The atomic masses of chemical elements were calculated for the first time. Relative atomic mass

DEFINITION

Iron- the twenty-sixth element of the Periodic Table. Designation - Fe from the Latin "ferrum". Located in the fourth period, VIIIB group. Refers to metals. The nuclear charge is 26.

Iron is the most common metal on the globe after aluminum: it makes up 4% (wt.) of the earth's crust. Iron is found in the form of various compounds: oxides, sulfides, silicates. Iron is found in its free state only in meteorites.

The most important iron ores include magnetic iron ore Fe 3 O 4 , red iron ore Fe 2 O 3 , brown iron ore 2Fe 2 O 3 × 3H 2 O and spar iron ore FeCO 3 .

Iron is a silvery (Fig. 1) ductile metal. It lends itself well to forging, rolling and other types of mechanical processing. The mechanical properties of iron strongly depend on its purity - on the content of even very small quantities of other elements in it.

Rice. 1. Iron. Appearance.

Atomic and molecular mass of iron

Relative molecular weight of the substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times the average mass of atoms of a chemical element is greater than 1/12 the mass of a carbon atom.

Since in the free state iron exists in the form of monatomic Fe molecules, the values ​​of its atomic and molecular masses coincide. They are equal to 55.847.

Allotropy and allotropic modifications of iron

Iron forms two crystalline modifications: α-iron and γ-iron. The first of them has a body-centered cubic lattice, the second has a face-centered cubic lattice. α-Iron is thermodynamically stable in two temperature ranges: below 912 o C and from 1394 o C to the melting point. The melting point of iron is 1539 ± 5 o C. Between 912 o C and from 1394 o C γ-iron is stable.

The temperature ranges of stability of α- and γ-iron are determined by the nature of the change in the Gibbs energy of both modifications with temperature changes. At temperatures below 912 o C and above 1394 o C, the Gibbs energy of α-iron is less than the Gibbs energy of γ-iron, and in the range of 912 - 1394 o C it is greater.

Isotopes of iron

It is known that in nature iron can be found in the form of four stable isotopes 54 Fe, 56 Fe, 57 Fe and 57 Fe. Their mass numbers are 54, 56, 57 and 58, respectively. The nucleus of an atom of the iron isotope 54 Fe contains twenty-six protons and twenty-eight neutrons, and the remaining isotopes differ from it only in the number of neutrons.

There are artificial isotopes of iron with mass numbers from 45 to 72, as well as 6 isomeric states of nuclei. The longest-lived among the above isotopes is 60 Fe with a half-life of 2.6 million years.

Iron ions

The electronic formula demonstrating the orbital distribution of iron electrons is as follows:

1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 .

As a result of chemical interaction, iron gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:

Fe 0 -2e → Fe 2+ ;

Fe 0 -3e → Fe 3+.

Iron molecule and atom

In the free state, iron exists in the form of monoatomic Fe molecules. Here are some properties characterizing the iron atom and molecule:

Iron alloys

Until the 19th century, iron alloys were mainly known for their alloys with carbon, called steel and cast iron. However, later new iron-based alloys containing chromium, nickel and other elements were created. Currently, iron alloys are divided into carbon steels, cast irons, alloy steels and steels with special properties.

In technology, iron alloys are usually called ferrous metals, and their production is called ferrous metallurgy.

Examples of problem solving

The physical properties of iron depend on its purity. Pure iron is a fairly ductile metal with a silvery-white color. The density of iron is 7.87 g/cm3. The melting point is 1539 ° C. Unlike many other metals, iron exhibits magnetic properties.

Pure iron is quite stable in air. In practical activities, iron is used containing impurities. When heated, iron is quite active towards many non-metals. Let's consider the chemical properties of iron using the example of interaction with typical non-metals: oxygen and sulfur.

