Chemical Formulas - Knowledge Hypermarket. How to find out the qualitative and quantitative composition of a substance Examples of determining the composition

During the lesson, you will learn about the qualitative and quantitative composition of organic substances, about what is the simplest, molecular, structural formula.

One simple formula can correspond to many molecular formulas.

A formula that shows the order in which atoms are connected in a molecule is called a structural formula.

Hexene and cyclohexane have the same molecular formula C 6 H 12 , but they are two different substances with different physical and chemical properties. See table. 1.

Tab. 1. The difference between the properties of hexene and cyclohexane

To characterize organic matter, it is necessary to know not only the composition of the molecule, but also the arrangement of atoms in the molecule - the structure of the molecule.

The structure of substances is reflected by structural (graphic) formulas in which covalent bonds between atoms are indicated by dashes - valence strokes.

In organic compounds, carbon forms four bonds, hydrogen one, oxygen two, and nitrogen three.

Valence. The number of covalent non-polar or polar bonds that an element can form is called valence

A bond formed by one pair of electrons is called simple or single connection.

A bond formed by two pairs of electrons is called double connection, it is denoted by two dashes, like an "equal" sign. Three electron pairs form triple connection, which is indicated by three dashes. See table. 2.

Tab. 2. Examples of organic compounds with different bonds

In practice, it is usually used abbreviated structural formulas, in which the bonds of carbon, oxygen and other atoms with hydrogen are not indicated:

Rice. 1. Volumetric model of the ethanol molecule

Structural formulas convey the order in which atoms are connected to each other, but do not convey the arrangement of atoms in space. Structural formulas are a two-dimensional drawing, and molecules are three-dimensional, i.e. are voluminous, this is shown by the example of ethanol in Fig. 1.

The lesson covered the question of the qualitative and quantitative composition of organic substances, about what constitutes the simplest, molecular, structural formula.

Bibliography

1. Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Chemistry. Grade 10. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2008. - 463 p.

3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2010. - 462 p.

4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering the universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.

Homework

1. No. 6-7 (p. 11) Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. -M.: Enlightenment, 2012.

2. Why do organic substances whose composition is reflected by the same molecular formula have different chemical and physical properties?

3. What does the simplest formula show?

Consider the qualitative and quantitative composition of substances. Let us determine its features for compounds of organic and inorganic origin.

What shows the qualitative composition of the substance

It demonstrates the types of atoms that are in the analyzed molecule. For example, water is made up of hydrogen and oxygen.

The molecule includes sodium and oxygen atoms. Sulfuric acid contains hydrogen, oxygen, and sulfur.

What shows the composition of the quantitative

It demonstrates the quantitative content of each element within a complex substance.

For example, water contains two hydrogen atoms and one oxygen. Sulfuric acid consists of two hydrogens, one sulfur atom, four oxygens.

It consists of three hydrogen atoms, one phosphorus, four oxygen atoms.

Organic substances also have a qualitative and quantitative composition of substances. For example, methane contains one carbon and four hydrogens.

Methods for determining the composition of a substance

The qualitative and quantitative composition of substances can be determined chemically. For example, when a molecule of a complex compound is decomposed, several molecules with a simpler composition are formed. Thus, when calcium carbonate, consisting of calcium, carbon, four oxygen atoms, is heated, two and carbon atoms can be obtained.

And the compounds formed in the course of chemical decomposition can have a different qualitative and quantitative composition of substances.

Simple and complex compounds can be of molecular as well as non-molecular composition.

The first group is in different states of aggregation. For example, sugar is a solid, water is liquid, and oxygen is a gas.

Compounds of a nonmolecular structure under standard conditions are in solid form. These include salts. In the process of heating, they melt, go from a solid to a liquid state.

Composition Determination Examples

"Describe the qualitative and quantitative composition of the following substances: sulfur oxide (4), sulfur oxide (6)". Such a task is typical in the school course of inorganic chemistry. In order to cope with it, you first need to formulate the proposed compounds using valencies or oxidation states.

Both proposed oxides contain the same chemical elements, therefore, their qualitative composition is the same. They include sulfur and oxygen atoms. But quantitatively, the results will differ.

The first compound contains two oxygen atoms, while the second has six.

Let's complete the following task: "Describe the qualitative and quantitative composition of H2S substances."

The hydrogen sulfide molecule consists of a sulfur atom and two hydrogens. The qualitative and quantitative composition of the H2S substance makes it possible to predict its chemical properties. Since the composition contains a hydrogen cation, hydrogen sulfide is able to exhibit oxidizing properties. For example, similar characteristics appear in interaction with the active metal.

