44th Latvian National Chemistry Olympiad (2003)
Theoretical problems

Honorable olympian!

Organizing committee of the 44th 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.

Good luck!

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

 

1.

9

5 p.

5.0 g of calcium carbonate (containing 3.0 % of alloy) were heated. The obtained gas was absorbed by previously weighted sodium hydroxide.

1. What was the increase of mass if 98% of the gas were absorbed by the sodium hydroxide?

2. Write down equations of proper reactions!

3. What will be mass of the calcium carbonate after heating?

4. Name a compound besides gas emerged while heating calcium carbonate. What will be the fraction of mass of it in the remains?

2.

9

7 p.

Acetic acid and citric acid are comonly used compounds. Their formulae are as follows:

Acetic acid is a weak one-valent acid while citric acid is a weak three-valent acid.

  1. Cicle residuals in the formulae of the acids.

    2. Both acetic and citric acids are appropriate means for removal of incrustation from dishes. 10.0 g of incrustation containing 80.0% calcium carbonate, 19.0% magnesium carbonate and 1.00% ferrum(III) oxide was formed in some vessel.

  2. Calculate the volume of 70% acetic acid (density - 1.07 g/ml) needed to clean the vessel. Write down equations for appropriate equations.

  3. Calculate mass of citric acid monohydrate C6H8O7·H2O needed to clean the vessel. Write down equations for appropriate equations.

  4. Explain which acid is more comfortable to use.

 

3.

9

7 p.

If the USA decides to attack Iraq one of the possible ways of retaliation of Iracis is burning the oilfields as it was in the first Persian Gulf War.

What wolume of CO2 and CO and how many tonns of water and grim would emerge if Iraq will burn down "just" 0,6 km3 (cubic kilometers) of oil with density of 750 kg/m3? The elemental composition of oil is as follows: 87% C and 13% H. On average 80% of carbon will form CO2, 7% - CO and 13% - grim.

4.

9

11 p.

200ml of limewater was neccessary for a practical task. Technician decided to make it using calcium oxide. Bottle containing calcium oxide was not firmly closed. And was covered by a thick layer of dust formed during years. The technician decided not to use this substance. He weighted 2.0g of calcium and put it in a mensure containing 200ml of water.

  1. Write down equations of reactions for prepearing limewater from calcium oxide.

  2. Why the technician decided not to use calcium oxide?

  3. Write down equations of reactions for process emerged in the mensure and calculate mass for all compounds emerged.

  4. Is it possible to use the content of the mensure for the practical task? Base your answer on calculations noting that at 20°C 0.16g of calcium hidroxide can be solved in 100g of water.

  5. Did the technician use the reagents economically? Explain your answer.

  6. What would you do in his position?

  7. What is fraction of mass in a saturated limewater at 20°C?

  8. What are storage procedures for the obtained solution?

  9. In what proval reactions limewater is used?

 

5.

9-10

7 p.

The art of fireworks was formed in the East while in the Europe the black powder was created which is the main compound for the pyrotechnics materials. The Franciscan monk Roger Beakon was exploring properities of saltpetre (KNO3) and discovered a way to create the black powder. The composition of it remains practically unchanged since 1242: 75% of Indian saltpetre (KNO3), 10% of sulfur, and 15% - carbon.

In explosion of the black powder potassium sulfide, the main compound of the air and carbon dioxide are formed.

  1. Write down equations for reaction corresponding to explosion of the black powder.

  2. For creating of 30 grams of the black powder a charcoal containing 20% of non-flammable alloy was used. The presence of alloy was taken into account while making the powder. Calculate the mass and fraction of mass of carbon in the freshly made poeder.

  3. 50 grams of the black powder were made by using only pure reagents. Will there be enough oxygen to burn all the carbon according to the equation of reaction?

  4. Will all the sulfur be converted into potassium sulfide? Base your answers on calculations!

 

6.

10

4 p.

The comb present in the mucous membrane of a human stomach secrete ~0.8% muriatic acid. The acidous medium is essential for successful action of enzymes splitting nutritives. The optimum pH in the stomach is -2.

  1. Calculate the optimum concentration of muriatic acid in the stomach.

    2. Enlarged concentration of acid causes burning and may cause serial affections of stomach. To decrease concentration of acid one may use some medical preperates containing bases. An example of such preperates is Rennie. One pastille of Rennie vontains 680 mg of calcium carbonate and 80 mg magnesium carbonate

  2. Calculate the less number of pastilles needed to elevate pH from 1 to 2, assuming stomach to contain 0.3 l of liquid.

 

 

 

7.

10

6 p.

Some unidentified crystalline substance X was explored. To identify this substance one needed to determine its molar weight. Physical methods of exploration were used to determine the crystalline structure of the substance. It proved to be a ionic type criystal grid. The element forming crystal grid is cube with length of edge 4.13·10-8 cm and the cube is formed by only one formular unit of substance X (it's like the cation is inside the cube and each peak of cube is formed by 1/8 of the anion). Later was discovered the density to be 3.97 g/cm3.

