mole(Russian designation: mole ; international: mol ; obsolete name gram molecule(in relation to the number of molecules); from lat. moles - quantity, mass, countable set) - a unit for measuring the amount of a substance in the International System of Units (SI), one of the seven basic SI units.
The mole was adopted as the basic SI unit by the XIV General Conference on Weights and Measures (CGPM) in 1971.
So far, the definition of a mole is related to mass. However, the XXVI General Conference on Weights and Measures (November 13-16, 2018) approved a new definition of the mole based on fixing the numerical value of the Avogadro constant. The decision will come into force on World Metrology Day on May 20, 2019.
Definition
The exact definition of a mole is formulated as follows:
Mole - the amount of substance of a system containing as many structural elements as there are atoms in carbon-12 weighing 0.012 kg. When using a mole, the structural elements must be specified and may be atoms, molecules, ions, electrons and other particles, or specified groups of particles.
It follows directly from the definition of a mole that the molar mass of carbon-12 is 12 g/mol exactly.
The number of specified structural elements in one mole of a substance is called the Avogadro constant (Avogadro number), usually denoted as N A. Thus, 0.012 kg of carbon-12 contains N A atoms. The value of the Avogadro constant recommended by the Committee on Data for Science and Technology (CODATA) in 2014 is 6.022140857(74)⋅10 23 mol −1 . Hence, 1 atom of carbon-12 has a mass of 0.012 / N A kg = 12/ N A g. 1/12 of the mass of the carbon-12 atom is called the atomic mass unit (abm notation), and, therefore, 1 a. e.m. = 0.001/ N A kg =1/ N A g. Thus, the mass of one mole of a substance (molar mass) is equal to the mass of one particle of a substance, atom or molecule, expressed in a. e. m. and multiplied by N A.
For example, the mass of 1 mole lithium, having an atomic crystal lattice, will be equal to
7 a. e.m.x N A \u003d 7 x 1 / N A g x N A mol −1 = 7 g/mol,
and the mass is 1 mole oxygen, consisting of diatomic molecules
2 x 16 a. e.m.x N A \u003d 2 x 16 x 1 / N A g x N A mol −1 \u003d 32 g / mol.
That is, from definition a. e. m. it follows that the molar mass of a substance, expressed in grams per mole, numerically is equal to the mass of the smallest particle (atom or molecule) of this substance, expressed in atomic mass units.
The mole will remain the unit of quantity of matter; but its magnitude will be established by fixing the numerical value of the Avogadro constant to exactly 6.02214X⋅10 23 when it is expressed in the SI unit mol −1 .
Here, X replaces one or more significant digits, which will be determined in the future based on the best recommendations of CODATA.
XXV CGPM, held in 2014, decided to continue work on the preparation of a new revision of the SI, including the redefinition of the mole, and planned to complete this work by 2018 in order to replace the existing SI with an updated version at XXVI CGPM in the same year.
Multiples and submultiples
Decimal multiples and submultiples are formed using standard SI prefixes. Moreover, the unit of measurement “ioktomol” can only be used formally, since such small amounts of a substance must be measured by individual particles (1 imol is formally equal to 0.602 particles).
| Multiples | Dolnye | ||||||
|---|---|---|---|---|---|---|---|
| magnitude | title | designation | magnitude | title | designation | ||
| 10 1 mol | decamole | damol | Damol | 10 −1 mol | decimol | dmol | dmol |
| 10 2 mol | hectomol | gmol | hmol | 10 −2 mol | centimole | resin | cmol |
| 10 3 mol | kilomole | kmol | kmol | 10 −3 mol | millimole | mmol | mmol |
| 10 6 mol | megamol | mmol | mmol | 10 −6 mol | micromol | µmol | mmol |
| 10 9 mol | gigamole | Gmol | gmol | 10 −9 mol | nanomol | nmol | nmol |
| 10 12 mol | theramol | Tmol | tmol | 10 −12 mol | picomole | pmol | pmol |
| 10 15 mol | petamol | Pmol | pmol | 10 −15 mol | femtomol | fmol | fmol |
| 10 18 mol | examol | emol | emol | 10 −18 mol | attomol | amol | amol |
| 10 21 mol | zettamol | Zmol | Zmol | 10 −21 mol | zeptomol | zmol | zmol |
| 10 24 mol | yottamol | Imol | Ymol | 10 −24 mol | yoktomol | imol | ymol |
| application is not recommended | |||||||
Holiday "Day of the Mole"
see also
Notes
- Term gram atom as applied to the mole of atoms, it is also little used at present.
