IChO 2009, Експеримент — Задача 2. Practical problem2 — Analysis of a Copper(II) Complex
41st International Chemistry Olympiad — Cambridge2009 — Practical problems (Volume3 collection edition2014)
Внимание: Бележка към темата
Source edition: The Competition Problems from the International Chemistry Olympiads, Volume 3, edited by Anton Sirota, IUVENTA / IChO International Information Centre, Bratislava, 2014. The collection normalizes task numbering and omits some original grading points, practical instructions and data-dependent key material. Every item actually printed in this Cambridge 2009 section is retained. The referenced practical answer booklet, TLC template and Practical 1 carbon NMR spectrum were not obtained. Reproduced for teaching purposes with the printed Volume 3 attribution and rights notice; commercial publication/distribution requires permission from the copyright holder.
Collection edition and native section
The Competition Problems from the International Chemistry Olympiads, Volume3,41st–45th IChO2009–2013. Edited by Anton Sirota. IUVENTA–Slovak Youth Institute, Bratislava2014. This scoped paper preserves only the41st International Chemistry Olympiad,18–27July2009, Cambridge, United Kingdom.

Native rights statement and archive qualification
You are free to copy, distribute, transmit or adapt this publication or its parts for unlimited teaching purposes, you are obliged, however, to attribute your copies, transmissions or adaptations with a reference to "The Competition Problems from the International Chemistry Olympiads, Volume 3" as it is commonly required in the chemical literature. The problems copied cannot be published and distributed for commercial proposes. The above conditions can only be waived if you get permission from the copyright holder.
Copyright2014IUVENTA–IChO International Information Centre; ISBN978-80-8072-154-1. Attribution: The Competition Problems from the International Chemistry Olympiads, Volume3, edited by Anton Sirota, Bratislava2014.
Source note: This wording permits copying, distribution, transmission and adaptation for unlimited teaching purposes with the required Volume3 reference. It forbids commercial publication/distribution unless the copyright holder waives the conditions. This free educational archive retains the required Volume 3 attribution and this rights notice. Reproduction is presented for teaching purposes under the printed grant; no commercially unrestricted permission is claimed. The publisher must assess this obtained statement before adoption.

Issued by IUVENTA – Slovak Youth Institute in 2014 with the financial support of the Ministry of Education of the Slovak Republic. Number of copies: 250. Not for sale.
International Chemistry Olympiad, International Information Centre, IUVENTA – Slovak Youth Institute, Búdková 2, SK 811 04 Bratislava 1, Slovakia. E-mail: anton.sirota@stuba.sk. Web: www.icho.sk.
Full native editorial preface
Preface This publication contains 39 theoretical and 14 practical competition problems from the 41st – 45th International Chemistry Olympiads (IChO) organized in the years 2009 – 2013. It has been published by the IChO International Information Centre in Bratislava (Slovakia) as a continuation of the preceding Volumes 1 and 2 published on the occasion of the 40th anniversary of the IChO with the titles: • The competition problems from the International Chemistry Olympiads, Volume 1, 1st – 20th IChO, 1968 – 1988 (IUVENTA, Bratislava, 2008). • The competition problems from the International Chemistry Olympiads, Volume 2, 21st – 40th IChO, 1989 – 2008 (IUVENTA, Bratislava, 2009). Not less than 318 theoretical and 110 practical problems were set in the IChO during the forty-five years of its existence. In the elaboration of this collection the editor had to face certain difficulties because the aim was not only to make use of past recordings but also to give them such a form that they may be used in practice and further chemical education. Consequently, it was necessary to make some corrections in order to unify the form of the problems (numbering the tasks of the particular problems, solution inserted immediately after the text of the problem, solutions without grading points and special graphs used for grading of practical problems). Nevertheless, the mentioned corrections and changes do not concern the contents and language of the competition problems. The practical problems set in the IChO competitions, contain as a rule some instructions, list of apparatuses available, chemicals on each desk and those available in the laboratory, and the risk and safety phrases with regard to the chemicals used. All of these items are important for the competitors during the competition but less important for those who are going to read the competition tasks of this collection and thus, they are omitted. Some parts of the solutions of practical problems are also left out since they require the experimental data which could be obtained by experiments during the practical part of the IChO competition. In this publication SI quantities and units are preferred. Only some exceptions have been made when, in an effort to preserve the original text, the quantities and units have been used that are not SI. Although the numbers of significant figures in the results of some solutions do not obey the criteria generally accepted, they were left without change.
Unfortunately, the authors of the particular competition problems are not known and due to the procedure of creation of the IChO competition problems, it is impossible to assign any author's name to a particular problem. Nevertheless, responsibility for the scientific content and language of the problems lies exclusively with the organizers of the particular International Chemistry Olympiads. This review of the competition problems from the 41st – 45th IChO should serve to both competitors and their teachers as a source of further ideas in their preparation for this difficult competition. For those who have taken part in some of these International Chemistry Olympiads the collection of the problems could be of help as archival and documentary material. In the previous forty-five years many known and unknown people - teachers, authors, pupils, and organizers proved their abilities and knowledge and contributed to the success of this already well known and world-wide competition. We wish to all who will organize and attend the future International Chemistry Olympiads, success and happiness. Bratislava, July 2014 Anton Sirota, editor
Source note: The preface describes general editorial omissions. This Cambridge section nevertheless prints practical chemical lists, procedures and selected grading graphs, all retained. Missing original answer-booklet/NMR material is not invented.
Условие
You are provided with a sample of an inorganic copper(II) complex, the anion of which is made from copper, chlorine, and oxygen. The counter ion is the tetramethyl ammonium cation. There is no water of crystallisation. You are required to determine proportions of copper ions and chloride ions by titration and hence determine the composition of the complex.
