MINISTRY OF EDUCATION AND SCIENCE
UNIVERSITY OF LATVIA
RIGA TECHNICAL UNIVERSITY

43rd Latvian National Chemistry Olympiad (2002)
Theoretical problems

Honorable olympian!

Organizing committee of the 42nd Latvian National Chemistry Olympiad congratulates you with success in preliminary round and wishes good luck in the final round!

To provide opportunity for each participant completely demonstrate his knowledge in chemistry and its boundaries we offer quite large number of problems. To achieve the best result and not lose points because of misunderstanding or laxity read carefully these instructions before start with solving problems!

1. Every participant receives a complete set of all problems with 26 problems and a test with 7 questions different for each class.
2. After the index number of problem there are indicated classes which should solve it. If more than one class is indicated all should solve the problem. There are 9 problems and a test for each participant.
3. Participants from younger classes may solve problems of elder.
   Example.

   13.

   10-11

   5+x p.

   
   This problem is for classes 10 and 11 participants from class 9 also may solve it while those from class 12 may not.

4. Behind classes the maximal number of points for the problem is indicated. If there is added + or +x there is a chance to get extra points answering the extra questions in the problem beginning with "if you can ...".
5. If you have any questions regarding the problems be sure to ask the responsible person from organizing committee who is present during the theoretical examination. Mutual consultations between participants are not allowed.
6. After complete or partial solving of a problem describe the respective number on your participant's card and cross the numbers which you didn't try to solve. This eases the job of estimators and will prevent potential misunderstandings.

Preparatory problems, results and other information you can find on our web page: http://www.liis.lv/chem/!

Little Andrew in the school lab was pouring concentrated (98%) sulfuric acid in the graduated cylinder. After 55 mL of acid were poured into the cylinder, it happened that it felt down and all the acid now was on the flour. Luckily, Andrew had 3% NaHCO₃ solution in the shelf for the neutralization of the acid.

What volume of the soda solution is necessary for complete neutralization of the spilled acid?

Density of concentrated sulfuric acid is 1.837 g/ml, density of NaHCO₃ solution is 1.03 g/ml.

10.0 g of calcium reacted with 1.00 liter of water. Which compounds will present in the solution after the reaction? Calculate their mass parts. Solubility of calcium hydroxide is 0.165 g in 100 g of water.

Chemical compounds of an element Q are widely found in the nature. This element has no allotropic forms and with oxygen it forms two compounds A and B with different composition. Compound A is neutral and in the reactions with binary compounds it may form either acids of bases.

Compound B decomposes in the presence of a catalyst, MnO₂, into the compounds A and Y.

When compound A reacts with active metals, simple compound X of element Q is formed besides the compounds, which reacts with acids and acidic oxides.

1. Identify element Q.

2. Write the chemical formulas for the compounds A, B, X and Y.

3. Write the equations for the following chemical reactions:

a) Preparation of A,

b) Preparation of base from A,

c) Preparation of acid from A,

d) Decomposition of B in the presence of catalyst MnO₂,

e) Reaction of A with the active metals.

Content of radon in seawater is 6´ 10- ¹⁹ g/l.

How many atoms of radon are present in 1 liter of seawater?

Lithium usually is obtained from the mineral spodumen, Li₂O·Al₂O₃·4SiO₂, according to the following scheme:

Element X may be found in the nature either as a simple compound or in different minerals. Element X containing species are formed usually during the volcanic activities. Element X is also important as an insecticide. In high temperature it reacts with many metals, hydrogen and oxygen.

Large amounts of this element are used in the production of a strong acid Y, which is used in many branches of manufacturing.

To an excess of diluted aqueous solution of acid Y, 1.28 g of zinc was added and the formed gas was burned.

Calculate the mass of the compound formed after the burning of the evolved gas.

Write the equations that show the production steps of the acid Y.

3.38 g mixture of sodium chloride, potassium carbonate and sodium sulfate dissolved in water and barium chloride solution (taken in excess) was added to the obtained solution. 4.30 g precipitate was formed (after drying). When excess of hydrochloric acid was added to this precipitate, 224 ml gas was evolved.

Write the equations for the chemical reactions.

Calculate the composition of the initial mixture (in mass parts).

