What is the constant avogadro. What is Avogadro's number

N A = 6.022 141 79(30)×10 23 mol −1 .

Avogadro's law

At the dawn of the development of atomic theory (), A. Avogadro put forward a hypothesis according to which, at the same temperature and pressure, equal volumes of ideal gases contain the same number molecules. Later it was shown that this conjecture is a necessary consequence of kinetic theory, and is now known as Avogadro's law. It can be formulated as follows: one mole of any gas at the same temperature and pressure occupies the same volume, under normal conditions equal to 22,41383 . This quantity is known as the molar volume of the gas.

Avogadro himself did not make estimates of the number of molecules in a given volume, but he understood that this was a very large quantity. The first attempt to find the number of molecules occupying a given volume was made by J. Loschmidt; it was found that 1 cm³ of an ideal gas under normal conditions contains 2.68675 10 19 molecules. By the name of this scientist, the indicated value was called the Loschmidt number (or constant). Since then, a large number of independent methods for determining the Avogadro number have been developed. The excellent agreement of the obtained values is a convincing evidence of the real existence of molecules.

Relationship between constants

Through the product of the Boltzmann constant, the Universal gas constant, R=kN A.
Through the product of an elementary electric charge and the Avogadro number, the Faraday constant is expressed, F=en A.

Avogadro's constant- Avogadro konstanta statusas T sritis Standartizacija ir metrologija apibrėžtis Apibrėžtį žr. priede. priedas(ai) Grafinis formatas atitikmenys: engl. Avogadro constant vok. Avogadro Konstante, f; Avogadrosche Konstante, f rus. Avogadro's constant... Penkiakalbis aiskinamasis metrologijos terminų žodynas

Avogadro's constant- Avogadro konstanta statusas T sritis fizika atitikmenys: engl. Avogadro's constant; Avogadro's number vok. Avogadro Konstante, f; Avogadrosche Konstante, f rus. Avogadro's constant, f; Avogadro's number, n pranc. constante d'Avogadro, f; nombre… … Fizikos terminų žodynas

Avogadro's constant- Avogadro konstanta statusas T sritis Energetika apibrėžtis Apibrėžtį žr. priede. priedas(ai) MS Word formatas atitikmenys: engl. Avogadro's constant vok. Avogadro Konstante, f; Avogadrosche Konstante, f rus. Avogadro's constant, f; constant... ... Aiškinamasis šiluminės ir branduolinės technikos terminų žodynas

- (Avogadro number) (NA), the number of molecules or atoms in 1 mole of a substance; NA \u003d 6.022? 1023 mol 1. Named after A. Avogadro ... Modern Encyclopedia

Avogadro constant- (Avogadro number) (NA), the number of molecules or atoms in 1 mole of a substance; NA=6.022´1023 mol 1. Named after A. Avogadro. … Illustrated Encyclopedic Dictionary

Avogadro Amedeo (08/09/1776, ‒ 07/09/1856, ibid.), Italian physicist and chemist. He received a law degree, then studied physics and mathematics. Corresponding member (1804), ordinary academician (1819), and then director of the department ... ...

- (Avogadro) Amedeo (08/09/1776, Turin, 07/09/1856, ibid.), Italian physicist and chemist. He received a law degree, then studied physics and mathematics. Corresponding member (1804), ordinary academician (1819), and then director of the department of physics ... ... Great Soviet Encyclopedia

The fine structure constant, usually denoted as, is a fundamental physical constant that characterizes the strength of the electromagnetic interaction. It was introduced in 1916 by the German physicist Arnold Sommerfeld as a measure ... ... Wikipedia

- (Avogadro's number), the number of structural elements (atoms, molecules, ions or other h c) in units. count va to va (in one mole). Named after A. Avogadro, designated NA. A. p. one of the fundamental physical constants, essential for determining many ... Physical Encyclopedia

CONSTANT- a value that has a constant value in the area of its use; (1) P. Avogadro is the same as Avogadro (see); (2) P. Boltzmann universal thermodynamic quantity relating energy elementary particle with its temperature denoted by k,… … Great Polytechnic Encyclopedia

Books

Biographies of physical constants. Fascinating stories about universal physical constants. Issue 46
Biographies of physical constants. Fascinating stories about universal physical constants, O. P. Spiridonov. This book is devoted to the consideration of universal physical constants and their important role in the development of physics. The task of the book is to tell in a popular form about the appearance in the history of physics ...

