Written part

Written tasks and essays — checked by AI

Chemistry and biology tasks 41–43 and the native-language essay, by the official criteria. Write your solution — the AI scores every part and tells you what is missing.

Chemistry

24 tasks

01A closed 2 L vessel was charged with 0.6 mol of nitrogen and 1.8 mol of hydrogen and heated with a catalyst: N₂ + 3H₂ ⇌ 2NH₃. At equilibrium the total amount of gas had decreased by 20 % compared with the initial amount (constant temperature).25 points
02In the lab, 400 g of an 8 % copper(II) sulfate solution (density 1.084 g/mL) was prepared from blue vitriol (CuSO₄·5H₂O) and water. The solution was then electrolysed with inert electrodes at 5 A, and electrolysis was stopped when the mass of the solution had decreased by 12 g.25 points
03Complete combustion of 5.6 L (STP) of a methane–ethane mixture (liquid water formed) released 289.515 kJ of heat. Enthalpies of formation (kJ/mol): ΔfH°(CH₄) = −74.8; ΔfH°(C₂H₆) = −84.7; ΔfH°(CO₂) = −393.5; ΔfH°(H₂O, l) = −285.8.25 points
04The reaction 2NO(g) + O₂(g) → 2NO₂(g) is treated as elementary, i.e. its rate obeys the law of mass action. Initially [NO] = 0.4 mol/L and [O₂] = 0.3 mol/L; after 20 s the NO concentration fell to 0.2 mol/L. The temperature coefficient of the reaction is γ = 2.5.25 points
05Natural chlorine consists of the isotopes ³⁵Cl and ³⁷Cl; its mean relative atomic mass is 35.45 (take the isotopic masses equal to the mass numbers). A 22.18 g sample of calcium chloride made from natural chlorine is also given (Ar(Ca) = 40).25 points
06The dissociation constant of acetic acid is Ka = 1.8·10⁻⁵ (25 °C). Treat the weak electrolyte with α ≪ 1. Helpful values: √1.8 ≈ 1.342; lg 1.342 ≈ 0.128; lg 4.243 ≈ 0.628; lg 1.8 ≈ 0.255; lg 5.27 ≈ 0.722; Kw = 1·10⁻¹⁴.25 points
07When 7.8 g of a magnesium–aluminium mixture was completely dissolved in excess hydrochloric acid, 8.96 L (STP) of hydrogen was evolved.25 points
08A mixture of carbon(II) oxide and carbon(IV) oxide has a density relative to helium of 9. 11.2 L (STP) of the mixture is taken.25 points
09The following sequence is given:25 points
10Among the compounds with formula C₄H₈O₂ there are carboxylic acids and esters. Some give the silver-mirror test, others release a gas with sodium hydrogen carbonate.25 points
112-Methylbutane was reacted with bromine in light (1 : 1 mole ratio). The main product was isolated and heated with alcoholic potassium hydroxide.25 points
12The following sequence is given:25 points
13Glucose is the most abundant monosaccharide. It gives reactions typical both of aldehydes and of polyhydric alcohols, and in solution exists mainly in cyclic forms. Fermentation of glucose yields various substances.25 points
14Three isomers A, B and C of composition C₃H₈O are given. A and B release hydrogen with sodium; C does not react with sodium. Hot copper(II) oxide converts A into a substance that gives the silver-mirror test and B into one that does not. Heating A or B with concentrated sulfuric acid at 170 °C gives the same gaseous product.25 points
15Aniline is an important raw material for dyes and drugs. The transformations are:25 points
16Polymers are everywhere: polyethylene bags, PVC pipes, polystyrene containers, rubber tyres, capron (nylon-6) and lavsan (PET) fibres. They are made by addition polymerisation or polycondensation.25 points
1711.2 g of iron was dissolved in excess dilute sulfuric acid. A 5 % potassium permanganate solution was then added (in the presence of sulfuric acid) until it stopped being decolourised. Excess sodium hydroxide solution was then added; the precipitate was separated and heated to constant mass.25 points
18Two identical 19.2 g copper samples were taken. The first was completely dissolved in excess concentrated nitric acid, the second in excess dilute nitric acid. The salt from the second experiment was isolated and heated to constant mass, and the evolved gases were completely absorbed in 200 g of water.25 points
19Sodium hydroxide solution was added stepwise to a solution containing 26.7 g of aluminium chloride: first 0.6 mol, then another 0.1 mol of NaOH. Alkali was then added until the precipitate dissolved completely, and carbon dioxide was passed through the solution until no more precipitate formed. The precipitate was separated and heated.25 points
20Excess concentrated hydrochloric acid acted on 15.8 g of potassium permanganate. The chlorine evolved was split into two equal parts. The first was passed through hot (excess) potassium hydroxide solution; the oxygen-containing salt formed was isolated and heated with MnO₂. The second part reacted with excess heated iron.25 points
21To model sulfuric acid production, 24 g of pyrite (FeS₂) was completely roasted in excess air. The sulfur(IV) oxide formed was oxidised by oxygen over a catalyst (80 % yield), and the sulfur(VI) oxide obtained was completely absorbed in 100 g of 9.8 % sulfuric acid.25 points
22Chemical analysis. A solution brought to the laboratory is thought to contain barium, iron(III), copper(II) and ammonium nitrates. Adding excess dilute sulfuric acid to 100 mL of it gave 2.33 g of a white crystalline precipitate insoluble in acids. Excess ammonia solution was added to one portion of the filtrate; other portions were tested with separate reagents.25 points
23Chemical analysis (permanganatometry). A sample containing an iron(II) salt was dissolved in dilute sulfuric acid and made up to the mark in a 250 mL volumetric flask. A 20.00 mL aliquot was titrated, in the presence of sulfuric acid, with 0.0200 mol/L potassium permanganate; 16.00 mL of titrant was used to the equivalence point.25 points
24Chemical analysis. Four unlabelled tubes contain solutions of sodium chloride, sodium iodide, sodium carbonate and sodium sulfate. Only dilute nitric acid, barium chloride, silver nitrate and ammonia solution are available. The sodium chloride solution was made by dissolving 1.17 g of the salt.25 points

Native language and literature

16 tasks

Biology

24 tasks