- Mole (unit of amount of substance) // Moesia - Morshansk. - M.: Soviet Encyclopedia, 1974. - (Great Soviet Encyclopedia: [in 30 volumes] / ch. ed.
Yesterday I promised to explain it in an accessible language. Something important for understanding chemistry. Once you understand, then you will never forget.
Chemistry has its own language, like any science. 2H 2 + O 2 → 2H 2 O - in chemical terms, a record of the reaction of the formation of water from simple substances, hydrogen (H) and oxygen (O). Small numbers refer to the number of atoms (They are after the symbol of the chemical element), large - to the number of molecules. It can be seen from the equation that two hydrogen molecules combine with one oxygen molecule and as a result two water molecules. Attention - this is very important to understand! It is molecules that connect with molecules, not “gram with gram”, but molecule with molecule.
This proportion will always remain:
Everything would be fine, but there are two problems. The first is that in real life we will not be able to measure one million molecules of oxygen or hydrogen. We will be able to measure one gram or one ton of reagents. Second, the molecules are very small. There are 6.7 10 24 of them in one glass of water. Or, in the usual notation, 6.7 trillion trillion (that's right - almost seven trillion times over a trillion molecules). It is inconvenient to operate with such figures.
What is the way out? Molecules, after all, also have mass, albeit a very small one. We just take mass of one molecule, multiply by number of molecules and we get the mass we need. We agreed so - we take a very large number of molecules (600 billion trillion pieces) and invent for this amount special unit of measure mole. Like there is a special name for 12 pieces of something "dozen", and when they talk about "ten dozen", they mean 120 pieces. 5 dozen eggs = 60 pieces. So with moles. 1 mole is 600 billion trillion molecules or, in mathematical notation, 6.02 10 23 molecules. That is, when we are told “1 mole” of hydrogen, we know that we are talking about 600 billion trillion hydrogen molecules. When talking about 0.2 moles of water, we understand that we are talking about 120 billion trillion water molecules.
Once again - the mole is just like that counting unit, only specifically for molecules. Like a "ten", "dozen" or "million", only much more.
Continuing the table above, one can write:
We solved the first problem, writing 1 mole or 2 moles is much more convenient than 600 billion trillion molecules or 1.2 trillion trillion molecules. But for one convenience, it was not worth fencing the garden. The second problem, as we remember, is the transition from number of molecules(do not count them individually!) to mass of matter, to what we can measure on the scales. Such a number of molecules in one mole (after all, it is a little strange, non-circular - 6.02 10 23 molecules) was chosen for a reason. One mole of carbon molecules weighs exactly 12 grams.
It is clear that all molecules are different. There are large and heavy ones - they may have many atoms, or not very many, but the atoms themselves are heavy. And there are small and light molecules. For each atom and for many molecules there are tables in the reference books with their molar mass. That is, with the weight of one mole of such molecules (if not, you can easily calculate it yourself by adding the molar masses of all the atoms that make up the molecule). Molar mass is measured in grams / mol (how many grams weigh one mole, that is, how many grams weigh 6.02 10 23 molecules). We remember that the mole is just a counting unit. Well, as if they wrote in the reference book - 1 dozen chicken eggs weigh 600 grams, and 1 dozen ostrich eggs weigh 19 kilograms. A dozen is just a quantity (12 pieces), and the eggs themselves, chicken or ostrich, weigh differently. And a dozen of these or other eggs also weigh differently.
