MINISTRY OF EDUCATION AND SCIENCE
UNIVERSITY OF LATVIA
RIGA TECHNICAL UNIVERSITY

42nd Latvian National Chemistry Olympiad (2001)
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 32 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/!

How to realize chemical reactions shown in schema below? Write down equations of the reactions and characterize conditions for reactions

		HCl
NH3		H2		H2S
		H2O

An unknown salt consists of 4 elements. The amount ratio between sulfur and oxygen is 1:2. Ratio between oxygen and hydrogen is also 1:2. Oxygen forms 21,20% by mass of the compound.

Find the formula of this compound and name it. If it's possible mention the compound's acquisition and usage in industry

While preparing meal for party 1 m³ of natural gas was used. Everybody was in hurry. Grandmother was taking active part and being afraid of draught kitchen was not ventilated. Panes were transpired. How changed the composition of air in the kitchen? How hosts of party were feeling? Write down equations of reactions.

Calculate volume of used and formed gaseous compounds. Assume natural gas to consist only from methaneCH₄.

It was winter and a fireplace was heated. There were a lot of carbon blazing in fireplace somewhere blue flames were to see. Soon a merrily party began.

What could frustrate fun of the party? Write down equations of reactions as well as chemical and common names of all compounds

5,60 g of the Fe, Zn, Cu alloy was treated with the excess of diluted sulfuric acid. A 92,8 g of liquid was obtained. There was 5,32% of zinc sulfate and 5,86% of iron sulfate in the obtained liquid.

Find composition of given fusion!

NH₄Cl is a salt - a solid substance while all the elements forming it are gaseous

Write equations showing how to obtain NH₄Cl from elementary substances!

Calculate volumes of required gases to obtain 50,0 g of NH₄Cl!

There was 0,284 g of chlorine in the reaction vessel. Burning hydrogen was introduced in it. After the was reaction completed 10 ml of water was added and all obtained gas was completely dissolved

Calculate the composition of solution in mass %!

One flask contains 90 ml of water, the second one - 90 ml of hydrogen and the third one - 90 ml of 20% sodium hydroxide solution (density - 1,186 g/ml).

Which flask contains the highest and which the lowest amount of substance? By what factor they differ?

Mixture of magnesium and magnesium carbonate reacted with an excess of formic acid, 2,91 liters of gas was formed. Identical sample was heated up to 600 ^oC and the mass dropped by 34,8%.

Calculate the composition of given sample!

In temperature of 20^oC saturated solution of some univalent metal chloride contains 45,3 % by mass of the chloride. From 60,0 g of solution 5,00 g water was vaporized and as a result of this, 9,26 g of substance precipitated from the solution.

What compound was in the solution and what compound precipitated from it?

If there are more than one compound that fits the conditions note how can one experimentally determine which one is in the solution!

Ag AgNO₃ Zn(NO₃)₂ ZnS ZnSO₄ Na₂[Zn(OH)₄] Zn(OH)₂ [Zn(NH₃)₄]Cl₂

There are four salts dissolved in water. Masses of their ions are given: Na⁺ 5,52g , K⁺ 21,8g , NH4⁺ 18,0g , SO₄^2- 59,5g , CO₄^2- 10,8g , NO₄^- 12,4g.

Find the formulae and quantity of dissolved salts

A thin tape of a silver-white metal was burned in air and immediately put in a blow of water steam. The metal kept on burning. Two products formed - a simple substance and a compound. The compound (containing 39,7 % oxygen by mass) easily dissolves in diluted acid. When barium chloride was added to the obtained solution white insoluble precipitate was formed.

Which metal fits these conditions?

Write equations for reactions happened!

Try to explain why in some rust (rust uranium oxide; rust of thorium oxide etc.) in form of [micro inclusion] helium is found. Base your answer on appropriate equations (examples)!

There are the physical properties of NF₃, PF₃, AsF₃ and SbF₃ given in a table below.

Find the matching row for each compound. Motivate your answer.

In a durable vessel N₂ and H₂ in stoichiometric ratio for obtaining ammonia is refilled. In temperature of 237^oC the pressure in the vessel was 2119 kPa. Catalyzer was introduced and the reaction took place. After the reaction in temperature of 227^oC the pressure was 1298 kPa.

Determine the composition of gas mixture after reaction and degree of conversation!

Mixture of zinc, zinc oxide and zinc carbonate with mass of 49,1 g was dissolved in the excess of formic acid. 6,72 l (normal conditions discounting the solubility of gases) of gas was formed. The obtained solution was vaporized until its mass was 500,0 g. The mass part of zinc chloride in the obtained solution by mass was 14,5 %.