When iron burns in oxygen, a compound of iron and oxygen is formed, which is called iron scale. The reaction is accompanied by the release of heat and light. Let's create an equation for a chemical reaction:

3Fe + 2O 2 = Fe 3 O 4

When heated, iron reacts violently with sulfur to form ferrum(II) sulfide. The reaction is also accompanied by the release of heat and light. Let's create an equation for a chemical reaction:

Iron is widely used in industry and everyday life. The Iron Age is an era in the development of mankind, which began at the beginning of the first millennium BC in connection with the spread of iron smelting and the manufacture of iron tools and military weapons. The Iron Age replaced the Bronze Age. Steel first appeared in India in the tenth century BC, cast iron only in the Middle Ages. Pure iron is used to make transformer cores and electromagnets, as well as in the production of special alloys. The most commonly used iron alloys in practice are cast iron and steel. Cast iron is used in the production of castings and steel, steel is used as structural and tool materials that are resistant to corrosion.

Under the influence of atmospheric oxygen and moisture, iron alloys turn into rust. The rusting product can be described by the chemical formula Fe 2 O 3 · xH 2 O. One sixth of the cast iron smelted dies from rusting, so the issue of combating corrosion is very relevant. Methods of corrosion protection are very diverse. The most important of them: protecting the metal surface with a coating, creating alloys with anti-corrosion properties, electrochemical agents, changing the composition of the environment. Protective coatings are divided into two groups: metal (coating iron with zinc, chromium, nickel, cobalt, copper) and non-metal (varnishes, paints, plastics, rubber, cement). By introducing special additives into the composition of alloys, stainless steel is obtained.

Iron. Occurrence of iron in nature

Iron. The prevalence of iron in nature. Biological role of iron

The second important chemical element after oxygen, the properties of which will be studied, is Ferum. Iron is a metallic element that forms the simple substance iron. Iron is part of the eighth group of the secondary subgroup of the periodic table. According to the group number, the maximum valence of iron should be eight, however, in Ferum compounds it more often exhibits valency two and three, as well as known compounds with an iron valence of six. The relative atomic mass of iron is fifty-six.

In terms of its abundance in the earth's crust, Ferum ranks second among metallic elements after aluminum. The mass fraction of iron in the earth's crust is almost five percent. Iron is found very rarely in its native state, usually only in the form of meteorites. It was in this form that our ancestors were able to first become acquainted with iron and appreciate it as a very good material for making tools. It is believed that iron is the main component of the earth's core. Ferum is most often found in nature in ores. The most important of them are: magnetic iron ore (magnetite) Fe 3 O 4, red iron ore (hematite) Fe 2 O 3, brown iron ore (limonite) Fe 2 O 3 nH 2 O, iron pyrite (pyrite) FeS 2, spar iron ore ( siderite) FeСO3, goethite FeO (OH). The waters of many mineral springs contain Fe (HCO 3) 2 and some other iron salts.

Iron is a vital element. In the human body, like animals, ferrum is present in all tissues, but the largest part of it (about three grams) is concentrated in blood cells. Iron atoms occupy a central position in hemoglobin molecules; hemoglobin owes its color and ability to attach and remove oxygen to them. Iron is involved in the process of transporting oxygen from the lungs to the tissues of the body. The body's daily need for Ferum is 15-20 mg. Its total amount enters the human body with plant foods and meat. With blood loss, the need for Ferum exceeds the amount that a person receives from food. A lack of iron in the body can lead to a condition characterized by a decrease in the number of red blood cells and hemoglobin in the blood. Iron supplements should only be taken as prescribed by a doctor.