The Italian scientist Amedeo Avogadro, a contemporary of A. S. Pushkin, was the first to understand that the number of atoms (molecules) in one gram-atom (mole) of a substance is the same for all substances. Knowledge of this number opens the way to estimating the size of atoms (molecules). During the life of Avogadro, his hypothesis did not receive due recognition. The history of the Avogadro number is the subject of a new book by Evgeny Zalmanovich Meilikhov, professor at the Moscow Institute of Physics and Technology, chief researcher at the National Research Center "Kurchatov Institute".

If, as a result of some world catastrophe, all the accumulated knowledge would be destroyed and only one phrase would come to the future generations of living beings, then what statement, composed of the least number of words, would bring most information? I believe this is the atomic hypothesis:<...>all bodies are made up of atoms - small bodies that are in constant motion.

R. Feynman, "The Feynman Lectures on Physics"

The Avogadro number (Avogadro's constant, Avogadro's constant) is defined as the number of atoms in 12 grams of the pure isotope carbon-12 (12 C). It is usually denoted as N A, less often L. The value of the Avogadro number recommended by CODATA (working group on fundamental constants) in 2015: N A = 6.02214082(11) 1023 mol −1 . A mole is the amount of a substance that contains N A structural elements (that is, as many elements as there are atoms in 12 g 12 C), and the structural elements are usually atoms, molecules, ions, etc. By definition, the atomic mass unit (amu) is 1/12 the mass of a 12 C atom. One mole (gram-mol) of a substance has a mass (molar mass) that, when expressed in grams, is numerically equal to the molecular weight of that substance (expressed in atomic mass units). For example: 1 mol of sodium has a mass of 22.9898 g and contains (approximately) 6.02 10 23 atoms, 1 mol of calcium fluoride CaF 2 has a mass of (40.08 + 2 18.998) = 78.076 g and contains (approximately) 6 .02 10 23 molecules.

At the end of 2011, at the XXIV General Conference on Weights and Measures, a proposal was unanimously adopted to define the mole in a future version of the International System of Units (SI) in such a way as to avoid its linkage to the definition of the gram. It is assumed that in 2018 the mole will be determined directly by the Avogadro number, which will be assigned an exact (without error) value based on the measurement results recommended by CODATA. So far, the Avogadro number is not accepted by definition, but a measured value.

This constant is named after the famous Italian chemist Amedeo Avogadro (1776–1856), who, although he himself did not know this number, understood that it was a very large value. At the dawn of the development of atomic theory, Avogadro put forward a hypothesis (1811), according to which, at the same temperature and pressure, equal volumes of ideal gases contain the same number of molecules. This hypothesis was later shown to be a consequence of the kinetic theory of gases, and is now known as Avogadro's law. It can be formulated as follows: one mole of any gas at the same temperature and pressure occupies the same volume, under normal conditions equal to 22.41383 liters (normal conditions correspond to pressure P 0 = 1 atm and temperature T 0 = 273.15 K). This quantity is known as the molar volume of the gas.

The first attempt to find the number of molecules occupying a given volume was made in 1865 by J. Loschmidt. It followed from his calculations that the number of molecules per unit volume of air is 1.8 10 18 cm −3 , which, as it turned out, is about 15 times less than the correct value. Eight years later, J. Maxwell gave an estimate much closer to the truth - 1.9 · 10 19 cm −3 . Finally, in 1908, Perrin gives an already acceptable assessment: N A = 6.8 10 23 mol −1 Avogadro's number, found from experiments on Brownian motion.

Since then, a large number of independent methods have been developed to determine the Avogadro number, and more precise measurements have shown that in reality there are (approximately) 2.69 x 10 19 molecules in 1 cm 3 of an ideal gas under normal conditions. This quantity is called the Loschmidt number (or constant). It corresponds to Avogadro's number N A ≈ 6.02 10 23 .