Information about the qualitative and quantitative composition of a substance is also relevant for organic compounds. For example, knowing the quantitative content of components in a hydrocarbon molecule, one can determine whether it belongs to a certain class of substances.

Such information makes it possible to predict the chemical and physical characteristics of the analyzed hydrocarbon, to identify its specific properties.

For example, knowing that there are four carbon atoms and ten hydrogens in the composition, we can conclude that this substance belongs to the class of saturated (saturated) hydrocarbons having the general formula SpH2n + 2. All representatives of this homologous series are characterized by a radical mechanism, as well as oxidation by atmospheric oxygen.

Conclusion

Any inorganic and organic substance has a certain quantitative and qualitative composition. Information is necessary to establish the physical and chemical properties of the analyzed inorganic compound, and for organic substances, the composition allows you to establish belonging to a class, to identify characteristic and specific chemical properties.

Mass fractions are usually expressed as a percentage:

ω% (O) \u003d 100% - ω% (H) \u003d 100% - 11.1% \u003d 88.9%.

Questions to control

1. What particles are usually formed as a result of the combination of atoms?

2. What is the composition of any molecule?

3. What are called indices in chemical formulas?

4. What do chemical formulas show?

5. How is the law of constancy of composition formulated?

6. What is a molecule?

7. What is the mass of a molecule?

8. What is relative molecular weight?

9. What is the mass fraction of this element in this substance?

1. Describe the qualitative and quantitative composition of the molecules of the following

working substances: methane CH4, soda Na2 CO3, glucose C6 H12 O6, chlorine Cl2, aluminum sulfate Al2 (SO4) 3.

2. The phosgene molecule consists of one carbon atom, one oxygen atom and two chlorine atoms. The urea molecule consists of one carbon atom, one oxygen atom and two NH atomic groups. 2. Write the formulas for phosgene and urea.

3. Count the total number of atoms in the following molecules: (NH 4 )3 PO4 , Ca(H2 PO4 )2 , 2 SO4 .

4. Calculate the relative molecular weights of the substances shown in exercise 1.

5. What are the mass fractions of elements in the following substances: NH 3 , N2 O, NO2 , NaNO3 , KNO3 , NH4 NO3 ? In which of these substances the mass fraction of nitrogen is the largest and in which is the smallest?

§ 1.5. Simple and complex substances. Allotropy.

Chemical compounds and mixtures

All substances are divided into simple and complex.

Simple substances are substances that are made up of atoms of one element.

In some simple substances, atoms of one element

connect with each other and form molecules. Such simple substances are molecular structure. Relative to them

are: hydrogen H 2, oxygen O 2, nitrogen N 2, fluorine F 2, chlorine Cl 2, bromine Br 2, iodine I 2. All these substances are composed of diatomic

molecules. (Note that the names of simple substances

match the element names!)

Other simple substances have atomic structure, i.e., they consist of atoms, between which there are certain bonds (we will consider their nature in the section “Chemical bond and structure of matter”). Examples of such simple substances are all metals (iron Fe, copper Cu, sodium Na, etc.) and some non-metals (carbon C, silicon Si, etc.). Not only the names, but also the formulas of these simple substances coincide with the symbols of the elements.

There is also a group of simple substances called noble gases. These include: helium He,

neon Ne, argon Ar, krypton Kr, xenon Xe, radon Rn. These simple substances are atoms that are not chemically bonded to each other.

Each element forms at least one simple substance. Some elements may form more than one,

but two or more simple substances. This phenomenon is called allotropy.

Allotropy is the phenomenon of the formation of several simple substances by one element.

Different simple substances that are formed by the same chemical element are called allotropic

modifications (modifications).

Allotropic modifications may differ from each other the composition of the molecules. For example, the element oxygen forms

two simple things. One of them consists of diatomic O2 molecules and has the same name as the element oxygen. Another simple substance consists of triatomic O3 molecules and has its own name - ozone:

Oxygen O2 and ozone O3 have different physical and chemical properties.

Allotropic modifications may be solids that have different structure of the crystal

thallus. An example is allotropic modifications carbon C - diamond and graphite.

The number of known simple substances (about 400) is much greater than the number of chemical elements, since many elements can form two or more allotropic modifications.

Compounds are substances that are made up of atoms of different elements.

Examples of complex substances: HCI, H 2 O, NaCl, CO 2,

H2 SO4 , Cu(NO3 )2 , C6 H12 O6 etc.

Compounds are often called chemical compounds. In chemical compounds, the properties of simple substances from which these compounds are formed do not

yutsya. The properties of a complex substance differ from the properties of the simple substances from which it is formed.