Chemical properties of oxygen. Compound reactions

Chemical properties of oxygen. Compound reactions. The concept of oxides, oxidation and combustion. Conditions for the initiation and cessation of combustion

When heated, oxygen reacts vigorously with many substances. If you add hot charcoal C into a vessel with oxygen, it becomes white-hot and burns. Let's create an equation for a chemical reaction:

C + ONaHCO 2 = CONaHCO 2

Sulfur S burns in oxygen with a bright blue flame to form a gaseous substance - sulfur dioxide. Let's create an equation for a chemical reaction:

S + ONaHCO 2 = SONaHCO 2

Phosphorus P burns in oxygen with a bright flame to produce thick white smoke, which consists of solid particles of phosphorus (V) oxide. Let's create an equation for a chemical reaction:

4P + 5ONaHCO 2 = 2PNaHCO 2 ONaHCO 5

The reaction equations for the interaction of oxygen with coal, sulfur and phosphorus are united by the fact that in each case one substance is formed from two starting substances. Such reactions, as a result of which only one substance (product) is formed from several starting substances (reagents), are called communication reactions.

The products of the interaction of oxygen with the substances considered (coal, sulfur, phosphorus) are oxides. Oxides are complex substances containing two elements, one of which is oxygen. Almost all chemical elements form oxides, with the exception of some inert elements: helium, neon, argon, krypton and xenon. There are some chemical elements that do not combine directly with oxygen, such as Aurum.

Chemical reactions of substances interacting with oxygen are called oxidation reactions. The concept of "oxidation" is more general than the concept of "combustion". Combustion is a chemical reaction in which substances are oxidized, accompanied by the release of heat and light. For combustion to occur, the following conditions are necessary: ​​close contact of air with the flammable substance and heating to the ignition temperature. For different substances, the ignition temperature has different values. For example, the ignition temperature of wood dust is 610 ° C, sulfur - 450 ° C, white phosphorus 45 - 60 ° C. In order to prevent combustion, it is necessary to excite at least one of these conditions. That is, it is necessary to remove the flammable substance, cool it below the ignition temperature, and block the access of oxygen. Combustion processes accompany us in everyday life, so every person must know the conditions for the occurrence and cessation of combustion, as well as follow the necessary rules for handling flammable substances.

Oxygen cycle in nature

Oxygen cycle in nature. The use of oxygen, its biological role

About a quarter of the atoms of all living matter are oxygen. Since the total number of oxygen atoms in nature is constant, as oxygen is removed from the air due to respiration and other processes, it must be replenished. The most important sources of oxygen in inanimate nature are carbon dioxide and water. Oxygen enters the atmosphere mainly through the process of photosynthesis, which involves this-o-two. An important source of oxygen is the Earth's atmosphere. Some of the oxygen is formed in the upper parts of the atmosphere due to the dissociation of water under the influence of solar radiation. Some of the oxygen is released by green plants during the process of photosynthesis with al-two-o and this-in-two. In turn, atmospheric this-o-two is formed as a result of reactions of combustion and respiration of animals. Atmospheric o-two is spent on the formation of ozone in the upper parts of the atmosphere, the oxidative processes of rock weathering, in the process of animal respiration and in combustion reactions. The transformation of this-o-two into tse-o-two leads to the release of energy; accordingly, energy must be expended to transform this-o-two into o-two. This energy turns out to be the Sun. Thus, life on Earth depends on cyclic chemical processes made possible by solar energy.

The use of oxygen is due to its chemical properties. Oxygen is widely used as an oxidizing agent. It is used for welding and cutting metals, in the chemical industry - to obtain various compounds and intensify some production processes. In space technology, oxygen is used to burn hydrogen and other types of fuel, in aviation - when flying at high altitudes, in surgery - to support patients with difficulty breathing.

The biological role of oxygen is determined by its ability to support respiration. A person, when breathing within one minute, consumes on average 0.5 dm3 of oxygen, during the day - 720 dm3, and during the year - 262.8 m3 of oxygen.
1. The reaction of thermal decomposition of potassium permanganate. Let's create an equation for a chemical reaction:

The substance potassium-manganese-o-four is widely distributed in everyday life under the name "potassium permanganate". The oxygen that is formed is manifested by a smoldering splinter, which flashes brightly at the opening of the gas outlet tube of the device in which the reaction is carried out, or when introduced into a vessel with oxygen.