The name chlorophyll is derived from the Greek words chloros (green) and phyllon (leaves). For the complete burning one mole of chlorophyll (M = 892.3 g/mol) 71 moles of oxygen are needed. In this burning process the following compounds are formed: 2420 g of gas A (this gas is found in all the sparkling drinks), 648 g of compound B (which is the main component of all the sparkling drinks), 44.8 l of gas D (which is the main component of atmosphere) and a solid residual - 40.3 g of diatomic oxide E.

1. Write the formulas of compounds A, B, D and E and name these species.

2. Calculate the amount of each element in one mole of chlorophyll.

3. Find the molecular formula of chlorophyll.

On the pans of the scale 100 g 10% sodium hydroxide solution and 100 g 10% sodium chloride solution were placed. Scale, of course, was in equilibrium, but after a while one of the pans started to move down.

Purple-red powdered compound A was boiled with concentrated sulfuric acid. At the beginning gas B (which is very good soluble in water) was evolved. Precipitate turned to black and its mass increased roughly by one-fourth – compound C was formed. After a while precipitate gradually turned almost white and it mass increased roughly twice – compound D was formed. Precipitate was filtered and water was added to it. Blue solution was obtained. After evaporating of this solution, blue crystals of compound E were obtained. Their mass was roughly four times higher than that of compound A. Heating of substance E gives D. However, if the heating is continued at higher temperature, compound D converts to the compound C and a gas, which contains compound B.

Derive the mathematical formula for calculation the volume (in normal conditions) of hydrogen evolved when a grams of magnesium reacts with b grams of sulfuric acid. Plot this relationship.

White pigment lithopone consists of barium sulfate and zinc sulfide. It may be obtained according to the following scheme:

a) Barium sulfide is prepared from barite, BaSO₄, by reducing it at high temperatures with carbon monoxide.

b) Zinc sulfate is prepared from zinc blend, ZnS, by burning and eventual treatment with sulfuric acid.

Obtained solution is treated with chlorine in weak acidic medium for the oxidation of iron(II) species to the iron(III) and manganese(II) species to the manganese(IV) species. In these conditions, iron(III) hydroxide and manganese(IV) oxide are precipitated from the solution.

In the following table density of hydrochloric acid solutions is shown for different mass parts of HCl.

w, %	r , g mL- 1		w, %	r , g mL- 1
2	1,01		22	1,11
4	1,02		26	1,13
8	1,04		36	1,18
18	1,09		38	1,19

What is the density of 30% hydrochloric acid solution?

For the exploration of different processes radioactive or so-called labeled elements often are used.

Using labeled atoms of radioactive chlorine (³⁶Cl), propose the method how one can proof that equilibrium between solid BaCl₂ and dissolved BaCl₂ is dynamic, e.g. dissolved BaCl₂ all the time returns into the precipitate and vice versa.

By addition of 2 volumes hydrochloric acid (c = 0.1 mol/l) to one volume of mercury (II) acetate (salt of acetic acid) solution (c = 0,1 mol/l), electric conductivity of obtained solution decreased by factor of several thousands in comparison to the initial solution of hydrochloric acid.

Solubility of compounds in water (g / 100g): Hg(CH₃COO)₂ – 50, HgCl₂ – 7, acid constant for acetic acid K_a = 1.74·10^-5.

Aqueous solutions of compounds A, B and C have the same chemical composition, and all the solutions are strongly acidic. If they are evaporated, at the beginning compound A is obtained (it contains 23.2% S), afterwards – compound B (26.7% S). Continuing the heating yields compound C which contains 28.9% S. If C is heated at high temperature white crystalline substance D is obtained. Aqueous solution of D is neutral, it colors the gas flame yellow and contains 22.6% S. In all the species sulfur exhibits the same oxidation state.

To the aqueous solution of a salt A bromine water was added at 5 ^oC temperature till the formation of precipitate B was completed. Obtained precipitate was filtered at 5 ^oC temperature. Colorless filtrate was heated up to 70 ^oC temperature and solution of salt A was added to it till the formation of precipitate B and the impurity C was completed. Mass of the precipitate B that formed at 70 ^oC temperature was 2/3 of that formed at 5 ^oC temperature. If an excess of potassium iodide is added to the solution containing 10 g of salt A, yellowish precipitate D is formed. D dissolves in the solution of sodium thiosulfate, but does not dissolve nor in the ammonia solution, either in diluted nitric acid.

Determine the compounds A, B, C and D, and write the equations for all the chemical reactions.