Avogadro's number is one of the important physical constants that played an important role in the development of the natural sciences. But is it a "universal (fundamental) physical constant"? The term itself is not defined and is usually associated with a more or less detailed table of the numerical values of physical constants that should be used in solving problems. In this regard, the fundamental physical constants are often considered those quantities that are not constants of nature and owe their existence only to the chosen system of units (such, for example, the magnetic and electric vacuum constants) or conditional international agreements (such as, for example, atomic unit masses). The fundamental constants often include many derived quantities (for example, the gas constant R, the classical electron radius r e= e 2 / m e c 2, etc.) or, as in the case of molar volume, the value of some physical parameter related to specific experimental conditions, which are chosen only for reasons of convenience (pressure 1 atm and temperature 273.15 K). From this point of view, the Avogadro number is a truly fundamental constant.

This book is devoted to the history and development of methods for determining this number. The epic lasted for about 200 years and at different stages was associated with a variety of physical models and theories, many of which have not lost their relevance to this day. The brightest scientific minds had a hand in this story - suffice it to name A. Avogadro, J. Loschmidt, J. Maxwell, J. Perrin, A. Einstein, M. Smoluchovsky. The list could go on and on...

The author must admit that the idea of the book does not belong to him, but to Lev Fedorovich Soloveichik, his classmate at the Moscow Institute of Physics and Technology, a man who was engaged in applied research and development, but remained a romantic physicist at heart. This is a person who (one of the few) continues “even in our cruel age” to fight for a real “higher” physical education in Russia, appreciates and, to the best of his ability, promotes the beauty and elegance of physical ideas. It is known that from the plot, which A. S. Pushkin presented to N. V. Gogol, a brilliant comedy arose. Of course, this is not the case here, but perhaps this book will also be useful to someone.

This book is not a "popular science" work, although it may seem so at first glance. It discusses serious physics against some historical background, uses serious mathematics, and discusses rather complex scientific models. In fact, the book consists of two (not always sharply demarcated) parts, designed for different readers - some may find it interesting from a historical and chemical point of view, while others may focus on the physical and mathematical side of the problem. The author had in mind an inquisitive reader - a student of the Faculty of Physics or Chemistry, not alien to mathematics and passionate about the history of science. Are there such students? The author does not know the exact answer to this question, but, based on his own experience, he hopes that there is.

Introduction (abbreviated) to the book: Meilikhov EZ Avogadro's number. How to see an atom. - Dolgoprudny: Publishing House "Intellect", 2017.

AVOGADRO NUMBER, NA = (6.022045±0.000031) 1023, the number of molecules in a mole of any substance or the number of atoms in a mole of a simple substance. Avogadro himself did not make estimates of the number of molecules in a given volume, but he understood that this was a very large quantity. 18 g H2O is the same number of H2O molecules (Mr = 18), etc. Since then, a large number of independent methods for determining the Avogadro number have been developed. One mole of a substance contains the number of molecules or atoms equal to the Avogadro constant.

At present (2016), the Avogadro number is still a measurable (rather than accepted by definition) quantity. Having such practically ideal objects, it is possible to count with high accuracy the number of silicon atoms in the ball and thereby determine the Avogadro number. This hypothesis was later shown to be a necessary consequence of the kinetic theory, and is now known as Avogadro's law.

Calculations using the Avogadro number.

Counting the number of particles at different heights in the suspension column gave the Avogadro number 6.82x1023. Using the Avogadro number, exact masses of atoms and molecules of many substances were obtained: sodium, 3.819×10–23 g (22.9898 g/6.02×1023), carbon tetrachloride, 25.54×10–23 g, etc. Avogadro) - the number of structural elements (atoms, molecules, ions or other particles) in 1 mole. Name in honor of A. Avogadro, designated. A. p. is one of the foundations.

The Avogadro constant is one of the fundamental physical constants. Named after A. Avogadro. In Avogadro's time, his hypothesis could not be proved theoretically. Thus, it followed from them that equal volumes of hydrogen and chlorine give twice the volume of hydrogen chloride. Avogadro with all experimental data. The number of molecules in one mole began to be called the Avogadro constant (it is usually denoted NA). This definition of a mole persisted for nearly a century.