2. The decomposition reaction of hydrogen peroxide in the presence of manganese (IV) oxide. Let's create an equation for a chemical reaction:

Hydrogen peroxide is also well known from everyday life. It can be used to treat scrapes and minor wounds (an al-two-o-two wt three percent solution should be in every emergency kit). Many chemical reactions are accelerated in the presence of certain substances. In this case, the decomposition reaction of hydrogen peroxide is accelerated by manganese-o-two, but manganese-o-two itself is not consumed and is not part of the reaction products. Manganese-o-two is a catalyst.

Catalysts are substances that speed up chemical reactions without being consumed. Catalysts are not only widely used in the chemical industry, but also play an important role in human life. Natural catalysts, called enzymes, involved in the regulation of biochemical processes.

Oxygen, as noted earlier, is slightly heavier than air. Therefore, it can be collected by displacing air into a vessel placed with the opening up.

They restored it with charcoal in a forge (see), built in a pit; they pumped bellows into the forge, the product, the kritsa, was separated from the slag by blows and various products were forged from it. As blowing methods improved and the height of the hearth increased, the process increased and part of it was carburized, i.e., cast iron was obtained; this relatively fragile product was considered a production waste. Hence the name of cast iron “pig”, “pig” - English pig iron. Later it was noticed that when loading cast iron rather than iron into the forge, low-carbon iron dough is also obtained, and such a two-stage process (see Krichny redistribution) turned out to be more profitable than the cheese-blowing process. In the 12th-13th centuries. the screaming method was already widespread. In the 14th century Cast iron began to be smelted not only as a semi-product for further processing, but also as a material for casting various products. The reconstruction of the furnace into a mine ("house"), and then into a blast furnace, also dates back to the same time. In the middle of the 18th century. In Europe, the crucible process for producing steel began to be used, which was known in Syria in the early Middle Ages, but later turned out to be forgotten. With this method, steel was produced by melting metal charges in small (crucibles) from a highly refractory mass. In the last quarter of the 18th century. The puddling process of converting cast iron into a fiery reflective hearth began to develop (see Pudling). The industrial revolution of the 18th - early 19th centuries, the invention of the steam engine, the construction of railways, large bridges and the steam fleet created a huge need for it. However, all existing production methods could not satisfy the needs of the market. Mass production of steel began only in the mid-19th century, when the Bessemer, Thomas and open-hearth processes were developed. In the 20th century The electric furnace melting process arose and became widespread, producing high-quality steel.

Prevalence in nature. In terms of content in the lithosphere (4.65% by mass) it ranks second among (first). It migrates vigorously in the earth's crust, forming about 300 (, etc.). takes an active part in magmatic, hydrothermal and supergene processes, which are associated with the formation of various types of its deposits (see Iron). - earthly depths, it accumulates in the early stages of magma, in ultrabasic (9.85%) and basic (8.56%) (in granites it is only 2.7%). B accumulates in many marine and continental sediments, forming sedimentary deposits.

The following are physical properties related primarily to those with a total impurity content of less than 0.01% by mass:

A peculiar interaction with. Concentrated HNO 3 (density 1.45 g/cm 3) passivates due to the appearance of a protective oxide film on its surface; more dilute HNO 3 dissolves to form Fe 2+ or Fe 3+, being reduced to MH 3 or N 2 O and N 2.

Receipt and application. Pure is obtained in relatively small quantities of aqueous it or it. A method is being developed to directly obtain from. The production of fairly pure metals is gradually increasing, either directly from ore concentrates or from coal at relatively low levels.