For example, sodium chloride NaCl can be formed from simple substances - metallic sodium Na And chlorine gas Cl 2. The physical and chemical properties of NaCl differ from those of Na and Cl 2 .

IN nature, as a rule, there are not pure substances,

but mixtures of substances. In practice, we also

we usually use mixtures of substances. Any mixture is

two or more substances, which are called com-

mixture components.

For example, air is a mixture of several gaseous substances: oxygen O 2 (21% by volume), nitrogen N 2 (78%), carbon dioxide CO 2 and others. Mixtures are

creations of many substances, alloys of certain metals, etc. Mixtures of substances are homogeneous (homogeneous) and ge-

terogenic (heterogeneous).

Homogeneous mixtures are mixtures in which there is no interface between the components.

Mixtures of gases (in particular, air), liquid solutions (for example, a solution of sugar in water) are homogeneous.

Heterogeneous mixtures are mixtures in which the components are separated by an interface.

TO heterogeneous aremixtures of solids(sand +

Chalk powder), mixtures of liquids insoluble in each other (water + oil), mixtures of liquids and solids insoluble in them (water + chalk).

liquid solutions, which are the most important representatives of homogeneous systems, we will study in detail in our course.

The most important differences between mixtures and chemical compounds:

1. In mixtures, the properties of individual substances (components)

are saved.

2. The composition of mixtures is not constant.

Questions to control

1. What are the two types of all substances?

2. What are simple substances?

3. What simple substances have a molecular structure (names and formulas)?

4. What simple substances have an atomic structure? Give examples.

5. What simple substances are made up of atoms that are not connected to each other?

6. What is allotropy?

7. What is called allotropic modifications (modifications)?

8. Why is the number of simple substances greater than the number of chemical elements?

9. What are complex substances?

10. Are the properties of simple substances preserved when they are formed into complex substances?

11. What are homogeneous mixtures? Give examples.

12. What are heterogeneous mixtures? Give examples.

13. How are mixtures different from chemical compounds?

Tasks for independent work

1. Write formulas known to you: a) simple substances (5 examples); b) complex substances (5 examples).

2. Divide the substances whose formulas are given below into simple and complex: NH 3 , Zn, Br2 , HI, C2 H5 OH, K, CO, F2 , C10 H22 .

3. The element phosphorus forms three simple substances that differ, in particular, in color: white, red and black phosphorus. What are these simple substances in relation to each other?

§ 1.6. Valency of elements. Graphic formulas of substances

Consider the chemical formulas of the compounds of some

As can be seen from these examples, the atoms of the elements chlorine, oxygen, nitrogen, carbon attach not any, but only a certain number of hydrogen atoms (1, 2, 3, 4 atoms, respectively).

Between atoms in chemical compounds there are chemical bonds. Let us write formulas in which each chi-

the mic connection is indicated by a dash:

Such formulas are called graphic.

Graphic formulas of substances - these are formulas that show the order of connection of atoms in molecules and the number of bonds that each atom forms.

The number of chemical bonds that one atom of a given element in a given molecule forms is called the valency of the element.

Valency is usually indicated by Roman numerals: I, II, III, IV, V, VI, VII, VIII.

In all the molecules under consideration, each hydrogen atom forms one bond: therefore, the valence of hydrogen is equal to one (I).

The chlorine atom in the HCl molecule forms one bond, its valence in this molecule is I. The oxygen atom in the H2 O molecule forms two bonds, its valency is II. Valence

nitrogen in NH3 is III, and the valence of carbon in CH4 is IV. Some elements have permanent valence.

Elements with constant valency are elements that in all connections exhibit the same valent

Elements with constant valency I are: hydrogen H, fluorine F , alkali metals: lithium Li, sodium Na,

potassium K, rubidium Rb, cesium Cs.

The atoms of these monovalent elements always form

only one chemical bond.

Elements with constant valency II:

oxygen O, magnesium Mg, calcium Ca, strontium Sr, barium Ba, zinc Zn.

Element with constant valence III - aluminum Al.

Most elements have variable valence.

Elements with variable valency are elements that in different compounds can have different valency values ​​*.

Consequently, the atoms of these elements in different compounds can form a different number of chemical bonds (Table 4).

* The physical meaning of valence, the reasons for the existence of elements with constant and variable valence, we will consider after studying the theory of the structure of atoms.

Table 4

The most characteristic valency values ​​of some elements

Elements	The most characteristic
	valency
	II, III, IV, VI, VII

To determine the valency of such elements in any given compound, you can use the va-

laziness.

