The quantity of matter is a physical quantity, representing an aggregate containing a certain number of particles, with the symbol n 。 The unit of quantity of substance is mole. Scientifically Avogadro constant (about 6.02 × 10 twenty-three ）The collective of particles as a unit is called mole 。 The number of particles in 1mol of different substances is the same, but because the mass of different particles is different, the mass of 1mol of different substances is also different. The 14th session in 1971 International Conference on Metrology There are two paragraphs about the definition of mole: "Mole is the amount of substances in a system, and Basic unit The number is equal to the number of atoms of 0.012kg carbon-12. " "When using the mole, the basic unit should be specified, which can be atom 、 molecule 、 Ions 、 Electronics And others particle , or a specific combination of these particles. " The last two paragraphs should be regarded as a whole. 0.012kg carbon-12 contained carbon atom Number is Avogadro constant（ N A ）, the approximate value measured in the experiment is N A =6.021687126645×10 twenty-three 。 The mole is different from the general unit. It has one characteristic: it measures the micro basic units, such as ions, but cannot be used to measure substances. 1mol substance is measured in Avogadro number, not in number of molecules, atoms, etc particulate Number of. It can also be used for measurement Microparticle For example, measure the amount of sulfuric acid by mole, that is, 1mol sulfuric acid contains 6.02 × 10 twenty-three Sulfuric acid molecules. Mole is the most widely used unit of measurement in chemistry, such as calculation of chemical reaction equation, calculation in solution, preparation and dilution of solution, calculation of chemical balance, Molar volume of gas And thermochemistry are inseparable from this basic unit. Molar mass is the mass of the substance divided by the amount of the substance, in grams per mole (g/mol), and molar volume is the volume of the substance divided by the amount of the substance, in cubic meters per mole (m three /mol）。
Molar mass and molar volume are derived quantities of substance quantity, and the basic unit must be specified in application. For the same substance, the basic unit is different, and the molar mass and molar volume are different.
Although Avogadro constant is a large value, it is very convenient to use the mole as the unit of material quantity. It is like a bridge Microparticle Associated with macroscopic matter. [1] The mass of a substance per unit of substance is called molar mass M express. When the mass of a substance is expressed in grams, it is numerically equal to the relative atomic mass or molecular mass of the substance. [1]
The common unit of molar mass is g/mol (grams per mole), and the international system unit of molar mass is kg/mol (kilograms per mole).
For a pure substance, its molar mass is fixed, and the mass of the substance changes with the amount of the substance. For example, 1mol O two 32 g, 2 mol O ₂ 64 g, but O two The molar mass of is still 32 g/mol. According to the accurate measurement of scientific experiments, it is known that 0.012kg carbon-12 atom contains about 6.02 × 10 carbon atoms twenty-three （ Avogadro constant ）。 [2] As we know, the number of molecules, atoms or ions in 1mol of different substances is the same, but the mass of different particles is different, so the mass of 1mol of different substances is also different. So, what is the mass of 1mol of different substances?
The relative atomic mass of one element is 1/12 of the mass of C-12 as the standard, and the atomic mass of other elements is compared with it. Through the study of the concept of material quantity, we also know that the mass of 1mol C-12 is 0.012kg, that is, 0.012kg is the mass of 6.02 × 10 ² ³ C-12. Using the relationship that any particle collective of 1mol contains the same number of particles, we can deduce the mass of any particle of 1mol. For example, the mass ratio of 1 ¹² C to 1 H is about 12:1, so the mass ratio of 1mol ¹² C to 1mol H is also about 12:1. Since the mass of 1mol ¹² C is 0.012kg, the mass of 1mol H is 0.001 kg.
For atoms, the mass of any atom in 1mol is in grams, which is numerically equal to the relative atomic mass of the atom. For example, the relative atomic mass of O is 16, and the mass of 1mol O is 16g; The relative atomic mass of Na is 23, and the mass of 1mol Na is 23g.
For molecules, 1mol, the mass of any molecule is in grams, which is numerically equal to relative molecular mass (formula quantity). For example, the relative molecular weight of O ₂ is 32, and the mass of 1mol O ₂ is 32g; The relative molecular weight of NaCl is 58.5, and the mass of 1mol NaCl is 58.5g. Ions are formed when atoms gain or lose electrons. Since the mass of the electron is very small, the mass of the atom after obtaining or losing the electron is still approximately equal to the mass of the atom. Therefore, for a simple ion, the mass of 1mol of any ion is in grams, which is numerically equal to the relative atomic mass of the atom forming the ion. For example, it can be inferred that the relative atomic mass of Na+is 23 and the mass of 1mol Na+is 23g; The relative atomic mass of Cl - is 35.5, and the mass of 1mol Cl - is 35.5g.
