answer. ()

3 Page Question (Continued) b) In the analysis of phosphoric acid by titration with NaOH, three different indicators were used to detect the endpoint. The following data was obtained for the concentration of phosphoric acid by the students of APCH31. Bromocresol Green Cresol Purple Thymolphthalein Median.111 mol/l.111 mol/l.113 mol/l Mean.119 mol/l.111 mol/l.1131 mol/l n s 5. x 1-3 mol/l 8.11 x 1 - mol/l.39 x 1 - mol/l i) Is there a significant difference at the 95% confidence level between the mean value obtained for cresol purple and thymolphthalein? Justify your answer. (3) ii) iii) iv) Which indicator proved to be the easiest endpoint to detect? Briefly justify your answer. (1) Which indicator proved to be the hardest endpoint to detect? Briefly justify your answer. (1) Is there a significant difference in precision between the indicators selected in part ii) and iii) above? Justify your answer. (3) v) From the bromocresol green data given in the table above, there is a difference between the median and mean value for the concentration of the phosphoric acid. What does this difference suggest about the data? ()

4 Page 5 Question (Continued) c) A method for the determination of chloride tested on pure NaCl gave the following results. NaCl in the sample (g) Error in Analysis Expected (g) Actual (g) Absolute difference (g) % relative difference % % % % i) What type of systematic error does the above data suggest for the gravimetric analysis? Give your reasoning () ii) If you wanted to correct the data to give values that gave a better estimate of the true value, what correction factor would you use? () iii) After this correction factor is applied, what is the remaining type of error? (1) [17]

5 Page Question 3 a) Lithium carbonate (Li CO 3 ) can be used as a standard material to prepare standard solutions of Li + ions. i) Describe in detail how you would prepare 1. L of a 1. x 1 3 mg/l lithium solution from a solid sample of lithium carbonate (73.89 g/mol). Include relevant calculations. (3) ii) From the above stock solution, describe how you would prepare the following set of aqueous standards: 1. x 1-1, 1., 1. x 1 1 and 1. x 1 mg/l Li +. Give precise experimental procedures, including pipettes used. () b) The titration curves and corresponding alpha plots were derived from the compounds listed below. Titration curves are derived from the addition of.1 mol/l NaOH to 5. ml aliquots of.1 mol/l solutions. Mandelic Acid C8H 8 O 3 pka1 = 3.1 Maleic acid C H O pk a1 = 1.97 pk a =.7 Fumaric acid C H O pk a1 = 3.5 pk a =.9 Citric acid C H 8 O 7 pk a1 = 3.13 pk a =.7 pk a3 =. Match the compound with the alpha plot and the corresponding titration curve. Explain fully your reasons for your choices. (8) [13]

6 Page 7 A B α α C D α. α E F volume base volume base G H volume base /ml volume base

7 Page 8 Question SECTION B a) A.77 g copper containing sample is dissolved and reacted with an excess of iodide. The liberated iodine is titrated with.7 ml of a.13 M standard thiosulphate solution to reach the endpoint. The relevant reactions are: I - + Cu + CuI + I I + S O 3 - S O - + I - i) Is sodium thiosulphate used as a primary or secondary standard in this procedure? Explain briefly. () ii) Calculate the percentage of Cu in the sample. () b) A.5 g sample of acetic acid (K a = 1.75 x 1-5 ) was dissolved in 9 ml of water. This was followed by the addition of 1 ml of a 1. M NaOH to this solution. Calculate the of the solution after the addition of NaOH. ()

8 Page 9 Question (Continued) c) Fumaric acid, HOOCCH=CHCOOH, (H M) is a diprotic acid that undergoes the following acid dissociation reactions: H M HM - + H + HM - M - + H + Fractional composition of fumaric acid solution as a function of i) At.5, 79% of the fumaric acid remains undissociated. What is the value of α at this? (1) ii) Calculate the equilibrium concentrations of the different species in a.1 M fumaric acid solution at.. For fumaric acid, pka 1 = 3.53 and pka =.9 (5)

9 Page 1 Question (Continued) iii) Which of the indicators shown in the table below would be suitable for the titration of fumaric acid with sodium hydroxide and what would be the colour at the endpoint? Explain briefly. () Indicator range Acid Colour Base Colour Methyl orange Red Orange Bromocresol green Yellow Blue Bromothymol blue.-7. Yellow Blue Phenolphthalein Colourless Pink d) You are required to make 5 ml of a buffer solution with = 5. and a total acetate concentration of.1 M. You are supplied with solid sodium actetate (CH 3 COONa) and. M hydrochloric acid. What mass of sodium acetate and what volume of hydrochloric acid would you use? Show all working. (For CH 3 COOH, K a = 1.75 x 1-5 ) (7) [5] Question 5 a) Consider the titration of 5. ml of.5 M Mg + with.5 M EDTA at 1, using Eriochrome black T as the indicator. Calculate the value of pmg after the addition of 1. ml of EDTA solution. Show all working. () b) The Eriochrome black T indicator forms complexes of different stabilities with many different metal ions. What problem will arise if the metal-indicator complex is much more stable than the metal-edta complex? (1)

10 Page 11 Question 5 (Continued) c) Explain, with the aid of titration curves, the effect of in EDTA titrations. (3) d) Discuss briefly, the two major factors that affect the sharpness of the endpoint in an EDTA titration. (3) e) Chloride in a brine solution is determined by a precipitation titration. A 1. ml aliquot of the solution is titrated with 15.mL of standard.118 M AgNO 3 solution. The excess silver is titrated with standard.11 M KSCN solution, requiring.38 ml to reach the red Fe(SCN) + endpoint. Calculate the concentration of chloride in the brine solution, in g/1ml. () f) Explain clearly the mechanism by which an adsorption indicator works in a precipitation titration. (3)

11 Page 1 Question 5 (Continued) g) Consider the two titration curves, Curve A and Curve B for a precipitation titration. Explain briefly, which curve represents the more concentrated reagents? () h) In gravimetric analysis, it is desirable to obtain precipitates with a large particle size. i) Why are precipitates with a large particle size favoured? (1) ii) Name two factors that determine the particle size of precipitates? (1)

12 Page 13 Question 5 (Continued) i) In a gravimetric determination, nickel can be precipitated with dimethylglyoxime (DMG) according to the following reaction: Ni + + HDMG Ni(DMG) + H + The nickel in a sample is dissolved and the nickel precipitated as Ni(DMG) (88.9 g/mol). The percentage of nickel in the sample is.9 %. What weight of sample must be taken to obtain a precipitate that will weigh about. g? (3) [5]

13 Page 1 Data Sheet x μ t calculated = s / N t calculated = x a s x pooled b s F calculated = s n n a a n + n b b t 1 calculated s pooled = x a x b = s1 s + n n 1 a a b b s ( Na 1) + s ( Nb 1) +... N + N +... Nsets of data t calculated = d s d n Critical values of F at the 5% Probability Level (95% confidence) Degrees of Freedom (Denominator) Degrees of Freedom (Numerator)

14 Page 15 Critical Values for the t-statistic Confidence level degrees Freedom 5% 9% 95% 99% N Critical Values for the Q-statistic Rejection Quotient Qcrit (Reject if Q exp > Q crit ) 9% Confidence 95% Confidence 99% Confidence N = number of observations Indicator Name pka Quinaldine Red.75 Methyl Orange 3. Bromophenol Blue.1 Bromocresol Green. Methyl Red 5. Bromothymol Blue 7.1 Phenol Red 7.81 m-nitrophenol 8.15 o-cresolphthalein 8.8 Phenolphthalein 9.7

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