Chemicals
• Inorganic complex: three samples of approximately 0.1 g, accurately pre-weighed in vials labelled ‘Sample 1’, ‘Sample 2’, ‘Sample 3’.
- Inorganic complex: three samples of approximately 0.2 g, accurately pre-weighed in vials labelled ‘Sample 4’, ‘Sample 5’ and ‘Sample 6’.
- pH 10 ammonia buffer, 10 cm³.
- Murexide indicator, solution in H₂O, 10 cm³.
- EDTA disodium salt, aqueous solution, c = 0.0200 mol dm⁻³, 150 cm³.
- Ethanoic acid, 10 cm³.
- 2,7-Dichlorofluorescein indicator, solution in 7 : 3 EtOH : H₂O), 10 cm³.
- Dextrin, 2 % solution in H₂O, 25 cm³.
- Silver nitrate, aqueous solution, c = 0.1000 mol dm⁻³, 150 cm³.
Procedure
Titration to determine the proportion of copper ions
(a) You are provided with three accurately pre-weighed samples of copper complex, each of approximately 0.1 g. These are labelled "Sample 1", "Sample 2", "Sample 3", together with the exact mass of the copper complex. Take the first of these, note down the mass of the sample and quantitatively transfer the contents to a 250 cm³ conical flask using approximately 25 cm³ of water.
(b) Add pH 10 ammonia buffer solution until the precipitate which forms initially just redissolves (about 10 drops).
(c) Add 10 drops of the murexide indicator.
(d) Titrate with the 0.0200 mol dm⁻³ EDTA solution until the solution turns violet and the colour persists for at least 15 seconds. Record the volume of solution used in the titration.
(e) Repeat if necessary with samples 2 and 3. Note: You will be marked only on a single value you report in the answer booklet. This may either be an average value, or a single value you feel most confident in.
2.1 Calculate the volume of EDTA solution needed to react completely with 0.100 g of complex.
2.2 Give an equation for the titration reaction.
2.3 Calculate the percentage by mass of copper in the sample.
You will need to wash out your burette before you start the titration for the determination of chloride ions. Any remaining EDTA solution may be disposed of into the waste containers labelled ‘EDTA’. Titration to determine the proportion of chloride ions present
(f) You are provided with three accurately pre-weighed samples of copper complex each of approximately 0.2 g. These are labelled "Sample 4", "Sample 5", "Sample 6", together with the exact mass of the copper complex. Take the first of these, note down the mass of the sample and quantitatively transfer the contents to a 250 cm³ conical flask using approximately 25 cm³ of water.
(g) Add 5 drops of ethanoic acid, followed by 10 drops of dichlorofluorescein indicator and 5 cm³ dextrin (2 % suspension in water). Shake the bottle well before adding the dextrin suspension.
(h) Titrate with the 0.1000 mol dm⁻³ silver nitrate solution, swirling constantly until the white suspension turns pink and the colour does not disappear after swirling.
(i) Repeat if necessary. Note: You will be marked only on a single value you report in the answer booklet. This may either be an average value, or the value you feel most confident in.
2.4 Calculate the volume of silver nitrate solution needed to react completely with 0.200 g of complex.
2.5 Give an equation for the titration reaction.
2.6 Calculate the percentage by mass of chloride ions in the sample.
The percentage of carbon, hydrogen and nitrogen in the complex was determined by combustion analysis and found to be as follows: Carbon: 20.87 %; Hydrogen: 5.17 %; Nitrogen: 5.96 %.
2.7 Mark in the answer booklet, which element in the complex has the greatest percentage error in the determination of its proportion.
□Cu □Cl □O □C □H □N
2.8 Determine the formula of the copper complex. Show your working.
Решение
2.1 Full marks if answer is Vcorr.±0.1cm³. Zero marks if answer is less than(Vcorr.−0.5cm³)or greater than(Vcorr.+0.5cm³),linear point scale is applied in between.
2.2 Cu²⁺+EDTA⁴⁻→[Cu(EDTA)]²⁻.
2.3 n(EDTA)=0.0200mol dm⁻³×V(EDTA)(in dm³)=(0.0200mol dm⁻³×V)mol. n(Cu²⁺)=(0.0200mol dm⁻³×V)mol. m(Cu)=M(Cu)×n(Cu)=63.55g mol⁻¹×(0.0200mol dm⁻³×V)mol.
2.4 Full marks if answer is Vcorr.±0.1cm³. Zero marks if answer is less than(Vcorr.−0.5cm³)or greater than(Vcorr.+0.5cm³),linear point scale is applied in between.
2.5 Ag⁺+Cl⁻→AgCl.
2.6 n(Ag⁺)=0.100mol dm⁻³×V(Ag⁺)(in dm³)=(0.100mol dm⁻³×V)mol. n(Cl⁻)=(0.100mol dm⁻³×V)mol. m(Cl)=M(Cl)×n(Cl)=35.45g mol⁻¹×(0.100mol dm⁻³×V)mol.
2.7 If the percentage of oxygen is calculated by subtracting all the others from100,it has all the errors from the determination of the other elements combined. In practice this leads to a large error which affects the subsequent calculation of the formula unless this point is appreciated.
Cu Cl O(circled in the original) C H N.
2.8 Full marks if the formula is correctly calculated from the student data and the large error in oxygen is taken into account(either by using one of the proportions of the other elements to determine the elemental ratios,or by a consideration of charge balance).
Source note: Numerical Vcorr. and a unique final complex formula are not supplied in this obtained data-dependent key. No measured titration volumes or formula have been invented.
Оригинал в Архива: IChO 41st-45th.pdf · решение от архива: IChO 41st-45th.pdf · допълнителни данни: IChO 41st-45th.pdf