For more complex ions, such as atomic clusters, the mass of any atomic cluster of 1mol is in grams, which is numerically equal to the sum of the relative atomic masses of the atoms constituting the atomic cluster. For example, the sum of the relative atomic mass of SO ₄ ² - is 96, and the mass of 1mol SO ₄ ² - is 96g;
The sum of the relative atomic mass of NH ₄+is 18, and the mass of 1mol NH ₄+is 18g.
From the above analysis, it is not difficult to see that when the unit is gram, the mass of 1mol of any particle or substance is numerically equal to the relative atomic mass or molecular mass of the particle. We call the mass of a substance per unit mass molar mass. In other words, the molar mass of a substance is the ratio of the mass of the substance to the amount of the substance. The symbol of molar mass is M , the common unit is g/mol or kg/mol.
For the above example, their molar masses are:
The molar mass of O is 16g/mol;
The molar mass of Na is 23g/mol;
The molar mass of O ₂ is 32g/mol;
The molar mass of NaCl is 58.5g/mol;
The molar mass of Na+is 23g/mol;
The molar mass of Cl is 35.5g/mol;
The molar mass of is 96g/mol; The molar mass of is 18g/mol. Similarly, the molar mass of C-12 is 12g/mol.
Amount of substance（ n ）. Material quality（ m ）And molar mass of substance（ M ）There is a relationship between:
n = m / M 。 [1]
Why is it that when you add g/mol to the relative atomic mass (23) of Na, the mass of 1mol Na will be 23g.
① Since atoms are composed of neutrons, protons and electrons, the mass of atoms=the mass of neutrons in the nucleus+the mass of protons in the nucleus (the mass of electrons is too small, which can be ignored.) According to the data, the mass of one neutron is 1.6749 × 10 ⁻ ² ⁷ kg, and the mass of one proton is 1.6726 × 10 ⁻ ² ⁷ kg.
That is, the mass of Na is 12 neutrons × 1.6749 × 10 ⁻ ² ⁷ kg+11 protons × 1.6726 × 10 ⁻ ² ⁷ kg.
② 1/12 of the mass of carbon atom; It refers to a carbon atom with 6 protons and 6 neutrons, 1/12 of its mass. (equal to 1.66 × 10 ⁻ ² ⁷ kg)
That is, (6 protons × 1.6726 × 10 ⁻ ² ⁷ kg+6 neutrons × 1.6749 × 10 ⁻ ² ⁷ kg) × (1/12)=1.66 × 10 ⁻ ² ⁷ kg.
③ Because the number of atomic masses is too small, it is difficult to write, so the value obtained by taking 1/12 of the mass of carbon atom as the standard, compared with the mass of other atoms (1/12 of the mass of carbon atom), is called the relative atomic mass.
That is, the relative atomic mass of an atom=(1.6749 × 10 ⁻ ² ⁷ kg × neutron number+1.6726 × 10 ⁻ ² ⁷ kg × proton number) ÷ (1.66 × 10 ⁻ ² ⁷ kg)
④ The process of actually calculating the mass of 1 mol Na should be (11 × 1.6726 × 10 ⁻ ² ⁷ kg+12 × 1.6749 × 10 ⁻ ² ⁷ kg) × (6.02 × 10 ² ³)=23g. Since 1 ÷ (1.66 × 10 ⁻ ² ⁷) × 1000=6.02 × 10 ² ³, we can add g/mol after the relative atomic mass, which will also equal the actual mass.
Relationship between molar mass and relative atomic mass, relative molecular mass
(1) Difference: The molar mass is obtained by dividing the mass of the substance by the amount of the substance, in g/mol; The relative atomic mass or molecular mass of a substance is based on 1/12 of the atomic mass of C12, and the mass of other atoms or molecules is compared with it.
(2) Contact: when the molar mass is in g/mol, it is numerically equal to the relative atomic mass or molecular mass of the substance. Namely: 1mol
The molar mass of a substance and the relative molecular (atomic) mass of the substance are equal in value and have different units. [2]
