1) Chromium (III) oxide.

Chromium oxide can be obtained:

Thermal decomposition of ammonium dichromate:

(NH 4) 2 C 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O

Reduction of potassium dichromate with carbon (coke) or sulfur:

2K 2 Cr 2 O 7 + 3C 2Cr 2 O 3 + 2K 2 CO 3 + CO 2

K 2 Cr 2 O 7 + S Cr 2 O 3 + K 2 SO 4

Chromium(III) oxide has amphoteric properties.

Chromium (III) oxide forms salts with acids:

Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O

When chromium (III) oxide is fused with oxides, hydroxides and carbonates of alkali and alkaline earth metals, chromates (III) (chromites) are formed:

Сr 2 O 3 + Ba(OH) 2 Ba(CrO 2) 2 + H 2 O

Сr 2 O 3 + Na 2 CO 3 2NaCrO 2 + CO 2

With alkaline melts of oxidizing agents – chromates (VI) (chromates)

Cr 2 O 3 + 3KNO 3 + 4KOH = 2K 2 CrO 4 + 3KNO 2 + 2H 2 O

Cr 2 O 3 + 3Br 2 + 10NaOH = 2Na 2 CrO 4 + 6NaBr + 5H 2 O

Cr 2 O 3 + O 3 + 4KOH = 2K 2 CrO 4 + 2H 2 O

Cr 2 O 3 + 3O 2 + 4Na 2 CO 3 = 2Na 2 CrO 4 + 4CO 2

Сr 2 O 3 + 3NaNO 3 + 2Na 2 CO 3 2Na 2 CrO 4 + 2CO 2 + 3NaNO 2

Cr 2 O 3 + KClO 3 + 2Na 2 CO 3 = 2Na 2 CrO 4 + KCl + 2CO 2

2) Chromium(III) hydroxide

Chromium(III) hydroxide has amphoteric properties.

2Cr(OH) 3 = Cr 2 O 3 + 3H 2 O

2Cr(OH) 3 + 3Br 2 + 10KOH = 2K 2 CrO 4 + 6KBr + 8H 2 O

3) Chromium(III) salts

2CrCl 3 + 3Br 2 + 16KOH = 2K 2 CrO 4 + 6KBr + 6KCl + 8H 2 O

2CrCl 3 + 3H 2 O 2 + 10NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O

Cr 2 (SO 4) 3 + 3H 2 O 2 + 10NaOH = 2Na 2 CrO 4 + 3Na 2 SO 4 + 8H 2 O

Cr 2 (SO 4) 3 + 3Br 2 + 16NaOH = 2Na 2 CrO 4 + 6NaBr + 3Na 2 SO 4 + 8H 2 O

Cr 2 (SO 4) 3 + 6KMnO 4 + 16KOH = 2K 2 CrO 4 + 6K 2 MnO 4 + 3K 2 SO 4 + 8H 2 O.

2Na 3 + 3Br 2 + 4NaOH = 2Na 2 CrO 4 + 6NaBr + 8H 2 O

2K 3 + 3Br 2 + 4KOH = 2K 2 CrO 4 + 6KBr + 8H 2 O

2KCrO2 + 3PbO2 + 8KOH = 2K2CrO4 + 3K2PbO2 + 4H2O

Cr 2 S 3 + 30HNO 3 (conc.) = 2Cr(NO 3) 3 + 3H 2 SO 4 + 24NO 2 + 12H 2 O

2CrCl 3 + Zn = 2CrCl 2 + ZnCl 2

Chromates (III) easily react with acids:

NaCrO 2 + HCl (deficiency) + H 2 O = Cr(OH) 3 + NaCl

NaCrO 2 + 4HCl (excess) = CrCl 3 + NaCl + 2H 2 O

K 3 + 3CO 2 = Cr(OH) 3 ↓ + 3NaHCO 3

In solution they undergo complete hydrolysis

NaCrO 2 + 2H 2 O = Cr(OH) 3 ↓ + NaOH

Most chromium salts are highly soluble in water, but are easily hydrolyzed:

Cr 3+ + HOH ↔ CrOH 2+ + H +

СrCl 3 + HOH ↔ CrOHCl 2 + HCl

Salts formed by chromium (III) cations and a weak or volatile acid anion are completely hydrolyzed in aqueous solutions:

Cr 2 S 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3H 2 S

Chromium(VI) compounds

1) Chromium (VI) oxide.

Chromium(VI) oxide. Highly poisonous!