Even in the time of Cannizzaro, it was obvious that since atoms and molecules are very small and no one has yet seen them, Avogadro's constant must be very large. First of all, it was clear to them that both quantities are related to each other: the smaller the atoms and molecules are, the larger the Avogadro number will be. The Avogadro constant has been determined by many methods. By measuring the ratio of the intensities of direct sunlight to that scattered by the blue sky, one can determine the Avogadro constant.

The Avogadro constant is so large that it is difficult to imagine. N is the number of molecules in a given sample. In other words, one mole of a substance is contained in its mass, expressed in grams and equal to the relative molecular (or atomic) mass of this substance.

Find the molar mass of water (H2O). 1 mol of water is contained in its 0.018 kg, and therefore, MH2O = 0.018 kg / mol. Knowing the Avogadro number also makes it possible to estimate the size of molecules or the volume V0 per molecule.

Additional materials on the topic: Molecular physics. Moth. Avogadro constant. The amount of substance.

The first attempt to find the number of molecules occupying a given volume was made in 1865 by Y. Loschmidt. It followed from Loschmidt's calculations that for air the number of molecules per unit volume is 1.81 1018 cm-3, which is about 15 times less than the true value. In fact, 1 cm³ of an ideal gas under normal conditions contains 2.68675 1019 molecules.

Quantitative calculations in chemistry

The excellent agreement of the obtained values is a convincing evidence of the real number of molecules. One of the fundamental constants, which can be used to determine such quantities as, for example, the mass of an atom or molecule (see below), the charge of an electron, etc.

Physics calculators

The Faraday number can be determined by measuring the amount of electricity required to dissolve or precipitate 1 mole of silver. It can also be shown that 1 g of sodium should contain approximately 3×1022 atoms of this element. Boltzmann constant, Faraday constant, etc.). One of the best experiments.

Definition based on the measurement of the charge of an electron.

In general, I'm completely confused =) if someone can explain this to me, I will be very grateful! AT chemical processes the smallest particles are involved - molecules, atoms, ions, electrons. The molar mass of a substance (M) is the mass of one mole of that substance.

Perrin's experiments.

It enters into some other constants, for example, into the Boltzmann constant. The values of the relative molecular weight are calculated from the values of the relative atomic mass, taking into account the number of atoms of each element in the formula unit of a complex substance. Atoms and molecules are extremely small particles; therefore, the portions of substances that are taken for chemical reactions are characterized by physical quantities corresponding to a large number of particles.

The amount of matter is physical quantity, directly proportional to the number of particles that make up a given substance and are included in the taken portion of this substance. In chemical calculations, the mass of gaseous reactants and products is often replaced by their volumes. This physical constant is the molar volume of the gas under normal conditions.

It was Avogadro's law that helped scientists correctly determine the formulas of many molecules and calculate the atomic masses of various elements.

More than 20 independent methods for determining the Avogadro constant are known, for example. based on the measurement of the charge of an electron or the amount of electricity required for electrolytic. And when Napoleon's troops occupied northern Italy, Avogadro became secretary of the new French province. Indeed, if 1 liter of hydrogen contains the same number of molecules as 1 liter of oxygen, then the ratio of the densities of these gases is equal to the ratio of the masses of the molecules.

To do this, it was only necessary to analyze the results of other similar experiments. This is partly due to the lack of a simple and clear record of the formulas and equations of chemical reactions in those days. From the point of view of this theory, it was impossible to imagine an oxygen molecule consisting of two equally charged atoms!

Avogadro emphasized that the molecules in gases do not have to consist of single atoms, but may contain several atoms - the same or different.

The cornerstone of modern atomic theory, wrote Cannizzaro, is the theory of Avogadro... Who will not see in this long and unconscious circling of science around and in the direction of the goal of decisive proof in favor of the theory of Avogadro and Ampère?

The more atoms or molecules in a macroscopic body, the obviously more substance is contained in this body. The number of molecules in macroscopic bodies is enormous. This value was called the Loschmidt number (or constant). Equal volumes of different gases under the same conditions contain the same number of molecules.

Avogadro's law in chemistry helps to calculate the volume, molar mass, amount of a gaseous substance and the relative density of a gas. The hypothesis was formulated by Amedeo Avogadro in 1811 and was later confirmed experimentally.