The most important modern technology. In its pure form, due to its low value, it is practically not used, although in everyday life steel or cast iron products are often called “iron”. The bulk is used in the form of very different compositions and properties. It accounts for approximately 95% of all metal products. Rich (over 2% by weight) cast irons are smelted in blast furnaces from enriched iron (see Blast furnace production). Steel of various grades (content less than 2% by weight) is smelted from cast iron in open-hearth and electric converters by (burning out) excess, removing harmful impurities (mainly S, P, O) and adding alloying elements (see Open-hearth, Converter). High-alloy steels (with a high content of other elements) are smelted in electric arc and induction. New processes are used for the production of steels and for especially critical purposes - vacuum, electroslag remelting, plasma and electron beam melting, etc. Methods are being developed for steel smelting in continuously operating units that ensure high quality and automation of the process.

Based on this, materials are created that can withstand the effects of high and low, and high, aggressive environments, high alternating voltages, nuclear radiation, etc. Its production is constantly growing. In 1971, the USSR produced 89.3 million tons of iron and 121 million tons of steel.

L. A. Shvartsman, L. V. Vanyukova.

As an artistic material it has been used since antiquity in Egypt (for the head from the tomb of Tutankhamun near Thebes, mid-14th century BC, Ashmolean Museum, Oxford), Mesopotamia (daggers found near Carchemish, 500 BC, British Museum , London),

From the lesson materials you will learn that the atoms of some chemical elements differ from the atoms of other chemical elements in mass. The teacher will tell you how chemists measured the mass of atoms that are so small that you cannot see them even with an electron microscope.

Topic: Initial chemical ideas

Lesson: Relative Atomic Mass of Chemical Elements

At the beginning of the 19th century. (150 years after the work of Robert Boyle), the English scientist John Dalton proposed a method for determining the mass of atoms of chemical elements. Let's consider the essence of this method.

Dalton proposed a model according to which a molecule of a complex substance contains only one atom of different chemical elements. For example, he believed that a water molecule consists of 1 hydrogen atom and 1 oxygen atom. According to Dalton, simple substances also contain only one atom of a chemical element. Those. an oxygen molecule must consist of one oxygen atom.

And then, knowing the mass fractions of elements in a substance, it is easy to determine how many times the mass of an atom of one element differs from the mass of an atom of another element. Thus, Dalton believed that the mass fraction of an element in a substance is determined by the mass of its atom.

It is known that the mass fraction of magnesium in magnesium oxide is 60%, and the mass fraction of oxygen is 40%. Following the path of Dalton's reasoning, we can say that the mass of a magnesium atom is 1.5 times greater than the mass of an oxygen atom (60/40 = 1.5):

The scientist noticed that the mass of the hydrogen atom is the smallest, because There is no complex substance in which the mass fraction of hydrogen would be greater than the mass fraction of another element. Therefore, he proposed to compare the masses of atoms of elements with the mass of a hydrogen atom. And in this way he calculated the first values ​​of the relative (relative to the hydrogen atom) atomic masses of chemical elements.

The atomic mass of hydrogen was taken as unity. And the value of the relative mass of sulfur turned out to be 17. But all the values ​​obtained were either approximate or incorrect, because the experimental technique of that time was far from perfect and Dalton’s assumption about the composition of the substance was incorrect.

In 1807 - 1817 Swedish chemist Jons Jakob Berzelius conducted extensive research to clarify the relative atomic masses of elements. He managed to obtain results close to modern ones.

Much later than the work of Berzelius, the masses of atoms of chemical elements began to be compared with 1/12 of the mass of a carbon atom (Fig. 2).

Rice. 1. Model for calculating the relative atomic mass of a chemical element

The relative atomic mass of a chemical element shows how many times the mass of an atom of a chemical element is greater than 1/12 the mass of a carbon atom.

Relative atomic mass is denoted by A r; it has no units of measurement, since it shows the ratio of the masses of atoms.

For example: A r (S) = 32, i.e. a sulfur atom is 32 times heavier than 1/12 the mass of a carbon atom.

The absolute mass of 1/12 of a carbon atom is a reference unit, the value of which is calculated with high accuracy and is 1.66 * 10 -24 g or 1.66 * 10 -27 kg. This reference mass is called atomic mass unit (a.e.m.).