Chromium(VI) oxide can be prepared by the action of concentrated sulfuric acid on dry chromates or dichromates:

Na 2 Cr 2 O 7 + 2H 2 SO 4 = 2CrO 3 + 2NaHSO 4 + H 2 O

Acidic oxide that interacts with basic oxides, bases, water:

CrO 3 + Li 2 O → Li 2 CrO 4

CrO 3 + 2KOH → K 2 CrO 4 + H 2 O

CrO 3 + H 2 O = H 2 CrO 4

2CrO 3 + H 2 O = H 2 Cr 2 O 7

Chromium (VI) oxide is a strong oxidizing agent: it oxidizes carbon, sulfur, iodine, phosphorus, turning into chromium (III) oxide

4CrO 3 → 2Cr 2 O 3 + 3O 2.

4CrO 3 + 3S = 2Cr 2 O 3 + 3SO 2

Oxidation of salts:

2CrO 3 + 3K 2 SO 3 + 3H 2 SO 4 = 3K 2 SO 4 + Cr 2 (SO 4) 3 + 3H 2 O

Oxidation of organic compounds:

4CrO 3 + C 2 H 5 OH + 6H 2 SO 4 = 2Cr 2 (SO 4) 2 + 2CO 2 + 9H 2 O

Strong oxidizing agents are salts of chromic acids - chromates and dichromates. The reduction products of which are chromium (III) derivatives.

In a neutral environment, chromium (III) hydroxide is formed:

K 2 Cr 2 O 7 + 3Na 2 SO 3 + 4H 2 O = 2Cr(OH) 3 ↓ + 3Na 2 SO 4 + 2KOH

2K 2 CrO 4 + 3(NH 4) 2 S + 2H 2 O = 2Cr(OH) 3 ↓ + 3S↓ + 6NH 3 + 4KOH

In alkaline – hydroxochromates (III):

2K 2 CrO 4 + 3NH 4 HS + 5H 2 O + 2KOH = 3S + 2K 3 + 3NH 3 H 2 O

2Na 2 CrO 4 + 3SO 2 + 2H 2 O + 8NaOH = 2Na 3 + 3Na 2 SO 4

2Na 2 CrO 4 + 3Na 2 S + 8H 2 O = 3S + 2Na 3 + 4NaOH

In acidic – chromium (III) salts:

3H 2 S + K 2 Cr 2 O 7 + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4) 3 + 3S + 7H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6KI = Cr 2 (SO 4) 3 + 3I 2 + 4K 2 SO 4 + 7H 2 O

K 2 Cr 2 O 7 + 3H 2 S + 4H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4) 3 + 3S + 7H 2 O

8K 2 Cr 2 O 7 + 3Ca 3 P 2 + 64HCl = 3Ca 3 (PO 4) 2 + 16CrCl 3 + 16KCl + 32H 2 O

K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 = Cr 2 (SO 4) 3 + 3Fe 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

K 2 Cr 2 O 7 + 4H 2 SO 4 + 3KNO 2 = Cr 2 (SO 4) 3 + 3KNO 3 + K 2 SO 4 + 4H 2 O

K 2 Cr 2 O 7 + 14HCl = 3Cl 2 + 2CrCl 3 + 7H 2 O + 2KCl

K 2 Cr 2 O 7 + 3SO 2 + 8HCl = 2KCl + 2CrCl 3 + 3H 2 SO 4 + H 2 O

2K 2 CrO 4 + 16HCl = 3Cl 2 + 2CrCl 3 + 8H 2 O + 4KCl

The recovery product in various environments can be represented schematically:

H 2 O Cr(OH) 3 gray-green precipitate

K 2 CrO 4 (CrO 4 2–)

OH – 3 – emerald green solution

K 2 Cr 2 O 7 (Cr 2 O 7 2–) H + Cr 3+ blue-violet solution

Salts of chromic acid - chromates - are yellow, and salts of dichromic acid - dichromates - are orange. By changing the reaction of the solution, it is possible to carry out the mutual conversion of chromates into dichromates:

2K 2 CrO 4 + 2HCl (diluted) = K 2 Cr 2 O 7 + 2KCl + H 2 O

2K 2 CrO 4 + H 2 O + CO 2 = K 2 Cr 2 O 7 + KHCO 3

acidic environment

2СrO 4 2 – + 2H + Cr 2 O 7 2– + H 2 O

alkaline environment

Chromium. Chromium compounds.

1. Chromium (III) sulfide was treated with water, gas was released and an insoluble substance remained. A solution of sodium hydroxide was added to this substance and chlorine gas was passed through, and the solution acquired a yellow color. The solution was acidified with sulfuric acid, as a result the color changed to orange; The gas released when the sulfide was treated with water was passed through the resulting solution, and the color of the solution changed to green. Write the equations for the reactions described.

