International Chemistry Olympiad

IChO 1983

Timisoara, Romania · 7 theory problems with complete solutions

  1. Problem 1: Short questions A-J in inorganic and general chemistry

    A) Describe the thermal decomposition of the following ammonium salts in terms of chemical equations:

    Figure 1

    B) Indicate the right answer:

    a) Can the molar mass be determined by measuring the density of a gaseous compound at a given temperature and pressure?

    1. Yes, under any conditions.
    2. Yes, if the gaseous compound does not dissociate and associate.
    3. Yes, if the gaseous compound does not dissociate.
    4. Yes, if the gaseous compound does not associate.

    b) Is a liquid boiling at a constant temperature (at a given pressure) a pure substance?

    1. Yes, if the liquid is not azeotropic.
    2. Yes, if the liquid is azeotropic.

    C) Complete and balance the following equation: (in \(\ce{H2O}\))

    Figure 2

    D) The solubility of \(\ce{Hg2Cl2}\) in water is \(3.0\times10^{-5}\) g/100 ml solution.

    a) What is the solubility product?

    b) What is the solubility (in mol dm\(^{-3}\)) of this substance in a 0.01 M \(\ce{NaCl}\) solution?

    c) What is the volume of a 0.01 M \(\ce{NaCl}\) solution which dissolves the same quantity of mercurous chloride as that dissolved in one litre of pure water?

    \(A_\mathrm{r}(\ce{Hg})=200.61\qquad A_\mathrm{r}(\ce{Cl})=35.45\)

    E) Which of the following groups contains solid compounds at \(10\,^\circ\mathrm{C}\)?

    a) \(\ce{H2O, NH3, CH4}\)

    b) \(\ce{F2, Cl2, Br2}\)

    c) \(\ce{SO3, I2, NaCl}\)

    d) \(\ce{Si, S8, Hg}\)

    F) Which of the following salts forms an acidic aqueous solution?

    a) \(\ce{CH3COONa}\)

    b) \(\ce{NH4Cl}\)

    c) \(\ce{Na2HPO4}\)

    d) \(\ce{Na2CO3}\)

    e) \(\ce{NaHCO3}\)

    G) Write the electronic formulas for the following compounds so that the nature of the chemical bonds is evident:

    a) \(\ce{NaClO3}\), b) \(\ce{HClO3}\), c) \(\ce{SiF4}\), d) \(\ce{NH3}\), e) \(\ce{CaF2}\), f) \(\ce{H2O}\)

    H) Solid perchloric acid is usually written as \(\ce{HClO4.H2O}\). Based on experimental data showing four equal bonds, suggest a structure accounting for the experimental result.

    I) The compounds of the second row elements with hydrogen are as follows: \(\ce{LiH, BeH2, B2H6, CH4, NH3, H2O, HF}\).

    a) Which compounds are solid at room temperature? Explain.

    b) Which of them are ionic?

    c) Which are polymeric?

    d) Which ones do not react with water under normal conditions?

    e) Give products of the following reactions.

    Figure 3

    f) Supposing that \(\ce{NH3}\), \(\ce{H2O}\) and \(\ce{HF}\) are acids under some conditions, write their corresponding conjugated bases and arrange them in order of increasing basic strength.

    J) The following \(E^0\) values are given for the half-reactions:

    Figure 4

    Calculate \(E^0\) for the following reaction:

    Figure 5

    Official English text of the problem.

  2. Problem 2: Composition of a CO-CO2 mixture

    In a gaseous mixture of \(\ce{CO}\) and \(\ce{CO2}\), a mass ratio of carbon : oxygen = 1 : 2 was determined.

    2.1 Calculate the mass percent composition.

    2.2 Calculate the volume percent composition.

    2.3 Indicate values of the carbon: oxygen ratios for which both gases cannot be present simultaneously.

    Official English text of the problem.

  3. Problem 3: Analysis of a NaCl-KCl mixture with silver nitrate and a copper strip

    A sample containing a mixture of sodium chloride and potassium chloride weights 25 g. After its dissolution in water 840 ml of \(\ce{AgNO3}\) solution (\(c = 0.5\ \mathrm{mol\,dm^{-3}}\)) is added. The precipitate is filtered off and a strip of copper weighing 100.00 g is dipped into the filtrate. After a given time interval the strip weights 101.52 g.

    Calculate the mass percent composition of the mixture.