There is no need to memorize the values ​​of the relative atomic masses of chemical elements; they are given in any textbook or reference book on chemistry, as well as in the periodic table of D.I. Mendeleev.

When calculating, the values ​​of relative atomic masses are usually rounded to whole numbers.

The exception is the relative atomic mass of chlorine - for chlorine a value of 35.5 is used.

1. Collection of problems and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry, 8th grade” / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. – M.: AST: Astrel, 2006.

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 24-25)

3. Chemistry: 8th grade: textbook. for general education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§10)

4. Chemistry: inorg. chemistry: textbook. for 8th grade. general education institutions / G.E. Rudzitis, Fyu Feldman. – M.: Education, OJSC “Moscow Textbooks”, 2009. (§§8,9)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. – M.: Avanta+, 2003.

Additional web resources

1. Unified collection of digital educational resources ().

2. Electronic version of the journal “Chemistry and Life” ().

Homework

p.24-25 No. 1-7 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

To measure the mass of an atom, relative atomic mass is used, which is expressed in atomic mass units (amu). Relative molecular weight is made up of the relative atomic masses of substances.

Concepts

To understand what relative atomic mass is in chemistry, you should understand that the absolute mass of an atom is too small to be expressed in grams, much less in kilograms. Therefore, in modern chemistry, 1/12 of the mass of carbon is taken as an atomic mass unit (amu). Relative atomic mass is equal to the ratio of the absolute mass to 1/12 of the absolute mass of carbon. In other words, relative mass reflects how many times the mass of an atom of a particular substance exceeds 1/12 the mass of a carbon atom. For example, the relative mass of nitrogen is 14, i.e. The nitrogen atom contains 14 a. e.m. or 14 times more than 1/12 of a carbon atom.

Rice. 1. Atoms and molecules.

Among all the elements, hydrogen is the lightest, its mass is 1 unit. The heaviest atoms have a mass of 300 a. eat.

Molecular mass is a value indicating how many times the mass of a molecule exceeds 1/12 of the mass of carbon. Also expressed in a. e.m. The mass of a molecule is made up of the mass of atoms, therefore, to calculate the relative molecular mass it is necessary to add up the masses of the atoms of the substance. For example, the relative molecular weight of water is 18. This value is the sum of the relative atomic masses of two hydrogen atoms (2) and one oxygen atom (16).

Rice. 2. Carbon in the periodic table.

As you can see, these two concepts have several common characteristics:

• the relative atomic and molecular masses of a substance are dimensionless quantities;
• relative atomic mass is designated Ar, molecular mass - Mr;
• The unit of measurement is the same in both cases - a. eat.

Molar and molecular masses are the same numerically, but differ in dimension. Molar mass is the ratio of the mass of a substance to the number of moles. It reflects the mass of one mole, which is equal to Avogadro’s number, i.e. 6.02 ⋅ 10 23 . For example, 1 mole of water weighs 18 g/mol, and M r (H 2 O) = 18 a. e.m. (18 times heavier than one atomic mass unit).

How to calculate

To express relative atomic mass mathematically, one should determine that 1/2 part of carbon or one atomic mass unit is equal to 1.66⋅10 −24 g. Therefore, the formula for relative atomic mass is as follows:

A r (X) = m a (X) / 1.66⋅10 −24,

where m a is the absolute atomic mass of the substance.

The relative atomic mass of chemical elements is indicated in the periodic table of Mendeleev, so it does not need to be calculated independently when solving problems. Relative atomic masses are usually rounded to whole numbers. The exception is chlorine. The mass of its atoms is 35.5.

It should be noted that when calculating the relative atomic mass of elements that have isotopes, their average value is taken into account. Atomic mass in this case is calculated as follows:

A r = ΣA r,i n i ,

where A r,i is the relative atomic mass of isotopes, n i is the content of isotopes in natural mixtures.