2. After briefly heating an unknown powdery substance of an orange substance, an orange-colored substance begins a spontaneous reaction, which is accompanied by a change in color to green, the release of gas and sparks. The solid residue was mixed with potassium hydroxide and heated, the resulting substance was added to a dilute solution of hydrochloric acid, and a green precipitate was formed, which dissolves in excess acid. Write the equations for the reactions described.

3. Two salts turn the flame purple. One of them is colorless, and when it is slightly heated with concentrated sulfuric acid, the liquid in which copper dissolves is distilled off; the latter transformation is accompanied by the release of brown gas. When a second salt of a sulfuric acid solution is added to the solution, the yellow color of the solution changes to orange, and when the resulting solution is neutralized with alkali, the original color is restored. Write the equations for the reactions described.

4. Trivalent chromium hydroxide was treated with hydrochloric acid. Potash was added to the resulting solution, the precipitate that formed was separated and added to a concentrated solution of potassium hydroxide, as a result of which the precipitate dissolved. After adding excess hydrochloric acid, a green solution was obtained. Write the equations for the reactions described.

5. When dilute hydrochloric acid was added to the solution of a yellow salt, which colors the flame violet, the color changed to orange-red. After neutralizing the solution with concentrated alkali, the color of the solution returned to its original color. When barium chloride is added to the resulting mixture, a yellow precipitate forms. The precipitate was filtered and a solution of silver nitrate was added to the filtrate. Write the equations for the reactions described.

6. Soda ash was added to the solution of trivalent chromium sulfate. The resulting precipitate was separated, transferred to a solution of sodium hydroxide, bromine was added and heated. After neutralizing the reaction products with sulfuric acid, the solution acquires an orange color, which disappears after passing sulfur dioxide through the solution. Write the equations for the reactions described.

7) Chromium (III) sulfide powder was treated with water. The resulting gray-green precipitate was treated with chlorine water in the presence of potassium hydroxide. A solution of potassium sulfite was added to the resulting yellow solution, and a gray-green precipitate formed again, which was calcined until the mass was constant. Write the equations for the reactions described.

8) Chromium (III) sulfide powder was dissolved in sulfuric acid. At the same time, gas was released and a solution was formed. An excess of ammonia solution was added to the resulting solution, and the gas was passed through a lead nitrate solution. The resulting black precipitate turned white after treatment with hydrogen peroxide. Write the equations for the reactions described.

9) Ammonium dichromate decomposed when heated. The solid decomposition product was dissolved in sulfuric acid. A solution of sodium hydroxide was added to the resulting solution until a precipitate formed. Upon further addition of sodium hydroxide to the precipitate, it dissolved. Write the equations for the reactions described.

10) Chromium (VI) oxide reacted with potassium hydroxide. The resulting substance was treated with sulfuric acid, and an orange salt was isolated from the resulting solution. This salt was treated with hydrobromic acid. The resulting simple substance reacted with hydrogen sulfide. Write the equations for the reactions described.

11. Chrome was burned in chlorine. The resulting salt reacted with a solution containing hydrogen peroxide and sodium hydroxide. Excess sulfuric acid was added to the resulting yellow solution, and the color of the solution changed to orange. When copper(I) oxide reacted with this solution, the color of the solution turned blue-green. Write the equations for the reactions described.

12. Sodium nitrate was fused with chromium(III) oxide in the presence of sodium carbonate. The gas released reacted with an excess of barium hydroxide solution, forming a white precipitate. The precipitate was dissolved in an excess of hydrochloric acid solution and silver nitrate was added to the resulting solution until the precipitation stopped. Write the equations for the reactions described.

13. Potassium was fused with sulfur. The resulting salt was treated with hydrochloric acid. The gas released was passed through a solution of potassium dichromate in sulfuric acid. the precipitated yellow substance was filtered and fused with aluminum. Write the equations for the reactions described.

14. Chrome was burned in a chlorine atmosphere. Potassium hydroxide was added dropwise to the resulting salt until the precipitation stopped. The resulting precipitate was oxidized with hydrogen peroxide in sodium hydroxide and evaporated. An excess of a hot solution of concentrated hydrochloric acid was added to the resulting solid residue. Write the equations for the reactions described.

Chromium. Chromium compounds.