Law

Joseph Gay-Lussac was the first to study the reactions of gases in 1808. He formulated the laws of thermal expansion of gases and volumetric ratios, having obtained from hydrogen chloride and ammonia (two gases) a crystalline substance - NH 4 Cl (ammonium chloride). It turned out that to create it, it is necessary to take the same volumes of gases. Moreover, if one gas was in excess, then the “extra” part after the reaction remained unused.

A little later, Avogadro formulated the conclusion that at the same temperatures and pressures, equal volumes of gases contain the same number of molecules. In this case, gases can have different chemical and physical properties.

Rice. 1. Amedeo Avogadro.

Two consequences follow from Avogadro's law:

first - one mole of gas under equal conditions occupies the same volume;
second - the ratio of the masses of equal volumes of two gases is equal to the ratio of their molar masses and expresses the relative density of one gas in terms of another (denoted by D).

Normal conditions (n.s.) are pressure P=101.3 kPa (1 atm) and temperature T=273 K (0°C). Under normal conditions, the molar volume of gases (the volume of a substance to its amount) is 22.4 l / mol, i.e. 1 mole of gas (6.02 ∙ 10 23 molecules - Avogadro's constant number) occupies a volume of 22.4 liters. Molar volume (V m) is a constant value.

Rice. 2. Normal conditions.

Problem solving

The main significance of the law is the ability to carry out chemical calculations. Based on the first consequence of the law, you can calculate the amount of gaseous matter through the volume using the formula:

where V is the volume of gas, V m is the molar volume, n is the amount of substance, measured in moles.

The second conclusion from Avogadro's law concerns the calculation of the relative density of a gas (ρ). Density is calculated using the m/V formula. If we consider 1 mole of gas, then the density formula will look like this:

ρ (gas) = M/V m ,

where M is the mass of one mole, i.e. molar mass.

To calculate the density of one gas from another gas, it is necessary to know the density of the gases. The general formula for the relative density of a gas is as follows:

D(y)x = ρ(x) / ρ(y),

where ρ(x) is the density of one gas, ρ(y) is the density of the second gas.

If we substitute the density calculation into the formula, we get:

D (y) x \u003d M (x) / V m / M (y) / V m.

The molar volume decreases and remains

D(y)x = M(x) / M(y).

Consider practical use law on the example of two tasks:

How many liters of CO 2 will be obtained from 6 mol of MgCO 3 in the reaction of decomposition of MgCO 3 into magnesium oxide and carbon dioxide (n.o.)?
What is the relative density of CO 2 for hydrogen and for air?

Let's solve the first problem first.

n(MgCO 3) = 6 mol

MgCO 3 \u003d MgO + CO 2

The amount of magnesium carbonate and carbon dioxide is the same (one molecule each), therefore n (CO 2) \u003d n (MgCO 3) \u003d 6 mol. From the formula n \u003d V / V m, you can calculate the volume:

V = nV m , i.e. V (CO 2) \u003d n (CO 2) ∙ V m \u003d 6 mol ∙ 22.4 l / mol \u003d 134.4 l

Answer: V (CO 2) \u003d 134.4 l

Solution of the second problem:

D (H2) CO 2 \u003d M (CO 2) / M (H 2) \u003d 44 g / mol / 2 g / mol \u003d 22;
D (air) CO 2 \u003d M (CO 2) / M (air) \u003d 44 g / mol / 29 g / mol \u003d 1.52.

Rice. 3. Formulas for the amount of substance by volume and relative density.

The formulas of Avogadro's law only work for gaseous substances. They do not apply to liquids and solids.

What have we learned?

According to the formulation of the law, equal volumes of gases under the same conditions contain the same number of molecules. Under normal conditions (n.c.), the value of the molar volume is constant, i.e. V m for gases is always 22.4 l/mol. It follows from the law that the same number of molecules of different gases under normal conditions occupy the same volume, as well as the relative density of one gas in another - the ratio of the molar mass of one gas to the molar mass of the second gas.