For example, oxygen has three isotopes - 16 O, 17 O, 18 O. Their relative mass is 15.995, 16.999, 17.999, and their content in natural mixtures is 99.759%, 0.037%, 0.204%, respectively. Dividing the percentages by 100 and substituting the values, we get:

A r = 15.995 ∙ 0.99759 + 16.999 ∙ 0.00037 + 17.999 ∙ 0.00204 = 15.999 amu

Referring to the periodic table, it is easy to find this value in the oxygen cell.

Rice. 3. Periodic table.

Relative molecular mass is the sum of the masses of the atoms of a substance:

When determining the relative molecular weight value, symbol indices are taken into account. For example, calculating the mass of H 2 CO 3 is as follows:

M r = 1 ∙ 2 + 12 + 16 ∙ 3 = 62 a. eat.

Knowing the relative molecular weight, you can calculate the relative density of one gas from the second, i.e. determine how many times one gaseous substance is heavier than the second. To do this, use the equation D (y) x = M r (x) / M r (y).

What have we learned?

From the 8th grade lesson we learned about relative atomic and molecular mass. The unit of relative atomic mass is taken to be 1/12 of the mass of carbon, equal to 1.66⋅10 −24 g. To calculate the mass, it is necessary to divide the absolute atomic mass of the substance by the atomic mass unit (amu). The value of the relative atomic mass is indicated in the periodic table of Mendeleev in each cell of the element. The molecular mass of a substance is the sum of the relative atomic masses of the elements.

Test on the topic

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One of the main characteristics of any chemical element is its relative atomic mass.

(An atomic mass unit is 1/12 of the mass of a carbon atom, the mass of which is taken to be 12 amu and is1,66 10 24 G.

By comparing the masses of atoms of elements per amu, the numerical values ​​of the relative atomic mass (Ar) are found.

The relative atomic mass of an element shows how many times the mass of its atom is greater than 1/12 the mass of a carbon atom.

For example, for oxygen Ar (O) = 15.9994, and for hydrogen Ar (H) = 1.0079.

For molecules of simple and complex substances, determine relative molecular weight, which is numerically equal to the sum of the atomic masses of all atoms that make up the molecule. For example, the molecular weight of water is H2O

Mg (H2O) = 2 1.0079 + 1 15.9994 = 18.0153.

Avogadro's law

In chemistry, along with units of mass and volume, a unit of quantity of a substance is used, called the mole.

!MOL (v) - a unit of measurement of the amount of a substance containing as many structural units (molecules, atoms, ions) as there are atoms contained in 0.012 kg (12 g) of the carbon isotope “C’’.

This means that 1 mole of any substance contains the same number of structural units, equal to 6,02 10 23 . This quantity is called Avogadro's constant(designation NA, dimension 1/mol).

The Italian scientist Amadeo Avogadro put forward a hypothesis in 1811, which was later confirmed by experimental data and was subsequently called Avogadro's law. He drew attention to the fact that all gases are equally compressed (Boyle-Marriott's law) and have the same coefficients of thermal expansion (Gay-Lussac's law). In this regard, he suggested that:

equal volumes of different gases under the same conditions contain the same number of molecules.

Under the same conditions (usually we talk about normal conditions: the absolute pressure is 1013 millibars and the temperature is 0 ° C), the distance between the molecules of all gases is the same, and the volume of the molecules is negligible. Considering all of the above, we can make the following assumption:

!if equal volumes of gases under the same conditions contain the same number of molecules, then the masses containing the same number of molecules must have the same volumes.

In other words,

Under the same conditions, 1 mole of any gas occupies the same volume. Under normal conditions, 1 mole of any gas occupies a volume v, equal to 22.4 l. This volume is calledmolar volume of gas (dimension l/mol or m³ /mol).

The exact value of the molar volume of gas under normal conditions (pressure 1013 millibars and temperature 0 ° C) is 22.4135 ± 0.0006 l/mol. Under standard conditions (t=+15° C, pressure = 1013 mbar) 1 mole of gas occupies a volume of 23.6451 liters, and att=+20° C and a pressure of 1013 mbar, 1 mole occupies a volume of about 24.2 liters.