Malachite is a basic carbonate of some metal, and its composition is the following: 5.47% C, 36.17% O, 0.91% H and 57.48% of some unknown metal. 25.0 g of malachite was heated and the black residual was dissolved in the hydrochloric acid. The obtained salt (in the solution) was subjected to the complete electrolysis. 13.89 g of metal was obtained.

How many percents of impurities contain this sample of malachite? Write the equations for the chemical reactions. What is the amount of electricity that is spend during this electrolysis, if the voltage was 5.0 V? (Faraday constant F = 96500 C/mol)

Substance X is an easily volatile solid compound and is of great importance in the producing of energy as well as in the military industry. It may be obtained in the direct reaction of fluorine with a pure silver white metal. Relative density of this compound against the air is 12.18. What is this compound? And why it is volatile despite its high molecular weight?

Density of air is 1.29 g/l.

Calculate the volume of carbon (IV) oxide, v (in liters, normal conditions), which is formed by burning m grams of hydrocarbon (or their mixture) with mass part of carbon w % in the excess of oxygen.

Using the obtained equation, calculate the volume of CO₂, which would be released by burning of 10.0 g hexadecane C₁₆H_34.

To100 ml of sodium acetate solution 1 g of white crystalline compound A was added. Solution turned to solid substance and its temperature raised. The elemental composition of the obtained substance was the same as that of compound A – it contained 16.9% Na, 17.7% C, 6.67% H and oxygen.

Determine compound A.

0.96 of an simple compound A was heated at 150 ^oC temperature with 20 g of an heavy colorless liquid B. After the heating 0.48 g of compound A and approximately 18 g liquid B remained unreacted. Gas, evolved during the heating, was trapped into the 36 g of 10% sodium hydroxide solution. Solution I was obtained. This solution was boiled and 0.48 of the previously remained substance A was added to it. Solution II was obtained. This solution contains compounds that are widely used in photography. If aqueous solution of barium chloride is added to the solution II, which has been standing in the air for a while, precipitate C is formed. C cannot be completely dissolved into the hydrochloric acid.

Most widely common drugs against fever are aspirine (acetylsalicylic acid) and paracetamol (para-acetylaminophenol).

Identify and name the various functional groups in aspirin and paracetamol.

Write the equation for one chemical reaction involving aspirin and for one involving paracetamol which cold serve as unique tests for the discrimination of these two drugs

Many anti-fever drugs however contain both – aspirine and parecetamol. Describe how to separate the mixture containing both of these chemicals and obtain both substances in pure forms separately. Write the equations for these chemical reactions.

Aspirin usually is prepared from phenol according to the following scheme:

Paracetamol could be prepared from phenol as well:

Write the equations for all the reactions and name all the organic compounds involved in these reactions according to the IUPAC nomenclature.

1.00 g organic substance A was dissolved 3.00 mL of hot water. Obtained solution contained only organic substance B. Solution was cooled and filtered. Filtered substance B was dried and its mass was 1.00 g. In 100 mL volumetric flask solution from 1.00 g of compound B was prepared. For titration of 10.00 ml of the prepared solution 16.95 ml 0.1 M sodium hydroxide solution are necessary. If the obtained compound B is heated, compound A is obtained.

Determine the compounds A and B, write their names according to the IUPAC nomenclature, and write the equations for all the chemical reactions.

Sodium hydrosulfide, NaHS, is obtained by bubbling the hydrogen sulfide through the ethanol solution of sodium etoxide and the formed sodium hydrosulfide is precipitated from the solution by adding of diethyl ether, in which it is not soluble.

Oxidation of alkyl groups attached to the aromatic rings is of great importance in organic synthesis. For instance, nicotic acid (vitamin PP) may be obtained this way from 3-methylpyridine.
3-Methylpyridine is oxidized with sodium permanganate in weak basic medium (by adding the potassium carbonate to the reaction mixture). During the reaction, manganese(IV) oxide is formed from the permanganate.

Calculate the mass of potassium permanganate necessary for preparation of 10 g nicotic acid if the reaction yield is 80%.

For the complete oxidation of 3-methylpyridine, potassium permanganate is usually taken in excess. Before the extraction of nicotic acid in this case, unreacted potassium permanganate is removed by adding of potassium sulfite.

Write the equation for this reaction.

In terms to extract nicotic acid after the reaction is completed, reaction mixture is acidified with hydrochloric acid and nicotic acid is precipitated.

Calculate the volume of 6.0 M hydrochloric acid for the complete precipitation of 10 g nicotic acid. If hydrochloric acid is added in excess, nicotic acid dissolves.