The vapour of an organic compound was burnt in oxygen; both gaseous substances were taken at the same pressure and temperature. After the reaction, the system was returned to the original conditions and it turned out that its volume had not changed. The products of combustion contained 50 % carbon dioxide and 50 % water vapour by volume. Identify the organic compound in question.

Problem 2.

80 g of a 0.548 % (m/m) solution of HCl reacted with a piece of potassium; 6.153 kJ of heat evolved during the process. Calculate the amounts of all ions present in the solution after the reaction.

Heats of formation: Q(H₂O) = -285.9 kJ/mol; Q(H⁺) = 0.00 kJ/mol;

Q(OH^-) = -228.6 kJ/mol; Q(K⁺) = -252.5 kJ/mol.

Problem 3.

100.0 g of a 10.00 % (m/m) sodium chloride solution and 100.0 g of a 5.00 % (m/m) sodium hydroxide solution were consecutively connected into an electric circuit and electrolysed with a current of 1.00 A until the concentration of NaOH in the two solutions became equal. Calculate the time required for this process, supposing that the efficiency of the electrolysis was 100 % and all the gaseous products quantitatively left the solutions.

Problem 4.

A metallic cylinder containing pale greyish tablets was confiscated from a dangerous international criminal organization. After opening the cylinder, the tablets were covered with paraffin oil since they were rapidly decomposing in air. When a tablet was dropped into water, a violent reaction took place with the tablet rapidly moving on the surface of the water emitting gas and emetic smoke. The gas formed around the tablet burned with a beautiful scarlet flame when ignited. Finally, the tablet completely dissolved and the solution turned purple when phenolphtaleine was added to it. - Quantitative experiments brought astonishing results: 10.00 g of the unknown substance reacted with excess water to give 30.58 dm³ of gas at 25 °C and atmospheric pressure! The aqueous solution formed in this reaction was neutralized with hydrogen fluoride and evaporated until dryness. The remaining white substance, when completely dry, weighed exactly 31.25 g.

Do you know what the unknown substance was? What kind of danger does it represent in the hands of a criminal organization?

Problem 5.

Nine chemical equations with eight unknown substances:

1. A (g) + B (g) --> C (l) + D (s)

2. A (g) + B (g) --> C (g) + E (g)

3. B (g) + D (s) --> E (g)

4. A (g) + E (g) --> C (l) + D (s)

5. B (g) + F (g) --> C (g)

6. D (g) + F (g) --> A (g)

7. B (g) + E (g) --> G (g)

8. C (l) + G (l) --> H (l)

9. A (g) + H (l) --> C (l) + D (s) + E (g)

Your task is to substitute chemical substances for the letters so that the resulting equations correspond to real processes that can take place among moderate conditions (atmospheric pressure, 0...1000 °C). Indicate the reaction conditions where they are known to you. Balance the equations.

Problem 6.

1.000 g of a chloride of a metal was slowly heated in air until red glow. 0.618 g of a solid residue was formed which, according to analysis, contained no chlorine but its metal content was equal to that of the original chloride. Identify these substances and write the balanced equation of the reaction that occurred during the heating.

All atomic weights should be rounded to integers except Cl = 35.5.

Problem 7.

A dipeptide like alanylglycine (H-Ala-Gly-OH) cannot be synthesized simply by adding a dehydrating agent to an equimolar mixture of the two amino acids since this procedure would lead to the formation of numerous side products (H-Gly-Ala-OH,

H-Gly-Gly-OH, H-Ala-Ala-OH, H-Gly-Ala-Gly-OH, etc.). A single product can only be obtained by the use of so-called protecting groups. A protecting group is a moiety which remains intact during the condensation reaction and thus protects the functional group it had been attached to but, in a following step, it can be removed from the molecule without affecting the peptide bond.

The dipeptide H-Ala-Gly-OH, for example, may be synthesized by the following sequence of reactions:

 									 -H2O

X-O-CO-NH-CH-CO-OH        +        NH2-CH2-CO-O-X ------>

          |

          CH3



  (X-O-CO-Ala-OH)                    (H-Gly-O-X)



------>   X-O-CO-NH-CH-CO-NH-CH2-CO-O-X

                       |

                       CH3



           (X-O-CO-Ala-Gly-O-X, protected dipeptide)

				    H+

X-O-CO-NH-CH-CO-NH-CH2-CO-O-X     ------>       NH2-CH-CO-NH-CH2-CO-OH

          |                                         |

          CH3                                       CH3



(X-O-CO-Ala-Gly-O-X, protected dipeptide)           (H-Ala-Gly-OH)

NH₂-CH₂-CO-OH + C_nH_2n ------> NH₂-CH₂-CO-O-X

a) What is the structure of group X, if the protected dipeptide mentioned in the above reactions contains 9.26 % (m/m) nitrogen, and the hydrogen atoms in X are indistinguishable?

b) Draw the structure of a molecule of the gas used for the preparation of H-Gly-O-X.