1) Cr 2 S 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3H 2 S

2Cr(OH) 3 + 3Cl 2 + 10NaOH = 2Na 2 CrO 4 + 6NaCl + 8H 2 O

Na 2 Cr 2 O 7 + 4H 2 SO 4 + 3H 2 S = Cr 2 (SO 4) 3 + Na 2 SO 4 + 3S↓ + 7H 2 O

2) (NH 4) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O

Cr 2 O 3 + 2KOH 2KCrO 2 + H 2 O

KCrO 2 + H 2 O + HCl = KCl + Cr(OH) 3 ↓

Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O

3) KNO 3 (tv.) + H 2 SO 4 (conc.) HNO 3 + KHSO 4

4HNO 3 + Cu = Cu(NO 3) 2 + 2NO 2 + 2H 2 O

2K 2 CrO 4 + H 2 SO 4 = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O

4) Cr(OH) 3 + 3HCl = CrCl 3 + 3H 2 O

2CrCl 3 + 3K 2 CO 3 + 3H 2 O = 2Cr(OH) 3 ↓ + 3CO 2 + 6KCl

Cr(OH) 3 + 3KOH = K 3

K 3 + 6HCl = CrCl 3 + 3KCl + 6H 2 O

5) 2K 2 CrO 4 + 2HCl = K 2 Cr 2 O 7 + 2KCl + H 2 O

K 2 Cr 2 O 7 + 2KOH = 2K 2 CrO 4 + H 2 O

K 2 CrO 4 + BaCl 2 = BaCrO 4 ↓ + 2 KCl

KCl + AgNO 3 = AgCl↓ + KNO 3

6) Cr 2 (SO 4) 3 + 3Na 2 CO 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3CO 2 + 3K 2 SO 4

2Cr(OH) 3 + 3Br 2 + 10NaOH = 2Na 2 CrO 4 + 6NaBr + 8H 2 O

2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O

Na 2 Cr 2 O 7 + H 2 SO 4 + 3SO 2 = Cr 2 (SO 4) 3 + Na 2 SO 4 + H 2 O

7) Cr 2 S 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3H 2 S

2Cr(OH) 3 + 3Cl 2 + 10KOH = 2K 2 CrO 4 + 6KCl + 8H 2 O

2K 2 CrO 4 + 3K 2 SO 3 + 5H 2 O = 2Cr(OH) 2 + 3K 2 SO 4 + 4KOH

2Cr(OH) 3 Cr 2 O 3 + 3H 2 O

8) Cr 2 S 3 + 3H 2 SO 4 = Cr 2 (SO 4) 3 + 3H 2 S

Cr 2 (SO 4) 3 + 6NH 3 + 6H 2 O = 2Cr(OH) 3 ↓ + 3(NH 4) 2 SO 4

H 2 S + Pb(NO 3) 2 = PbS + 2HNO 3

PbS + 4H 2 O 2 = PbSO 4 + 4H 2 O

9) (NH 4) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 4H 2 O

Cr 2 O 3 + 3H 2 SO 4 = Cr 2 (SO 4) 3 + 3H 2 O

Cr 2 (SO 4) 3 + 6NaOH = 2Cr(OH) 3 ↓ + 3Na 2 SO 4

Cr(OH) 3 + 3NaOH = Na 3

10) CrO 3 + 2KOH = K 2 CrO 4 + H 2 O

2K 2 CrO 4 + H 2 SO 4 (diluted) = K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O

K 2 Cr 2 O 7 + 14HBr = 3Br 2 + 2CrBr 3 + 7H 2 O + 2KBr

Br 2 + H 2 S = S + 2HBr

11) 2Cr + 3Cl 2 = 2CrCl 3

2CrCl 3 + 10NaOH + 3H 2 O 2 = 2Na 2 CrO 4 + 6NaCl + 8H 2 O

2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O

Na 2 Cr 2 O 7 + 3Cu 2 O + 10H 2 SO 4 = 6CuSO 4 + Cr 2 (SO 4) 3 + Na 2 SO 4 + 10H 2 O

12) 3NaNO 3 + Cr 2 O 3 + 2Na 2 CO 3 = 2Na 2 CrO 4 + 3NaNO 2 + 2CO 2

CO 2 + Ba(OH) 2 = BaCO 3 ↓ + H 2 O

BaCO 3 + 2HCl = BaCl 2 + CO 2 + H 2 O

BaCl 2 + 2AgNO 3 = 2AgCl↓ + Ba(NO 3) 2

13) 2K + S = K 2 S

K 2 S + 2HCl = 2KCl + H 2 S

3H 2 S + K 2 Cr 2 O 7 + 4H 2 SO 4 = 3S + Cr 2 (SO 4) 3 + K 2 SO 4 + 7H 2 O

3S + 2Al = Al 2 S 3

14) 2Cr + 3Cl 2 = 2CrCl 3

CrCl 3 + 3KOH = 3KCl + Cr(OH) 3 ↓

2Cr(OH) 3 + 3H 2 O 2 + 4KOH = 2K 2 CrO 4 + 8H 2 O

2K 2 CrO 4 + 16HCl = 2CrCl 3 + 4KCl + 3Cl 2 + 8H 2 O

Non-metals.

IV A group (carbon, silicon).

Carbon. Carbon compounds.