Calculate molar weight of substnce X and determine Aprēķiniet vielas X molmasu un nosakiet, kas tā ir par vielu, ja tās formulvienību veido IA grupas metāls un kāds halogēns.

 

8.

10

7 p.

The solution contained a soluble compound of a two valent metal Me, mass of the metal's ions in the solution was 1.00g.Sulfate of the metal is practically insoluble. Sulfuric acid (concentration c = 0.1 mol/l) was gradually added to the solution. Experiment was ended when 100 ml of the solution of sulfuric acid were added.

  1. Calculate mass of the precipitation after experiment if Me = Sr ; Ba.

  2. Construct a mathematical equation showing the mass of precipitation m depending on the volume of sulfuric acid v added during the experiment.

  3. Represent obtained equation graphically if Me = Sr ; Ba.

 

9.

10-11

6 p.

A plastic bottle in the lab contained an unidentified liquid. The liquid consistet of two layers which didn't mix together. They were seperated to identify.

One of the layers turned out to be an organic compound with molar wieght 119.5 g/mol. It consisted of 0.008 fractions by mass H, 0.891 - Cl and one other element.

1) Determine chemical composition of the organic substance.

The second layer was a water solution of two compounds coloring paper of universal indicator red.

2) What substances may cause coloring paper of universal indicator red?

10 ml of the solution was titrated with 0.05 M NaOH. 4 ml of NaOH were used for titration. Afterwards there was only one substance in the solution constitued of two elements in proportions 1:1 and the relation between molar wights of the elements is 0.648.

3) Determine which two substances were present in the initial sample.

The titrated sample was steamed and the dry remains weighted. Mass was 0.1287 g.

4) Determine amount of both substances in the initial sample.

5) Note the correct answer:

6) How do you think is it correct to store these substances in a plastic bottle? Motivate your answer.

 

10.

11

7 p.

Several features may attest a chemical reaction: ooze of light or heat, change of colour, presence of some odours, formation of precipitation or gas.

Write one chemical equation where as much as possible of the named features are present.

11.

11

5 p.

In a 5 liters gas-bag at a pressure of 2 atm (1 atm = 101,325 kPa) and at a temperature of 20oC a mixture of propane (C3H8) and butane (C4H10) was present. This mixture was completely burnt and 672 kJ of thermal energy was released.

Calculate composition of the mixture in molar parts assuming that burning one mole of butane 2877 kJ are released and one mole of propane - 1195 kJ.

R=8,314 kPa*l*mol-1*K-1

0o C = 273,15 K

 

12.

11

6 p.

A and B are elementar substances and in normal conditions are gases. They react with an explosion even at –250 oC and form a substance C which is a weak acid in a water solution. Boiling point of the substance is at 19,5 oC and in this temperature steam density ratio versus hydrogen is 60. Raising the temperature this ratio decreases and reaches minimum above 90 oC.

  1. What are gases A and B?

  2. Determine substance C assuming that transmitting it through solution of calcium hydrogencarbonate a white precipitation is formed and some gas oozes.

  3. Explain why the density ratio of C versus hydrogenium decreases while increasing temperature?

13.

11-12

11 p.

One of the oldest known metals is copper.

Our ancestry copper and its alloys used to make bijouterie, housecraft requisites and weapons. Today copper is widely used in technique.

    1. What for is used copper of very high purity (99,95%)? Why especially copper?

Ores usually contains less than 5% of copper. To determine exact fraction of mass of copper in a sample of ore containing copper oxides a chemical analysis is made. concentration of copper ions usually is done fotometrically exploring absorption of light in solutions. Absorption A is proportional to its concentration in solution:

A = ε b c,

where ε – molar absorption of the substance (if c= 1 mol/l), b – path of the light ray, cm, c – concentration of the substance, mol/l.

A sample with mass 1.000g was treated with excess of sulfuric acid. Colour of copper hydrate ion is not intensive enough Therefor excess of sulfuric acid was neutralized with NaOH and some ammonia was added and the solution was diluted to 500 ml with water. A dark blue solution was obtained.

2) Write equations of the chemical reactions noted above!

3) What are complex compounds, how do they form?

Absorption of the solution was investigated in a layer of 1 cm and obtained A = 0.150. Molar absorption of copper tetraaminocomplex is 120 l / (mol.cm)

4) Calculate fraction of mass for copper in the sample!

Nowdays copper usually is obtained in a hydrometalurgical process where ore containing copper oxide is extracted by sulfuric acid and then copper is obtained from the solution by electrolysis.

5) Write equation for electrolysis of copper sulfate!

Calculate time to obtain 1.00kg of copper in an electrolysis plant if current is 1500 A and outcome by current is 90%.

While exploring copper compounds a young chemist decided to obtain copper (II) hydroxide. To be sure he added a lot of concentrated NaOH to a solution of copper sulfate. In the beginning really a blue precipitation formed but they dissolved when more solution was added.