    Official English text of the problem.

  4. Problem 4: Kinetics and stereochemistry of the alkaline hydrolysis of chlorinated compounds

    The following data were gathered for the alkaline hydrolysis of certain chlorinated compounds:

    a) A certain volume of a solution of the neutral potassium salt of chlorosuccinic acid is mixed with an equal volume of hydroxide solution. The initial concentration of each solution is \(0.2\ \mathrm{mol\,dm^{-3}}\). The potassium hydroxide concentration in the reaction mixture was determined at different time intervals at \(25\,^\circ\mathrm{C}\). The following values were obtained:

    t (minutes) 10 20 30 45 60 80 100
    \(c(\ce{KOH})\) \((\mathrm{mol\,dm^{-3}})\) 0.085 0.074 0.065 0.056 0.049 0.042 0.036

    The experiment was repeated with the same initial solutions at \(35\,^\circ\mathrm{C}\). The hydroxide concentration is reduced to one half after 21 minutes.

    b) In the hydrolysis of 3-chloro-3-methylhexane with potassium hydroxide, the concentration of potassium hydroxide was found to have been reduced to one half after 32 minutes at \(25\,^\circ\mathrm{C}\) or 11 minutes at \(35\,^\circ\mathrm{C}\), regardless of the initial reactant concentrations (identical).

    c) In the alkaline hydrolysis of 3-chloro-2,4-dimethyl-3-isopropylpentane an identical reaction mechanism as for reaction b was found but the reaction rate was about 100 times faster under the same reaction conditions.

    Considering the above data answer the following questions:

    4.1 What is the reaction order in cases a, b, and c?

    4.2 What is the rate constant at \(25\,^\circ\mathrm{C}\) for reaction a? Indicate the units.

    4.3 Calculate the activation energies for reactions a and b.

    4.4 If in reaction a dipotassium salt of L-chlorosuccinic acid (which is levorotatory,) is used, what type of optical rotation will be exhibited by the corresponding salt of malic acid formed by hydrolysis?

    4.5 If the levorotatory isomer is also used in reaction b, what optical rotation will be exhibited by 3-methyl-3-hexanol formed in the hydrolysis reaction?

    4.6 Why is the rate of reaction c much faster than that of reaction b when both reactions are of the same type and occur under the same temperature and concentration conditions?

    Official English text of the problem.

  5. Problem 5: Ethanol dehydration equilibrium and its pressure dependence

    On passing ethanol over a catalyst at 400 K, a dehydration reaction occurs resulting in the formation of ethylene:

    Figure 1

    At the above temperature and \(p_0 = 101.325\ \mathrm{kPa}\), the conversion of ethyl alcohol is 90.6 mol %.

    5.1 Calculate the equilibrium constant \(K_p\) of the reaction under given conditions.

    5.2 Calculate the values of the equilibrium constants \(K_x\) and \(K_c\) at the above temperature.

    5.3 Calculate the ethanol conversion at the following pressures:

    \(5p_0\), \(10p_0\), \(50p_0\), \(100p_0\), and \(200p_0\).

    5.4 Plot the graph for the variation of conversion vs. pressure.

    Official English text of the problem.

  6. Problem 6: Identification of compounds A-G from a base-catalysed reaction sequence

    One mole of compound A reacts successively with 3 moles of compound B in aqueous solution in the presence of a basic catalyst (such as \(\ce{Ca(OH)2}\)):

    Figure 1

    Hydrogenation of compound E yields compound F:

    Figure 2

    F has the composition: \(\ce{C} = 44.18\,\%,\quad \ce{H} = 8.82\,\%,\quad \ce{O} = 47.00\,\%.\)

    Its molar mass: \(M = 136\ \mathrm{g\,mol^{-1}}\)

    Knowing that 13.6 g of F reacts with 40.8 g acetic anhydride to form product G and acetic acid write down all chemical equations and assign the letters A, B, C, D, E, F, and G to particular formulas of compounds.

    Official English text of the problem.

  7. Problem 7: Reaction scheme of the isomers A and B (C7H7NO): compounds A-S

    Knowing that compounds A and B are isomers with the molecular formula \(\ce{C7H7NO}\) and the relative molecular mass of compound M is 93, determine the formulae of compounds A to S taking in account the reactions given in the following reaction scheme:

    Figure 1

    Official English text of the problem.