I. Carbon.

Carbon can exhibit both reducing and oxidizing properties. Carbon exhibits reducing properties with simple substances formed by nonmetals with a higher electronegativity value (halogens, oxygen, sulfur, nitrogen), as well as with metal oxides, water and other oxidizing agents.

When heated with excess air, graphite burns to form carbon monoxide (IV):

when there is a lack of oxygen, you can get CO

Amorphous carbon reacts with fluorine already at room temperature.

C + 2F 2 = CF 4

When heated with chlorine:

C + 2Cl 2 = CCl 4

With stronger heating, carbon reacts with sulfur and silicon:

Under the action of an electric discharge, carbon combines with nitrogen, forming diacine:

2С + N 2 → N ≡ C – C ≡ N

In the presence of a catalyst (nickel) and upon heating, carbon reacts with hydrogen:

C + 2H 2 = CH 4

With water, hot coke forms a mixture of gases:

C + H 2 O = CO + H 2

The reducing properties of carbon are used in pyrometallurgy:

C + CuO = Cu + CO

When heated with oxides of active metals, carbon forms carbides:

3C + CaO = CaC 2 + CO

9C + 2Al 2 O 3 = Al 4 C 3 + 6CO

2C + Na 2 SO 4 = Na 2 S + CO 2

2C + Na 2 CO 3 = 2Na + 3CO

Carbon is oxidized by such strong oxidizing agents as concentrated sulfuric and nitric acids, and other oxidizing agents:

C + 4HNO 3 (conc.) = CO 2 + 4NO 2 + 2H 2 O

C + 2H 2 SO 4 (conc.) = 2SO 2 + CO 2 + 2H 2 O

3C + 8H 2 SO 4 + 2K 2 Cr 2 O 7 = 2Cr 2 (SO 4) 3 + 2K 2 SO 4 + 3CO 2 + 8H 2 O

In reactions with active metals, carbon exhibits the properties of an oxidizing agent. In this case, carbides are formed:

4C + 3Al = Al 4 C 3

Carbides undergo hydrolysis, forming hydrocarbons:

Al 4 C 3 + 12H 2 O = 4Al(OH) 3 + 3CH 4

CaC 2 + 2H 2 O = Ca(OH) 2 + C 2 H 2

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Chemical formula

Molar mass of Cr 2 S 3, chromium (III) sulfide 200.1872 g/mol

51.9961 2+32.065 3

Mass fractions of elements in the compound

Using the Molar Mass Calculator

• Chemical formulas must be entered case sensitive
• Subscripts are entered as regular numbers
• The point on the middle line (multiplication sign), used, for example, in the formulas of crystalline hydrates, is replaced by a regular point.
• Example: instead of CuSO₄·5H₂O in the converter, for ease of entry, the spelling CuSO4.5H2O is used.

Molar mass calculator

Mole

All substances are made up of atoms and molecules. In chemistry, it is important to accurately measure the mass of substances that react and are produced as a result. By definition, the mole is the SI unit of quantity of a substance. One mole contains exactly 6.02214076×10²³ elementary particles. This value is numerically equal to Avogadro's constant N A when expressed in units of mol⁻¹ and is called Avogadro's number. Amount of substance (symbol n) of a system is a measure of the number of structural elements. A structural element can be an atom, molecule, ion, electron, or any particle or group of particles.

Avogadro's constant N A = 6.02214076×10²³ mol⁻¹. Avogadro's number is 6.02214076×10²³.

In other words, a mole is an amount of substance equal in mass to the sum of the atomic masses of atoms and molecules of the substance, multiplied by Avogadro's number. The unit of quantity of a substance, the mole, is one of the seven basic SI units and is symbolized by the mole. Since the name of the unit and its symbol are the same, it should be noted that the symbol is not declined, unlike the name of the unit, which can be declined according to the usual rules of the Russian language. One mole of pure carbon-12 is equal to exactly 12 g.

Molar mass

Molar mass is a physical property of a substance, defined as the ratio of the mass of this substance to the amount of substance in moles. In other words, this is the mass of one mole of a substance. The SI unit of molar mass is kilogram/mol (kg/mol). However, chemists are accustomed to using the more convenient unit g/mol.

molar mass = g/mol

Molar mass of elements and compounds

Compounds are substances consisting of different atoms that are chemically bonded to each other. For example, the following substances, which can be found in any housewife’s kitchen, are chemical compounds:

• salt (sodium chloride) NaCl
• sugar (sucrose) C₁₂H₂₂O₁₁
• vinegar (acetic acid solution) CH₃COOH

The molar mass of a chemical element in grams per mole is numerically the same as the mass of the element's atoms expressed in atomic mass units (or daltons). The molar mass of compounds is equal to the sum of the molar masses of the elements that make up the compound, taking into account the number of atoms in the compound. For example, the molar mass of water (H₂O) is approximately 1 × 2 + 16 = 18 g/mol.