In numerical terms, molar mass coincides with the masses of atoms and molecules (in amu) and with relative atomic and molecular masses.

Consequently, 1 mole of any substance has a mass in grams that is numerically equal to the molecular mass of this substance, expressed in atomic mass units.

For example, M(O2) = 16 a. e.m. 2 = 32 a. e.m., thus, 1 mole of oxygen corresponds to 32 g. The densities of gases measured under the same conditions are referred to as their molar masses. Since when transporting liquefied gases on gas carriers the main object of practical problems are molecular substances (liquids, vapors, gases), the main sought-for quantities will be molar mass M(g/mol), amount of substance v in moles and mass T substances in grams or kilograms.

Knowing the chemical formula of a particular gas, you can solve some practical problems that arise when transporting liquefied gases.

Example 1. A deck tank contains 22 tons of liquefied ethylene (WITH2 N4 ). It is necessary to determine whether there is enough cargo on board to blow through three cargo tanks with a volume of 5000 m 3 each, if after blowing the temperature of the tanks is 0 ° C and the pressure is 1013 millibars.

1. Determine the molecular weight of ethylene:

M = 2 12.011 + 4 1.0079 = 28.054 g/mol.

2. Calculate the density of ethylene vapor under normal conditions:

ρ = M/V = 28.054: 22.4 = 1.232 g/l.

3. Find the volume of cargo vapor under normal conditions:

22∙10 6: 1.252= 27544 m3.

The total volume of cargo tanks is 15,000 m3. Consequently, there is enough cargo on board to purge all cargo tanks with ethylene vapor.

Example 2. It is necessary to determine how much propane (WITH3 N8 ) will be required for purging cargo tanks with a total capacity of 8000 m 3, if the temperature of the tanks is +15 ° C, and the pressure of propane vapor in the tank after the end of purging will not exceed 1013 millibars.

1. Determine the molar mass of propane WITH3 N8

M = 3 12,011 + 8 1,0079 = 44.1 g/mol.

2. Let’s determine the propane vapor density after purging the tanks:

ρ = M: v = 44.1: 23.641 = 1.865 kg/m 3.

3. Knowing the vapor density and volume, we determine the total amount of propane required to purge the tank:

m = ρ v = 1.865 8000 = 14920 kg ≈ 15 t.

Absolute masses of atoms One of the fundamental properties of atoms is their mass. Absolute (true) mass of an atom– the value is extremely small. It is impossible to weigh atoms on a balance because such precise scales do not exist. Their masses were determined using calculations. For example, the mass of one hydrogen atom is 0.000 000 000 000 000 000 000 001 663 grams! The mass of a uranium atom, one of the heaviest atoms, is approximately 0.000 000 000 000 000 000 000 4 grams. Writing and reading these numbers is not easy; You can make a mistake by missing a zero or adding an extra one. There is another way to write it - in the form of a product: 4 ∙ 10−22 (22 is the number of zeros in the previous number). The exact mass of the uranium atom is 3.952 ∙ 10−22 g, and the hydrogen atom, the lightest among all atoms, is 1.673 ∙ 10−24 g. It is inconvenient to carry out calculations with small numbers. Therefore, instead of the absolute masses of atoms, their relative masses are used.

Relative atomic mass

Ar(O) = 16; Ar(Na) = 23; Ar(P) = 31. The relative atomic mass of chlorine is usually written as 35.5! Ar(Cl) = 35.5

• Relative atomic masses are proportional to the absolute masses of atoms
• The standard for determining relative atomic mass is 1/12 of the mass of a carbon atom
• 1 amu = 1.662 ∙ 10−24 g
• Relative atomic mass is denoted by Ar
• For calculations, the values ​​of relative atomic masses are rounded to whole numbers, with the exception of chlorine, for which Ar = 35.5
• Relative atomic mass has no units of measurement