Then he decided to obtain copper (II) iodide mixing copper(II) sulfate with excess of potassium iodide. And again something failed instead of a nice blue colour a brown precipitation formed which turned violet when gasoline was added.

6) Help your collegue! Write equations for described reactions and explain why the youbg chemist failed to obtain wanted substances?

 

14.

12

5 p.

To describe fraction of unsaturated organic substances in some samples like fat and oil the so called iodine number is used which shows mass of iodine in grams which will react with 100 g of substance saturating the unsaturated bounds.

Sample of fat substances consists of glycerin tripalmitate and glycerin trioletate. Calculate amount of glycerin trioletate in the sample in mol% if the iodine number for the sample is J = 6.35.

Palmitic acid - C16H32O2, oleic acid - C18H34O2.

 

15.

12

17 p.

Just 50 years ago soda was widely used for washing now its completely replaced by washing powder. It always contains one or more surface - active substances whose molecule contains a long hydrofobe residual of hydrocarbon and a hydrofile group usually forming ions. Most of powders also contains enzymes, water mitigaters and some strong oxidiser. for example a washing powder might contain the following substances: peptidase, sodium salt of some alkylbenzenesulfonic acid, sodium perborate and sodium triphosphate.

  1. What is the medium in soda solution? Motivate your answer with equation of protolysis (hydrolysis) reaction.

    2. What mystical changes occours to fat in a water solution of soda while heating? What is the name for such reactions. Write what happens to such molecule of fat:

    Name all substances formed in the reaction.

  2. To wich of named substance classes belongs each substance constituing washing powder?

  3. Hidrofobe groupe of surface active substance during the washing is included inside the fat paricles while the hydrofile remains in the water. Show how the surface active substance places on the surface of fat and enables its removal from contexture.

  4. What class of substances is split by enzyme contained by washing powder? What is name for chemical bound split by enzyme? What class of substances belongs enzyme itself?

    5. Write an example for enzyme catalysed reaction. Choose any simple built compound which may be split by enzyme as substrate.

    What is optimum temperature for anzymatic reactions? How boiling affects structure and action of enzyme?

  5. Water mitigater included in the washing powder forms soluble complex compounds with ions of calcium and magnesium. Show structure for anion.

    6. In a hard water fatty acids forms insoluble salines of calcium and magnesium whic precipiates on surface of contexture. Write an example for such reaction (note the medium present in the solution).

  6. Determine degree of oxidation for each atom in the oxidiser added to washing powder.

The oxidiser slowly hydrolyses in water solution forming hydrogen peroxide and one valent anion where the central atom is bound to four equal groups. Write equation for this reaction (none of atoms changes degree of oxidation in this reaction).

Write equations for half-reaction of reduction of hydrogen peroxide during washing. In what temperature and medium hydrogen peroxide would be the most effective as oxidiser?

For what kind of smutchs to clean oxidiser is added to powder?

 

16.

12

14 p.

The sweet taste of honey is caused by fructose (fraction of mass in honey - 40%), glycose (30%), maltose (7%) etc.Maltose is a disacharide which is formed when from two molecules of a-D-glycose one molecule of water splits off.

  1. Calculate volume of carbon dioxide needed by plants in photosynthesis so that bees may collect 1 kg of honey (in warm and sunny summer days when temperature is 25 oC and pressure 780 mm Hg). Write equation for photosynthesis.

  2. In a human body honey is "burnt" to obtain energy. .Write equations for appropriate equations

    3. Calculate volume of air needed to inhale for "burning" of 100 g of honey (in cold and rainy autumn day when temperature is 10 oC and pressure - only 740 mm Hg). Note that oxygen volume part in the air is 16%.

    Human inhales and exhales on average 14 times per minute each time inhaling ~ 0.5 l of air. Calculate how long a human hav to breathe to "burn" 100 g of honey.

    Combustion heat for glycose is 2816 kJ/mol, fructose – 2827 kJ/mol and maltose – 5649 kJ/mol. Calculate how many liters of water might be heated from a room temperature (20 oC) to temperature of human body (36,5 oC) with energy obtained by eating 100 g of honey specific heat for water is 4,18 J/(g·K).

  3. Glycose and fructose in honey is mainly formed from disacharide as a result of action bee's enzymes and acids Molecule of sacharose is formed by residuals of a-D-glycose and b-D-fructose and it is an unreducing carbohydrate. Write formula of sacharose and explain why it is an unreducing carbohydrate.

    4. Write equation for reaction where a-D-glycose and b-D-fructose are formed from saccharose.

    Splitting of saccharose is catalysed by enzyme invertase. Its molar wieght is 270 kDa (270 000 g/mol). On the bottle is said that 1 mg of invertase coincides to 500 units of activity. Enzyme with activity one unit splits 1 mmol of saccharose per minute (55 oC, pH 4,5). Calculate number of saccharose molecules split by one molecule of invertase in one second.

  4. Maltose is a reducing carbohydrate. Draw structural formula for maltose.

 

Good luck in solving problems!