Molecular mass

Molecular mass (the old name is molecular weight) is the mass of a molecule, calculated as the sum of the masses of each atom that makes up the molecule, multiplied by the number of atoms in this molecule. Molecular weight is dimensionless a physical quantity numerically equal to molar mass. That is, molecular mass differs from molar mass in dimension. Although molecular mass is dimensionless, it still has a value called the atomic mass unit (amu) or dalton (Da), which is approximately equal to the mass of one proton or neutron. The atomic mass unit is also numerically equal to 1 g/mol.

Calculation of molar mass

Molar mass is calculated as follows:

• determine the atomic masses of elements according to the periodic table;
• determine the number of atoms of each element in the compound formula;
• The molar mass is determined by adding the atomic masses of the elements included in the compound, multiplied by their number.

For example, let's calculate the molar mass of acetic acid

It consists of:

• two carbon atoms
• four hydrogen atoms
• two oxygen atoms

• carbon C = 2 × 12.0107 g/mol = 24.0214 g/mol
• hydrogen H = 4 × 1.00794 g/mol = 4.03176 g/mol
• oxygen O = 2 × 15.9994 g/mol = 31.9988 g/mol
• molar mass = 24.0214 + 4.03176 + 31.9988 = 60.05196 g/mol

Our calculator performs exactly this calculation. You can enter the acetic acid formula into it and check what happens.

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The stability of sulfides of metals of the sixth group increases with a decrease in the oxidizing properties of the metal atom, that is, as the oxidation state decreases and when moving down the group. The impossibility of obtaining chromium(VI) chalcogenides is explained by the high oxidizing ability of chromium in the highest oxidation state, while such compounds are known for molybdenum and tungsten.

When chromium is fused with sulfur, a shiny black mass is formed, consisting of a mixture of sulfides - in addition to CrS and Cr 2 S 3, it also contains intermediate sulfide phases Cr 3 S 4, Cr 5 S 6, Cr 7 S 8 (Fig. 5.33 Phase diagram of the system Cr–S). (Footnote: Chromium disulfide CrS 2 is also known: A. Lafond, C. Deudon et al, Eur. J. Solid State Inorg. Chem., 1994, 31, 967) Black chromium(II) sulfide can be precipitated from an aqueous salt solution chromium(II) with sodium sulfide or obtained by passing hydrogen sulfide over anhydrous chromium(II) chloride at 440 ºC, reducing chromium(III) sulfide with hydrogen and carbon monoxide. Like sulfides of other doubly charged cations, it has the structure of nickel arsenide. In contrast, chromium(III) sulfide cannot be precipitated from aqueous solutions due to complete irreversible hydrolysis. Pure crystalline Cr 2 S 3 is obtained by passing a current of dry hydrogen sulfide over anhydrous chromium chloride:

3H 2 S + 2CrCl 3 = Cr 2 S 3 + 6HCl.

The sulfide obtained in this way is black hexagonal plate-like crystals, like chromium(II) sulfide, insoluble in water and non-oxidizing acids. Both sulfides are decomposed by concentrated solutions of alkalis, nitric acid and aqua regia:

Cr 2 S 3 + 24HNO 3 = 2Cr(NO 3) 3 + 18NO 2 + 3SO 2 + 12H 2 O.

Chromium(III) thiosalts, which are actually mixed sulfides, are also known. In aqueous solutions they are stable only in an alkaline environment and in an excess of sulfide ions. Dark gray sodium thiochromate(III) powder NaCrS 2 is obtained by reducing chromate with sulfur in molten sodium carbonate at 800 ºC or by fusing chromium(III) oxide with sulfur and sodium carbonate:

Cr 2 O 3 + 6S + Na 2 CO 3 = 2NaCrS 2 + 2SO 2 + CO 2

The substance has a layered structure in which layers of CrS 6 octahedra, connected by edges, are separated by sodium ions. A similar lithium derivative LiCrS 2 has (B. van Laar, D. J. W. Ijdo, J. Solid State Chem., 1971, 3, 590). When boiling alkaline solutions of alkali metal thiochromates with salts of iron(II), cobalt, nickel, silver, zinc, cadmium, manganese(II) and other metals, thiochromates M I CrS 2 and M II Cr 2 S 4 precipitate. Cadmium thiochromate(III) is also formed by the reaction of thiourea with a chromium(III) salt and cadmium ammonia:

2Cr 3 + Cd(NH 3) 4 2+ + 4(NH 2) 2 CS + 8OH – = CdCr 2 S 4 + 4CH 2 N 2 + 8H 2 O + 4NH 3.