Determine the composition of initial mixture.

Determine the concentration of methane acid (HCOOH) knowing that the tenth part of all molecules are dissociated. K_A = 1,77·10^-4 mol/l.

Note. Formic acid splits off only the hydrogen attached to oxygen:

HCOOH HCOO^- + H⁺

18.

10

12 p.

Magnesium is usually obtained from dolomite. The dolomite is first heated and afterwards treated with seawater. precipite is obtained containing more magnesium and a lot less calcium as dolomite.

1. Write equations for all reactions. Accounting magnesium hydroxide to dissolve in water a lot worse than calcium hydroxide but magnesium carbonate to dissolve much better than calcium carbonate.
   1,00 t of dolomite containing 12,0% of magnesium (in form of carbonates) was heated.
2. Seawater contains 1,29 g magnesium, 0,412 g calcium and 0,267g HCO₃^- ions (on an average) per liter. Calculate the volume of seawater needed to treat the mixture obtained while heating dolomite. Assume that dolomite consists only of magnesium carbonate and calcium carbonate. If you use other assumptions in your calculations explain them.
3. Solubility of magnesium hydroxide in the water at 25 °C is 6,4·10-4 g in 100 ml. Calculate mass of magnesium remained in the water after treating the heated dolomite.
4. Write equations of reactions used to obtain magnesium from the product of reaction of dolomite with seawater.
5. Calculate mass of obtained magnesium. Explain assumptions used in calculations.
6. Explain what additions may contain magnesium obtained in this way. Write the appropriate equations.

Cu CuSO4 Cu(NO3)2 CuO CuCl2
Cu CuSO4 Cu(NO3)2 CuO (CuOH)2CO3

A surface of 24,0 cm² must be coated with a 12 mm thick layer of silver. How long the electrolysis must be carried out with density of current 0,8 A/cm² to coat the surface? What's the mass of silver precipitated during the electrolysis? (Ag)=10,5g/cm³

Prove that 1,0 gram of mixture of any proportions of Y₂(CO₃)₃ and CoCO₃ will release the same quantity carbon dioxide if treated with excess of formic acid.

In 1669 German alchemist Henning Brand while searching for philosopher's stone carried out the following experiment:

He filled retort with urine and evaporated it. Then he added carbon (charcoal) and sand to the dry residue and kept on heating. Volatile products of reaction were collected under water. After long heating the retort cracked and content of flask begun to shine.

The experiment is shown on the painting of Englishman Josef Wright of Derby (1734-1787) below.

What chemical element was obtained in the described way?
Base Your answer on logical judgments explaining the role of each substance and showing respective equations and schemes of reactions.

There are installed special systems to provide suitable air condition, ventilation, lighting and air humidity in the modern artwork's repositories such as Louvre in Paris or London National Gallery. These factors are important canvas to last protractedly.

It's known that many artists could not afford "good" paints so they bought the cheapest or made themselves using cheaper low quality staples. One of such "cheap" oil is blue paint containing mineral azurite 2CuCO₃·Cu(OH)₂. After a long quest a famous painting was found in a humid cellar and blue sky in that painting had become green.

Knowing artist has used the cheap blue oil in that picture try to explain this interesting observation.

What in Your opinion means term "good paint"?

Two students are making their homework and they have come to N₂H₄ Lewis structure. Janis put single bond between both atoms of nitrogen and a lone pair electrons at each of the nitrogen atoms, while Edgars puts a triple bond between two nitrogens. In both cases number of electrons at nitrogen's atoms is correct and molecule is symmetric.

Which structure is correct? Why? Name this compound!

Note: While solving this problem don't use ready-to-wear rules as for example octet rule but fundamental concepts linked to structure of atom, orbitals, substance's properties.

While heating mixture of aluminum and sulfur powder a chemical reaction took place resulting a grayish merged aggregate. Water was added to this product until another reaction ended. during this process:

1. gas X was liberated. It forms 1,34 g of precipite when reacts with an excess of iodine/potassium iodide solution;
2. in the vessel remained precipite which dissolves in the excess of formic acid liberating 0,269 l of gas (normal conditions).

Calculate the composition of initial mixture.

Bubling a colorless gas (density at 25,0^oC; 100 kPa r = 1,534 g/L) through the solution of KI, its volume volume remains the same but the solution becomes brown. Results of elemental analysis shows that the gas consist only of one element.

