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       #Post#: 2635--------------------------------------------------
       Chemistry olympiad past question- Analytical chemistry
   DIR By: Michel
       Date: July 13, 2013, 4:10 am
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       In the practical exam for the 2009 International Chemistry
       Olympiad held in Cambridge, students were asked to analyse
       compound X, a compound containing copper(II).  This compound
       consists of ions: the negatively charged ion is a complex ion
       containing copper, chlorine and oxygen.  The positive counter
       ion in compound X is the tetramethylammonium ion, (CH3)4N+.  The
       students were asked to determine the formula of the negative
       ion, using two titrations. The first titration was used to
       determine the proportion of copper ions in the complex.  In this
       titration, the complex was reacted with EDTA solution.  EDTA
       reacts with copper ions according to the equation: Cu2+(from the
       complex) + EDTA4–(aq) → [CuEDTA]2– (aq) The end-point of
       this titration was determined using an indicator called
       murexide, and one of the UK team found that 0.1000 g of X
       required 21.70 cm3 of 0.02000 mol dm–3 EDTA solution for
       complete reaction.
       (a) Calculate the % by mass of copper in compound X. The second
       titration determined the proportion of chloride ions in the
       complex.  The chloride ions from the complex were titrated
       against 0.1000 mol dm–3 silver(I) nitrate solution, and the
       student found that 0.2000 g of X required 21.70 cm3 of silver
       nitrate.
       (b) i)  Give a balanced equation that shows the reaction that
       takes place in this titration
       . ii) Calculate the % by mass of chlorine in compound X. The
       proportions by mass of carbon, hydrogen and nitrogen in compound
       X were determined by combustion analysis.  The results were
       found to be C 20.87%, H 5.17%, and N 5.96%.
       (c) i)  Which of the six elements in compound X has the greatest
       percentage error in the determination of its proportion?  Circle
       the element in the answer booklet.
       ii)  Ignoring this element, because of this uncertainty,
       determine the simplest whole number ratio of the remaining five
       elements in X.
       iii) Hence determine the formula of the negative ion in X
       
       Please post your solutions, and lets review them.
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