(R. S. Mane, B. R. Sankapal, K. M. Gadave, C. D. Lokhande, Mater. Res. Bull. 1999, 34, 2035).

Thiochromats(III) are semiconductors with antiferromagnetic properties and can be used as magneto-optical materials, the optical properties of which change under the influence of a magnetic field.

For molybdenum and tungsten, sulfides in various oxidation states from +2 to +6 are described. When hydrogen sulfide is passed through weakly acidified solutions of molybdates and tungstates, brown trisulfide hydrates precipitate:

(NH 4) 6 Mo 7 O 24 + 21H 2 S + 3H 2 SO 4 = 7MoS 3 ¯ + 3(NH 4) 2 SO 4 + 24H 2 O.

The structure of these compounds has not yet been studied. In a strongly acidic environment, the solution turns blue or brown due to the reduction of molybdate ions. If alkali is added to the initial molybdate solution, the oxygen atoms in the molybdate ions are successively replaced by sulfur atoms MoO 4 2–, MoSO 3 2–, MoS 2 O 2 2–, MoS 3 O 2–, MoS 4 2– – solution At the same time, it first turns yellow and then becomes dark red. In the cold, red crystals of thiosalt, for example, (NH 4) 2 MoS 4, can be isolated from it. Like other thiosalts, thiomolybdates and thiotungstates are stable only in a neutral and alkaline environment, and upon acidification they decompose, releasing hydrogen sulfide and turning into sulfides:

(NH 4) 2 MoS 4 + 2HCl = MoS 3 ¯ + 2NH 4 Cl + H 2 S.

Thiomolybdate and thiotungstate ions have the shape of a regular tetrahedron.

MoS 4 2– ions, due to the presence of sulfur atoms, are able to act as bridging ligands, forming complexes with transition metals that have a polymeric structure, for example, n n – . Interestingly, thioanalogs of isopolymolybdates and isopolytungstates have not yet been obtained.

The energies of the d-orbitals of Mo and W are closer in energy to the p-orbitals of sulfur than of oxygen, therefore the M═S bond turns out to be covalent and stronger than the M═O bond (M = Mo, W) due to strong pp-dp bonding. This explains the fact that soft bases, for example, S 2 - form strong compounds with molybdenum and tungsten, which are soft acids.

Anhydrous trisulfides are formed by gently heating ammonium thiosalts:

(NH 4) 2 MoS 4 = MoS 3 + 2NH 3 + H 2 S.

When heated strongly, they lose sulfur:

MoS 3 ¾¾→ MoS 2 + S.

Thiometalates are used for the synthesis of complex thiocomplexes, for example, cubanes containing an M 4 S 4 cluster.

Selenometalates are also known, formed by the interaction of potassium triselenide K 2 Se 3 with molybdenum and tungsten hexacarbonyls M(CO) 6 . Compounds containing ions were not obtained.

When molybdenum or tungsten interacts with sulfur over a wide temperature range, the most stable phase is MS 2 disulfides with double layers of sulfur atoms, in the center of which molybdenum atoms are located in trigonal prismatic voids (Fig. 5.34. Crystal structure of MoS 2: (a) general view, (b, c) projections along coordinate planes) (V.L. Kalikhman, Izv. AN SSSR, Inorganic Materials, 1983, 19(7), 1060). Double layers are connected to each other only by weak van der Waals forces, which causes a strong anisotropy in the properties of the substance - it is soft, like graphite, and is easily divided into individual flakes. The layered structure and chemical inertness explain the similarity of MoS 2 to graphite and its properties as a solid lubricant. Like graphite, disulfides form intercalated compounds with alkali metals, for example, Li x MoS 2. In water, the intercalates decompose, forming a fine powder of molybdenum disulfide.

The natural mineral molybdenite MoS 2 is so soft that it can leave a mark on a sheet of paper. Due to its low coefficient of friction, its powder is used as a component of lubricants for internal combustion engines, plain bearings, and instrument units operating under heavy loads. Disulfides are refractory (T mp. MoS 2 2100 o C) and fairly inert substances that decompose only under the action of alkalis and oxidizing acids - aqua regia, boiling concentrated sulfuric acid, a mixture of nitric and hydrofluoric acids. When strongly heated in air, they burn, oxidizing to higher oxides:

2MoS 2 + 7O 2 = 2MoO 3 + 4SO 2,

and in an atmosphere of chlorine - to the chlorides MoCl 5 and WCl 6.

Convenient methods for producing disulfides are the fusion of MO 3 oxides with excess sulfur in the presence of potash K 2 CO 3

2WO 3 + 7S = 2WS 2 + 3SO 2

interaction of molybdenum pentachloride with sodium sulfide (P.R. Bonneau et al, Inorg. Synth. 1995, 30, 33):

2MoCl 5 + 5Na 2 S = 2MoS 2 + 10NaCl + S.