So it is with molecules. 1 mole of small and light hydrogen molecules weighs 2 grams, and 1 mole of large sulfuric acid molecules weighs 98 grams. 1 mole of oxygen weighs 32 grams, 1 mole of water weighs 18 grams. Here is an example picture showing small hydrogen molecules and large oxygen molecules. This picture is a graphical representation of the reaction 2H 2 + O 2 → 2H 2 O.
We continue to fill in the table:
See the transition from number of molecules to their mass? See that the law of conservation of matter is fulfilled? 4 grams + 32 grams gave 36 grams.
Now we can solve simple problems in chemistry. Here is the most primitive one: There were 100 oxygen molecules and 100 hydrogen molecules. What will happen as a result of the reaction? We know that for 1 molecule of oxygen, 2 molecules of hydrogen are needed. Therefore, all 100 hydrogen molecules will react (and 100 water molecules are formed), but not all oxygen will react, another 50 molecules will remain. Oxygen is in excess.
Molecules are pieces, as I said above, no one considers. Substances are usually measured in grams. Now a task from a school textbook: there are 10 g of hydrogen and 64 g of oxygen, what will happen if they are mixed? We must first convert the masses to moles (that is, the number of molecules or the amount of substance, as chemists say). 10 g of hydrogen is 5 moles of hydrogen (1 mole of hydrogen weighs 2 grams). 64 g of oxygen is 2 moles (1 mole weighs 32 grams). We know that for 1 mole of oxygen, 2 moles of hydrogen are consumed in the reaction. This means that in our case, all oxygen (2 moles) and 4 out of five moles of hydrogen will react. You get 4 moles of water and still have 1 mole of hydrogen.
Let's convert the answer back to grams. All oxygen (64 grams) and 8 grams of hydrogen (4 mol * 2 g/mol) will react. 1 mole of hydrogen remains unreacted (that's 2 grams) and you get 72 grams of water (4 moles * 18 g/mol). The law of conservation of matter is again fulfilled - 64 + 10 = 72 + 2.
I think it should be clear to everyone by now. 1 mole is just the number of molecules. Molar mass is the mass of one mole. It is needed in order to move from the mass of a substance (with which we work in the real world) to the number of molecules, or the amount of substance needed for reactions.
Let's repeat again:
a) substances react in the ratio of n molecules of one to m molecules of the other. This proportion will be the same for 100 molecules of the original substance, and for a hundred trillion, or a hundred trillion trillion.
b) for convenience, in order not to consider molecules as pieces, they came up with a special counting unit - a mole, that is, immediately 6.02 10 23 molecules. The number of these moles is called the usual "amount of substance"
c) a mole of each substance weighs differently, because. the molecules and atoms that make up matter themselves weigh differently. The mass of one mole of a substance is called its molar mass. Another example is that ordinary and silicate bricks weigh differently. If we draw an analogy, then the “weight of a thousand bricks” is the “molar mass” (with the difference that there are not 1000 molecules, but more). The mass of this "thousand bricks" is different for silicate and ordinary bricks.
d) we fence this whole garden in order to easily switch from the mass of reagents to the amount of substance (the number of molecules, the number of moles) and vice versa. And you need to go back and forth because in the real world we measure reagents in grams, and chemical reactions proceed in proportion not to mass, but to the number of molecules.
P.S. Chemists and others - I specifically simplified a lot here. I don’t need to explain that 12 grams weighs not 1 mole of carbon, but 1 mole of C 12 isotope molecules, or about the fact that instead of “molecules” it would be necessary to write “structural units” (molecules, ions, atoms ...), specially not mentioned that 1 mole of gas occupies the same volume under the same conditions and much more
What I didn't like in the textbooks is only the formal definition of the mole, without specifying the meaning of this concept and what it is for.
One of the basic units in the International System of Units (SI) is the unit of quantity of a substance is the mole.
mole – this is such an amount of a substance that contains as many structural units of a given substance (molecules, atoms, ions, etc.) as there are carbon atoms in 0.012 kg (12 g) of a carbon isotope 12 FROM .
