VI Baltic Chemistry Olympiad, problems

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VI Baltic Chemistry Olympiad
May 12-14, 1998, Tartu
Theoretical problems

PROBLEM 1

The analysis laboratory received a white crystalline material for analysis. The analyst weighed out two samples of the material.

Sample 1. with a mass of 1.084 g was dissolved in water. As the material dissolved 26.17 ml of gas, having a mass of 37.39 mg, was released. The resulting solution was found to be very acidic. Addition hydrochloric acid caused a precipitate to form. After addition of 47.5 ml 0.0983 molar HCl no more precipitate was formed. The precipitate was recovered, dried and weighed. The mass of precipitate obtained was 0.6693 g.

Sample 2 with a mass of 1.120 g was heated to above 300^o C. During heating 324.6 ml of a gas, with a mass of 599.0 mg, was collected. After heating a light colored metal with a mass of 0.521 g remained. The gas was dissolved in 100 ml of water. Not all the gas dissolved, 54.10 ml, with a mass of 77.2 mg, remained. The pH of the solution was found to be 1.015.

a) What was the initial substance?
b) What reactions took place during the analysis?

PROBLEM 2

A sample of a compound was heated at 600^oC. A solid with a mass of 4.51 g remained after heating and 21.48 l of gas were liberated at 600^o. The solid contains 21.25% oxygen. After cooling to room temperature the gas occupies 2.69 l and has a density of 1.61 g/l. After passing the gas through alkaline solution, there remained 1.34 l. of a combustible gas having a density of 1.25 g/l.

Find the original compound and write all appropriate equations.

PROBLEM 3

R(+)-glycerol aldehyde can be transformed to (-)-malic acid (2-hydroxybutanedioic acid) according to the following scheme. R(+)-glycerol aldehyde was treated with hydrogen cyanide and two isomeric compounds were formed, namely A and B. Those compounds were separated by fractional crystallisation. Only isomer B was used for further transformations. This compound was converted to C (C₄H₈O₅) upon hydrolysis then oxidised by conc. nitric acid to produce compound D (C₄H₆O₆). Further compound D was treated with phosphorus tribromide thus compound E (C₄H₅BrO₅) was formed. Finally, compound E was converted to (-)-malic acid after reaction of zinc with an acid.

a) Write equations of all reactions using stereochemical (Fisher projections) formulae.
b) Explain why only one isomer E was formed from D (hint: consider mechanism of the reaction)? How many isomers of E can be formed if the starting compound for synthesis would be an isomer A?

PROBLEM 4

Mustard gas (2,2'-dichlorodiethylsulfyde) is a chemical weapon. In contact with humidity it produces gaseous hydrogen chloride. In comparison 1,5-dichloropenthane is hydrolysed much more slowly.

a) Write mechanism of the above described reaction for both compounds. Explain the difference in reactivity.
b) Which well-known compound might be used to destroy this gas? Write equation of the reaction.
c) Write synthetic pathway to produce the mustard gas.

PROBLEM 5

The thermal decomposition of azomethane (compound A) follows the equation:

H₃CN = NCH₃(g) Ž C₂H₆(g) + N₂(g)

The total pressure of the mixture of gases at the temperature of 298.0 °C was measured at different time periods:

t (min)	10.0	21.0	35.0	Ľ
p (Torr)	491.9	548.0	609.7	861.6

a) Prove that the process is a first order reaction.
b) Calculate the average value of the rate constant at 298.0 °C.
c) The half life of compound A at 320.6 °C is 9.5 min. Determine the activation energy of the reaction.

PROBLEM 6

Eriochrome Black T (H₃Ind) is a typical metal ion indicator that is used in complex-formation titrations with EDTA (Na₂H₂Y). This compound contains a sulfonic acid group, which is completely dissociated in water, and two phenolic groups, which dissociate only partially (K_a1=5.0ˇ10^-7 and K_a2=2.8ˇ10^-12).

H₂O + H₂Ind- = HInd^2- + H₃O⁺
red blue
H₂O + HInd^2- = Ind^3- + H₃O⁺
blue orange

a) The metal complexes of Eriochrome Black T are generally red. Calculate the minimum pH at which Eriochrome Black T can be used as a indicator in complex-formation titrations (assume, that the detectable color change requires tenfold excess of one or other colored specie).
b) Determine the transition range for Eriochrome Black T in titrations of Mg2+ and Ca2+ at pH 10.0, given that the (thermodynamic) formation constant for MgInd^- is 1.0 ˇ10⁷ and for CaInd^- is 2.58 ˇ10⁵.
c) 50.00 ml of 0.00500 M Mg²⁺ and Ca²⁺solutions were both titrated with 0.0100 M EDTA solution at pH 10. Conditional formation constants (Instead of the equilibrium concentration of Y^4- the total concentratioini of all forms of EDTA is used) at that pH are 1.72 ˇ10⁸ for MgY^2- and 1.75 ˇ10¹⁰ for CaY^2-. Calculate the equilibrium concentrations of the metal cations at the equivalence point. Determine the sign of the relative error, connected with assuming that stoichometry point is judged to be approached at the first change of color of solution.

Solution

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