Heat is required to initiate this reaction, but then the heat release causes the mixture of components to burn very quickly.

From solutions containing molybdenum(V) ions, for example, 2–, Mo 2 S 5 sulfide can be precipitated with hydrogen sulfide. MoS monosulfide is formed by heating stoichiometric amounts of molybdenum and sulfur in an evacuated ampoule.

Addition. Chevreul phases and other thiomolybene clusters. Mo 3 S 4 sulfide is a cluster compound consisting of Mo 6 S 8 groups in which molybdenum atoms are located at the vertices of a highly distorted octahedron. The reason for the distortion of Mo 6 S 8 is its electron-deficient nature - four electrons are missing to fill all bonding orbitals. That is why this compound easily reacts with electron donor metals. In this case, Chevreul phases M x Mo 6 S 8 are formed, where M is a d- or p-metal, for example, Cu, Co, Fe, Pb, Sn. Many of them have a crystal lattice of the CsCl type, at the nodes of which there are metal cations and cluster anions 2 - (Fig. 5.35. Structure of the Chevreul phase PbMo 6 S 8). The electronic transition Mo 6 S 8 + 2e - ¾® 2 - leads to strengthening of the crystal structure and strengthening of the Mo-Mo bond. Chevreul phases are of practical interest due to their semiconductor properties - they retain superconductivity up to a temperature of 14 K in the presence of strong magnetic fields, which allows them to be used for the manufacture of ultra-powerful magnets. The synthesis of these compounds is usually carried out by annealing stoichiometric amounts of elements:

Pb + 6Mo + 8S ¾¾® PbMo 6 S 8

Similar substances have been obtained in the case of selenium and tellurium, but tungsten analogues of the Chevreul phases are currently unknown.

A large number of thiomolybdenum clusters were obtained in aqueous solutions during the reduction of thiomolybdates. The most well-known is the tetranuclear cluster 5+ in which sulfur and molybdenum atoms occupy opposite vertices of the cube (Fig. 5.36. n+). The molybdenum coordination sphere is supplemented with up to six water molecules or other ligands. The Mo 4 S 4 group is preserved during oxidation and reduction:

E – – e –

4+ ¾ 5+ ¾® 6+ .

Molybdenum atoms can be replaced by atoms of other metals, for example, copper or iron, with the formation of heterometallic clusters like [Mo 3 CuS 4 (H 2 O) 10 ] 5+. Such thioclusters are the active centers of many enzymes, for example, ferrodoxin (Fig. 5.37. Active center of ferrodoxin). Studying the compounds they contain will reveal the mechanism of action of nitrogenase, an iron-molybdenum enzyme that plays a critical role in the fixation of air nitrogen by bacteria.

END OF ADDENDUM

5.11. Carbides, nitrides and borides of group 6 elements

With carbon, chromium, molybdenum and tungsten, like other d-metals, form carbides - hard and high-melting (2400-2800 o C) compounds with delocalized metallic bonds. They are obtained by the interaction of appropriate amounts of simple substances at high (1000-2000 o C) temperatures, as well as the reduction of oxides with carbon, for example,

2MoO 3 + 7C = Mo 2 C + 6CO.

Carbides are non-stoichiometric compounds with a wide (up to several at.% C) homogeneity range. In carbides of the M 2 C type, the metal atoms form a hexagonal close packing, in the octahedral voids of which C atoms are statistically embedded. MC monocarbides belong to the NiAs structural type and are not interstitial phases. Along with exceptional heat resistance and refractoriness, carbides have high corrosion resistance. For example, WC does not dissolve even in a mixture of nitric and hydrofluoric acids; up to 400 o C it does not react with chlorine. Based on these substances, superhard and refractory alloys are produced. The hardness of tungsten monocarbide is close to the hardness of diamond, so it is used to make the cutting part of cutters and drills.

Nitrides MN and M 2 N are obtained by reacting metals with nitrogen or ammonia, and phosphides MP 2 , MP 4 , M 2 P are obtained from simple substances, as well as by heating halides with phosphine. Like carbides, these are non-stoichiometric, highly hard, chemically inert and refractory (2000-2500 o C) substances.

Borides of metals of the sixth group, depending on the boron content, can contain isolated (M 2 B), chains (MB) and networks (MB 2) and three-dimensional frameworks (MB 12) of boron atoms. They are also characterized by high hardness, heat resistance and chemical resistance. Thermodynamically they are stronger than carbides. Borides are used for the manufacture of jet engine parts, gas turbine blades, etc.