Given that the value of the absolute atomic mass for carbon is m(C) \u003d 1.99 10 26 kg, you can calculate the number of carbon atoms N BUT contained in 0.012 kg of carbon.
A mole of any substance contains the same number of particles of this substance (structural units). The number of structural units contained in a substance with an amount of one mole is 6.02 10 23 and called Avogadro's number (N BUT ).
For example, one mole of copper contains 6.02 10 23 copper atoms (Cu), and one mole of hydrogen (H 2) contains 6.02 10 23 hydrogen molecules.
molar mass(M) is the mass of a substance taken in an amount of 1 mol.
The molar mass is denoted by the letter M and has the unit [g/mol]. In physics, the dimension [kg/kmol] is used.
In the general case, the numerical value of the molar mass of a substance numerically coincides with the value of its relative molecular (relative atomic) mass.
For example, the relative molecular weight of water is:
Mr (H 2 O) \u003d 2Ar (H) + Ar (O) \u003d 2 ∙ 1 + 16 \u003d 18 a.m.u.
The molar mass of water has the same value, but is expressed in g/mol:
M (H 2 O) = 18 g/mol.
Thus, a mole of water containing 6.02 10 23 water molecules (respectively 2 6.02 10 23 hydrogen atoms and 6.02 10 23 oxygen atoms) has a mass of 18 grams. 1 mole of water contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms.
1.3.4. The relationship between the mass of a substance and its quantity
Knowing the mass of a substance and its chemical formula, and hence the value of its molar mass, one can determine the amount of a substance and, conversely, knowing the amount of a substance, one can determine its mass. For such calculations, you should use the formulas:
where ν is the amount of substance, [mol]; m is the mass of the substance, [g] or [kg]; M is the molar mass of the substance, [g/mol] or [kg/kmol].
For example, to find the mass of sodium sulfate (Na 2 SO 4) in the amount of 5 mol, we find:
1) the value of the relative molecular weight of Na 2 SO 4, which is the sum of the rounded values of the relative atomic masses:
Mr (Na 2 SO 4) \u003d 2Ar (Na) + Ar (S) + 4Ar (O) \u003d 142,
2) the value of the molar mass of the substance numerically equal to it:
M (Na 2 SO 4) = 142 g/mol,
3) and, finally, a mass of 5 mol of sodium sulfate:
m = ν M = 5 mol 142 g/mol = 710 g
Answer: 710.
1.3.5. The relationship between the volume of a substance and its quantity
Under normal conditions (n.o.), i.e. at pressure R , equal to 101325 Pa (760 mm Hg), and temperature T, equal to 273.15 K (0 С), one mole of various gases and vapors occupies the same volume, equal to 22.4 l.
The volume occupied by 1 mole of gas or vapor at n.o. is called molar volumegas and has the dimension of a liter per mole.
V mol \u003d 22.4 l / mol.
Knowing the amount of gaseous substance (ν ) and molar volume value (V mol) you can calculate its volume (V) under normal conditions:
V = ν V mol,
where ν is the amount of substance [mol]; V is the volume of the gaseous substance [l]; V mol \u003d 22.4 l / mol.
Conversely, knowing the volume ( V) of a gaseous substance under normal conditions, you can calculate its amount (ν) :
mole(Russian designation: mole ; international: mol ; obsolete name gram molecule(in relation to the number of molecules); from lat. moles - quantity, mass, countable set) - a unit for measuring the amount of a substance in the International System of Units (SI), one of the seven basic SI units.
The mole was adopted as the basic SI unit by the XIV General Conference on Weights and Measures (CGPM) in 1971.
So far, the definition of a mole is related to mass. However, the XXVI General Conference on Weights and Measures (November 13-16, 2018) approved a new definition of the mole based on fixing the numerical value of the Avogadro constant. The decision will come into force on World Metrology Day on May 20, 2019.
Definition [ | ]
The exact definition of a mole is formulated as follows:
Mole - the amount of substance of a system containing as many structural elements as there are atoms in carbon-12 weighing 0.012 kg. When using a mole, the structural elements must be specified and may be atoms, molecules, ions, electrons and other particles, or specified groups of particles.