Problem 5/a. (for students specialized to chemistry)

Three samples of the same weight were taken from a mixture consisting of ammonium chloride, ammonium sulphate and ammonium nitrate.

a) The first sample was heated in concentrated potassium hydoxide solution. The gas liberated was absorbed in 100 cm³ 1.000 mol/dm³ sulphuric acid solution, which was diluted to 250 cm³ with distilled water. 10.00 cm³ portions of the resulting solution consumed an average of 16.48 cm³ when titrated with 0.0987 mol/dm³ NaOH solution.

b) The second sample was heated in concentrated potassium hydoxide solution with excess zinc metal. In this case the following reaction took place. Balance the equation.

Zn + OH^- + NO₃^- = [Zn(OH)₄]^2- + NH₃

The gas liberated was absorbed again in 100 cm³ 1.000 mol/dm³ sulphuric acid solution, but this time the solution was diluted to 200 cm³ with distilled water. 10.00 cm³ of this solution was neutralized with 16.08 cm³ 0.0987 mol/dm³ NaOH solution.

c) The third sample was reacted with an excess of BaCl₂ solution. The mass of the precipitate formed was 2.3340 g after washing and drying.

Calculate the weight of the samples and their composition in mol%.

(Disregard the reaction NH₄NO₃ = N₂O + 2 H₂O, which might take place upon heating.)

Problem 6/a. (for students specialized to chemistry)

An aqueous solution contains 2.00·10^-3 mol/dm³ acetic acid.

a) What should be the concentration of propionic acid in the same solution if we want the degree of dissociation of the acetic acid to be 0.080?

b) What should be the propionic acid concentration in the 2.00·10^-3 mol/dm³ acetic acid solution if we want the pH to be 3.28?

c) Can we make a solution from water, acetic acid and propionic acid so that the degree of dissociation of the two acids is equal? Explain your reasoning.

The dissociation constant of acetic acid: K_A = 1.8·10^-5 mol/dm³

propionic acid: K_P = 1.3·10^-5 mol/dm³

Practical problem 1.

You see three mixtures consisting of inorganic materials on your desk. Determine which compounds are present in the mixture and which are not. The possibilities are:

1) (NH₄)₂CO₃, AgNO₃, BaCl₂·2H₂O, NH₄NO₃, NiCl₂·2H₂O

2) ZnO, KI, Pb(NO₃)₂, BaSO₄, MnO₂, Mg

3) CaCO₃, NH₄I, FeSO₄·7H₂O, TiO₂, CuCl₂·2H₂O

You can use distilled water, 2 mol/dm³ HCl, 2 mol/dm³ HNO₃, 2 mol/dm³ NH₃, 2 mol/dm³ NaOH solution, pH paper, test tubes and a Bunsen burner.

Put down your experimental findings in detail. Explain every conclusion (positive or negative). Include reaction equations where possible.

[60 minutes]

Comment: You will determine the quantitative composition of one of the samples - remember your results.

[All competitors had the same samples.

1) AgNO₃, BaCl₂·2H₂O, NH₄NO₃

2) ZnO, Pb(NO₃)₂, Mg

3) CaCO₃, CuCl₂·2H₂O]

Practical problem 2.

Determine the composition of mixture 3 which contains two components. Do not forget to write down all the reactions involved.

Procedure:

Pipet 20 cm³ 1,059 M HCl solution to the sample (0.7000 g) in the numbered Erlenmeyer flask. Prepare 100.0 cm³ stock solution. Titrate 10.00 cm³ aliquots with 0.1092 M NaOH solution. Use 2 drops of the methyl red - methylene blue indicator mixture (purple in acidic, green in basic solution, the bluish grey transition color can be seen at pH=5.4).

First make some trial titrations. Mean consumption:

In the three final measurements add 25 cm³ 0.5 Na₂S₂O₃ solution to the sample appr. 0.4 cm³ before the end point.

The thiosulphate ions form a complex with the heavy metal ion present in the sample.

What is the effect of the thiosulphate on the titration and why?

Why do we use the thiosulphate?

Why didn't we add the thiosulphate at the beginning of the titration?

What is the composition of the sample in m/m%?

Estimate the acidic dissociation constant of the hydrated transition metal ion using your measurements.

[90 minutes]

Practical problem 3.

You find the following materials in 7 numbered vials:

sucrose, fructose, gelatin, sorbitol (reduced glucose - HOCH₂- (CHOH)₄- CH₂OH), paraformaldehyde, starch, PVC.

Determine which compound is in which vial. Try to identify every unknown with at least one positive reaction. Apart from the unknowns you can use distilled water, 10% NaOH, 0.27 M CuSO₄, 2 M HCl solution and pH paper.You find a Bunsen burner and a water bath for heating the samples.

Take into consideration that organic reactions usually take longer - in some cases you will find reaction times of 3 minutes. Do not forget that the Fehling test is done with Cu(OH)₂ dissolved in complex form in a basic solution.

In your report you should put down how you identified the unknowns, all your experimental findings and their explanations. Summarize your results.

[90 minutes]