According to this rule, in most binary compounds of type A m B n, the product of the valency of the element A (x) by the number of its atoms (t) is equal to the product of the valence of the element

ta B (y) by the number of its atoms (n):

x t = y n * .

Let us determine, for example, the valency of phosphorus in the following compounds:

x I	x" II
PH3	P2 O5
Hydrogen valence	Oxygen valency
constant and equal to I	constant and equal to II
x 1 = 1 3	x" 2 = 2 5
x = 3	x" = 5
PH3	P2 O5
Phosphorus in PH3 is	Phosphorus in P2 O5 is
trivalent	pentavalent
element	element

* The valency rule does not apply to binary compounds in which atoms of the same element are directly connected to each other. For example, the rule of valency does not obey per-

hydrogen oxide H2 O2, since in its molecule there is a bond between oxygen atoms: H-O-O-H.

Using the valency rule, one can formulate binary compounds, i.e., determine the indices in these formulas.

Let's write, for example, the formula of the compound aluminum with oxygen. Al and O have constant valency values,

responsibly III and II:

The least common multiple (LCM) of the numbers 3 and 2 is 6. Divide the LCM by the valency of Al:

6: 3 \u003d 2 and for valency O: 6: 2 \u003d 3

These numbers are equal to the indices of the symbols of the corresponding

elements in the compound formula:

Al2 O3

Let's look at two more examples.

Write formulas for compounds that consist of:

note that in most binary compounds

Atoms of the same element do not bond directly to each other.

Let's write the graphic formulas of all the compounds that we considered in this paragraph:

Compare the number of dashes for each element with its valence, which is indicated in the text of the paragraph.

Questions to control

1. What is the valency of an element?

2. What numbers usually indicate valence?

3. What are constant valency elements?

4. What elements have constant valency?

5. What are elements with variable valency? Specify the most typical valence values ​​for chlorine, sulfur, carbon, phosphorus, iron.

6. How is the rule of valency formulated?

7. What are the names of the formulas that show the order of connection of atoms in molecules and the valency of each element?

Tasks for independent work

1. Determine the valency of the elements in the following compounds: AsH 3 , СuО, N 2 O 3 , CaBr 2 , AlI 3 , SF 6 , K 2 S, SiO 2 , Mg 3 N 2 .

Make graphical formulas of these substances.

2. Define Indices m and n in the following formulas:

Hm Sen , Pm Cln , Pbm On , Om Fn , Fem Sn Write the graphic formulas for these substances.

3. Compose the molecular and graphical formulas of chromium compounds with oxygen, in which chromium exhibits valence II, III and VI.

4. Write formulas for compounds that consist of:

a) manganese (II) and oxygen; b) manganese (IV) and oxygen; c) manganese (VI) and oxygen; d) chlorine (VII) and oxygen; e) barium and oxygen. Write the graphical formulas of these substances.

§ 1.7. Moth. Molar mass

The mass of a substance is expressed in kg, g or other units

The unit of quantity of a substance is the mole.

Most substances are made up of molecules or atoms.

A mole is the amount of a substance that contains as many molecules (atoms) of this substance as there are atoms in 12 g (0.012 kg) of carbon C.

Let us determine the number of C atoms in 12 g of carbon. To do this, we divide 0.012 kg by the absolute mass of the carbon atom m a (C) (see § 1.3):

0.012 kg / 19.93 10–27 kg ≈ 6.02 1023 .

From the definition of the concept "mole" it follows that this number

equal to the number of molecules (atoms) in one mole of any substance. It is called the Avogadro number and is denoted by the symbol

ox N A :

(Note that Avogadro's number is a very large number!)

If a substance consists of molecules, then 1 mol is 6.02 1023 molecules of this substance.

For example: 1 mol of hydrogen H2 is 6.02 1023 H2 molecules; 1 mol of H2O water is 6.02 1023 H2O molecules;

1 mol of glucose C6 H12 O6 is 6.02 1023

C6 H12 O6 molecules.

If a substance consists of atoms, then 1 mole is 6.02 1023 atoms of this substance.

For example: 1 mole of iron Fe is 6.02 1023 Fe atoms;

1 mole of sulfur S is 6.02 1023 S atoms. Therefore:

1 mole of any substance contains the Avogadian number of particles that make up this substance, i.e. approximately 6.02 x 1023 molecules or atoms.

The amount of substance (i.e., the number of moles) is denoted by the Latin letter p (or the Greek letter v). Any given number of molecules (atoms) is denoted by the letter N.

The amount of substance n is equal to the ratio of a given number of molecules (atoms) N to the number of molecules (atoms) in 1 mole NA.