It follows directly from the definition of a mole that the molar mass of carbon-12 is 12 g/mol exactly.
The number of specified structural elements in one mole of a substance is called the Avogadro constant (Avogadro number), usually denoted as N A. Thus, 0.012 kg of carbon-12 contains N A atoms. The value of the Avogadro constant recommended by the Committee on Data for Science and Technology (CODATA) in 2014 is 6.022140857(74)⋅10 23 mol −1 . Hence, 1 atom of carbon-12 has a mass of 0.012 / N A kg = 12/ N A g. 1/12 of the mass of the carbon-12 atom is called the atomic mass unit (abm notation), and, therefore, 1 a. e.m. = 0.001/ N A kg =1/ N A g. Thus, the mass of one mole of a substance (molar mass) is equal to the mass of one particle of a substance, atom or molecule, expressed in a. e. m. and multiplied by N A.
For example, the mass of 1 mole lithium, having an atomic crystal lattice, will be equal to
7 a. e.m.x N A \u003d 7 x 1 / N A g x N A mol −1 = 7 g/mol,
and the mass is 1 mole oxygen, consisting of diatomic molecules
2 x 16 a. e.m.x N A \u003d 2 x 16 x 1 / N A g x N A mol −1 \u003d 32 g / mol.
That is, from definition a. e. m. it follows that the molar mass of a substance, expressed in grams per mole, numerically is equal to the mass of the smallest particle (atom or molecule) of this substance, expressed in atomic mass units.
The mole will remain the unit of quantity of matter; but its magnitude will be established by fixing the numerical value of the Avogadro constant to exactly 6.02214X⋅10 23 when it is expressed in the SI unit mol −1 .
Here, X replaces one or more significant digits, which will be determined in the future based on the best recommendations of CODATA.
XXV CGPM, held in 2014, decided to continue work on the preparation of a new revision of the SI, including the redefinition of the mole, and planned to complete this work by 2018 in order to replace the existing SI with an updated version at XXVI CGPM in the same year.
Multiples and submultiples[ | ]
Decimal multiples and submultiples are formed using standard SI prefixes. Moreover, the unit of measurement “ioktomol” can only be used formally, since such small amounts of a substance must be measured by individual particles (1 imol is formally equal to 0.602 particles).
| Multiples | Dolnye | ||||||
|---|---|---|---|---|---|---|---|
| magnitude | title | designation | magnitude | title | designation | ||
| 10 1 mol | decamole | damol | Damol | 10 −1 mol | decimol | dmol | dmol |
| 10 2 mol | hectomol | gmol | hmol | 10 −2 mol | centimole | resin | cmol |
| 10 3 mol | kilomole | kmol | kmol | 10 −3 mol | millimole | mmol | mmol |
| 10 6 mol | megamol | mmol | mmol | 10 −6 mol | micromol | µmol | mmol |
| 10 9 mol | gigamole | Gmol | gmol | 10 −9 mol | nanomol | nmol | nmol |
| 10 12 mol | theramol | Tmol | tmol | 10 −12 mol | picomole | pmol | pmol |
| 10 15 mol | petamol | Pmol | pmol | 10 −15 mol | femtomol | fmol | fmol |
| 10 18 mol | examol | emol | emol | 10 −18 mol | attomol | amol | amol |
| 10 21 mol | zettamol | Zmol | Zmol | 10 −21 mol | zeptomol | zmol | zmol |
| 10 24 mol | yottamol | Imol | Ymol | 10 −24 mol | yoktomol | imol | ymol |
| application is not recommended | |||||||
Holiday "Day of the Mole"[ | ]
see also [ | ]
Notes [ | ]
- Term gram atom as applied to the mole of atoms, it is also little used at present.
- Mole (unit of amount of substance) // Moesia - Morshansk. - M.: Soviet Encyclopedia, 1974. - (Great Soviet Encyclopedia: [in 30 volumes] / ch. ed.