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Doctor of Physical and Mathematical Sciences Evgeny Meilikhov

Introduction (abbreviated) to the book: Meilikhov EZ Avogadro's number. How to see an atom. - Dolgoprudny: Publishing House "Intellect", 2017.

The history of the Avogadro number is the subject of a new book by Evgeny Zalmanovich Meilikhov, professor at the Moscow Institute of Physics and Technology, chief researcher at the National Research Center "Kurchatov Institute".

If, as a result of some world catastrophe, all the accumulated knowledge would be destroyed and only one phrase would come to the future generations of living beings, then what statement, composed of the smallest number of words, would bring the most information? I believe that this is the atomic hypothesis: ... all bodies are composed of atoms - small bodies that are in constant motion.
R. Feynman. Feynman Lectures on Physics

The Avogadro number (Avogadro's constant, Avogadro's constant) is defined as the number of atoms in 12 grams of the pure isotope carbon-12 (12 C). It is usually denoted as N A, less often L. The value of the Avogadro number recommended by CODATA (working group on fundamental constants) in 2015: N A = 6.02214082(11) 10 23 mol -1. A mole is the amount of a substance that contains N A structural elements (that is, as many elements as there are atoms in 12 g 12 C), and the structural elements are usually atoms, molecules, ions, etc. By definition, the atomic mass unit (a.e. .m) is equal to 1/12 of the mass of a 12 C atom. One mole (gram-mol) of a substance has a mass (molar mass) that, when expressed in grams, is numerically equal to the molecular weight of that substance (expressed in atomic mass units). For example: 1 mol of sodium has a mass of 22.9898 g and contains (approximately) 6.02 10 23 atoms, 1 mol of calcium fluoride CaF 2 has a mass of (40.08 + 2 18.998) = 78.076 g and contains (approximately) 6 .02 10 23 molecules.

This constant is named after the famous Italian chemist Amedeo Avogadro (1776-1856), who, although he himself did not know this number, understood that it was a very large value. At the dawn of the development of atomic theory, Avogadro put forward a hypothesis (1811), according to which, at the same temperature and pressure, equal volumes of ideal gases contain the same number of molecules. This hypothesis was later shown to be a consequence of the kinetic theory of gases, and is now known as Avogadro's law. It can be formulated as follows: one mole of any gas at the same temperature and pressure occupies the same volume, under normal conditions equal to 22.41383 liters (normal conditions correspond to pressure P 0 \u003d 1 atm and temperature T 0 \u003d 273.15 K). This quantity is known as the molar volume of the gas.

The first attempt to find the number of molecules occupying a given volume was made in 1865 by J. Loschmidt. From his calculations it followed that the number of molecules per unit volume of air is 1.8·10 18 cm -3, which, as it turned out, is about 15 times less than the correct value. Eight years later, J. Maxwell gave a much closer estimate to the truth - 1.9·10 19 cm -3. Finally, in 1908, Perrin gives an already acceptable estimate: N A = 6.8·10 23 mol -1 Avogadro's number, found from experiments on Brownian motion.

Since then, a large number of independent methods have been developed to determine the Avogadro number, and more accurate measurements have shown that in reality there are (approximately) 2.69 x 10 19 molecules in 1 cm 3 of an ideal gas under normal conditions. This quantity is called the Loschmidt number (or constant). It corresponds to the Avogadro number N A ≈ 6.02·10 23 .

Avogadro's number is one of the important physical constants that played an important role in the development of the natural sciences. But is it a "universal (fundamental) physical constant"? The term itself is not defined and is usually associated with a more or less detailed table of the numerical values of physical constants that should be used in solving problems. In this regard, fundamental physical constants are often considered those quantities that are not constants of nature and owe their existence only to the chosen system of units (such, for example, the magnetic and electric vacuum constants) or conditional international agreements (such, for example, the atomic mass unit) . The fundamental constants often include many derived quantities (for example, the gas constant R, the classical electron radius r e \u003d e 2 /m e c 2, etc.) or, as in the case of molar volume, the value of some physical parameter related to specific experimental conditions that are chosen only for reasons of convenience (pressure 1 atm and temperature 273.15 K). From this point of view, the Avogadro number is a truly fundamental constant.

Information about the books of the Publishing House "Intellect" - on the site www.id-intellect.ru