Name the initial gas? Write equation for correspondent reaction.

It's known that at 3000 K equilibrium constants of reactions

are accordingly K₁ and K₂.

Calculate the equilibrium constant of reaction

Prove that equilibrium constant of reaction CO₂ (g) + H₂ (g) H₂O (g) + CO (g) K₃ =

Write down all equations and give the conditions of these reactions.

In certain conditions following reactions take place

     aA bB.

It's known that the speed of reaction A B is higher than that of B A.
In the picture below changes in A and B concentration are shown.

Which curve corresponds to A and which to B?
What could the values for the be coefficients a and b?
Give some specific example to satisfy the given data.

Equimolar mixture of two liquid organic compounds completely reacted with:

1. 25,0 ml of NaOH solution c = 1,00 mol/l;
2. sodium (metallic) evolving 0,560 l of gas (normal conditions);
3. both compounds reacted with each other in presence of strong acid forming volatile compound with specific odor.

Known that the mixture doesn't decolorizes bromine water while burning the same quantity of mixture 2,24 l of CO₂ and 2,25 g of H₂O occured.

Name both unknown substances X and Y! Calculate the composition of mixture.

2,0 liters of 0,10 M sodium hydrogen carbonate solution was made. Density of the solution was 1,0 g/ml and pH - 8,3. 1,0 l of the solution was electrolyzed with current of 1,0 A for 1,0 h using inert electrodes. During the electrolysis the solution was intensively mixed. Afterwards the pH remained the same as it was before.

The rest 1,0 l of initial solution was also electrolyzed for 1,0 h using inert electrodes. All conditions remained the same except the mixing and a porous membrane was put between the partitions of electrodes. After electrolysis the solutions from anode's and cathode's partitions were combined. pH of obtained solution was higher as that before electrolysis.

1. Explain why in the second case a solution with higher pH was obtained.
2. Calculate concentration of dissolved substances in both solutions after electrolysis assuming density to remain constant during electrolysis.
3. Calculate pH of the second solution.

F = 96500 C/mol. Acid constants of carbonic acid are 4,45·10^-7 mol/l and 4,69·10^-11 mol/l.

Food additive E282 prevents mildewing of bread. E282 is a white crystalline substance coloring the flame brick-red. E282 is water-soluble and solution of it is weak basic.

A 100,0 ml of water solution was made from 1,000 g of E282. To a portion of 10,00 ml 20,00 ml of 1,000·10^-1 formic acid was added. The excess of formic acid was titrated with 9,777·10^-2 M of sodium hydroxide. 9,47 ml of hydroxide solution was used.

After titration, an excess of sodium oxalate (ethanedionate) solution was added. The occured precipite was filtrated and heated at 1000^oC. 3,011·10^-1 g of white substance was obtained which easily dissolved in solution of some weak univalent acid. After evaporating the solution only one compound was obtained - 1,000 g of E282.

Write down all equations and determine structure of E282.

Compound A is a colorless liquid. Water solution of it is used in food industry and concentrated solutions has stinging taste. B is also a colorless liquid whose water solution is used in food industry and it tastes acetous. 9,2 g of compound A reacted with 12 g of compound B and 17,6 g of compound C and compound D occured. Compound C is a colorless liquid with a specific odor it's water insoluble and is widely used as eluent. Compound D also is a colorless liquid without taste or odor in nature it's found in gaseous, liquid or solid condition.

1. Determine compounds A - D.
2. Write reactions showing how compounds A and B are industrially obtained in Latvia.
3. Explain what concentration of A and B solutions are used in food industry.
4. Write the equation for reaction between A and B and point conditions of reaction.
5. Write the mechanism of reaction between A and B.
6. Reaction between A and B is reverse. Calculate the equilibrium constant for this reaction at 50 ^oC. Show the direction of equilibrium. Thermodynamic data of A, B, C and D are given in table below.
7. Explain how can the outcome of C be increased for equimolar quantities of A and B.

While hydrolyzing some tripeptide with a diluted acid only alanine was obtained. When treated with ferments aminopeptidase and carboxypeptidase the tripeptide doesn't hydrolyzes.

Alanine - CH₃CH(NH₃⁺)COO^-.
Aminopeptidase - a ferment splitting off N-end amino acid.
Carboxypeptidase - a ferment splitting off C-end amino acid.

1. Write structure of tripeptide and equations for its hydrolysis reaction.
2. Explain dependence of tripeptide's water solubility from pH.
3. Draw all optical isomers of tripeptide and determine absolute configuration of hiral atoms.