Let us carefully look at the table of D.I. Mendeleev and choose one of the elements. For example, the inhabitant of the sixth cell is carbon. What is depicted in this cell? The sign of the chemical element and its name. In the upper left corner is the ordinal (atomic) number of the element, under the symbol of the element is its name. And what does the number 12.011 written under the name mean?
This number is called the relative atomic mass of the element, in our case, carbon.
As you remember, one of the most important properties of atoms of chemical elements is their mass.
The dimension of mass is well known to you: it is expressed in grams, kilograms, milligrams. There are, of course, other units of mass: tons, for example, or centners. However, in chemistry it is more convenient to operate not by the exact mass of atoms or molecules, but by comparing it with something. Since the masses of atoms and molecules are extremely small, they are compared with the mass of the atom of the lightest element - hydrogen. This comparison suggests that the mass of the carbon atom is 12 times the mass of the hydrogen atom, the mass of the oxygen atom is 16 times, and the mass of the iron atom is 56 times.
The relative atomic mass of an element is denoted by the symbol Ar. The subscript r is the first letter of the English word relative, which means "relative." For example: Ar (H) \u003d 1, Ar (0) \u003d 16, Ar (C) \u003d 12, Ar (Fe) \u003d 56, Ar (N) \u003d 14, Ar (Cl) \u003d 35.5.
Usually, the relative atomic masses of all elements are rounded to integers, except for chlorine (Ar (C1) \u003d 35.5), although they are given with greater accuracy in the table of D. I. Mendeleev.
Note that the relative atomic mass of a dimension does not have what follows from the definition.
Do molecules have substances? Of course! Can you answer the question, how many times is a water molecule heavier than a hydrogen atom? It is very simple: you need to add the relative atomic masses of all the atoms that make up the molecule (Fig. 68):
Fig. 68.
Calculation of the relative molecular weight of water
Mr (H 2 0) \u003d 2 Ar (H) + Ar (0) \u003d 2 1 + 16 \u003d 18.
Mr (H 2 0) is the relative molecular weight of water. It would be more correct to call it the relative formula weight. Firstly, not all substances are composed of molecules (for example, iron, graphite or table salt). Secondly, when calculating this value, you need to carefully look at the formula of the substance and do not forget about the indices indicating the number of atoms or ions of each element that is part of the substance:
Mr (CH 4) \u003d Ar (C) + 4 Ar (H) \u003d 12 + 4 1 \u003d 16;
Mr (S0 2) \u003d Ar (S) + 2 Ar (0) \u003d 32 + 2 16 \u003d 64;
Mr (CuS0 4) \u003d Ar (Cu) + Ar (S) + 4 Ar (O) \u003d 64 + 32 + 4 16 \u003d 160.
Knowing the relative atomic masses, it is possible to determine the formulas of chemicals by the mass ratio of the elements included in its composition. How to make these calculations, we will tell in the next lesson.
Questions and Tasks
- What is relative atomic mass? Why does this quantity have no dimension?
- What is relative molecular weight? How is it calculated?
- The phosphoric acid molecule contains three hydrogen atoms, one phosphorus atom and four oxygen atoms. Find the relative molecular weight of this substance.
- Calculate the relative molecular weights of substances according to their formulas: Br 2, H 2 O, CO 2, H 2 SO 4, KOH, BaCl 2. Find the values \u200b\u200bof the relative atomic masses from the table of D. I. Mendeleev, round them to whole numbers (except for chlorine, Ar (C1) \u003d 35.5).
- The sulfur atom forms two complex substances (called oxides) of different composition with oxygen atoms. The relative molecular weight of the first oxide is 64, and the second is 80. Derive the formulas of these oxides.
- Without making calculations, but only based on the values \u200b\u200bof the relative atomic masses (find them according to the table of D. I. Mendeleev), determine which of the substances whose formulas are given below has the largest and smallest relative molecular masses: a) hydrogen selenide H 2 Se; b) water H 2 O; c) H 2 Te telluride; g) hydrogen sulfide H 2 S.
Subject: Fundamentals of Molecular Kinetic Theory
Lesson: A mass of molecules. Amount of substance
In previous lessons, it was already mentioned that the average size of atoms and molecules (with the exception of organic compounds) does not exceed cm (Fig. 1). Obviously, bodies with such dimensions cannot have a large mass. And there is.
Fig. 1. Atomic structure of a gold alloy under a microscope ()
The mass of a molecule is indicated and, like any other mass, is measured in kg. It is easy to guess that in order to find the described quantity it is necessary to measure the mass of a certain portion of the substance and divide it by the number of molecules included in this portion.
For example, one gram of water contains approximately molecules. Therefore, according to the above formula, the mass of the water molecule is approximately kg. And again, if organic substances are not taken into account, then the masses of molecules of all substances are comparable in magnitude.
Since it is always inconvenient to work with values \u200b\u200bthat are too large or too small, quantities such as relative atomic mass (OAM) and relative molecular mass (OMM) are introduced in physics and chemistry.
Definition Relative atomic mass- the ratio of the mass of an atom of a particular chemical element to one-twelfth of the mass of a carbon atom.
At the moment, all OAMs of known elements are calculated and known with fairly high accuracy. To find the relative mass of a certain molecule, it is enough to simply add the relative atomic masses of those elements that are part of the molecule, and as many times as many pieces of element atoms are included in the molecule. Using an example of a water molecule, it looks:
It is worth adding that the relative atomic and molecular masses are taken from the ratio to the mass of the carbon atom, because carbon is one of the most common elements in nature.
A certain portion of a substance (a certain body) is sometimes required to be described not from the side, what is the mass of this portion, but how many particles are contained in it. And, as mentioned earlier, such large numbers, which describe the absolute number of molecules and atoms in a substance, are inconvenient to use. Therefore, a quantity such as the quantity of substance is introduced.
Definition Amount of substance - a physical quantity characterizing the number of molecules entering a certain portion of a substance. Designation -.
Unit of measurement - mole.
1 mol- such an amount of substance that contains as many molecules (atoms) as 12 g of carbon.
Here: - the amount of all particles in a portion of a substance; - the number of particles in one mole, a value designated as the Avogadro number.
Definition Avogadro number - the number of atoms in 12 g of carbon calculated by the Italian scientist Amedeo Avogadro (Fig. 2). Based on the definition of a mole - the number of molecules or atoms in 1 mole. Unit of measurement - . The value of this important constant:
Fig. 2. Amedeo Avogadro ()
A certain portion of the substance in an amount of 1 mole already has dimensions that are visible to the eye (already a macro-object), and a tangible mass. Therefore, it makes sense to introduce the concept of molar mass.
Fig. 3. An example of the design of an element in the periodic table ()
Two rules for working with the periodic table should be remembered. First, all values \u200b\u200bshould be rounded to integer according to the rules of mathematics. The exception is chlorine, its molar mass \u003d 35.5. Secondly, all molar masses are given in dimension, therefore, in order to convert this value to the SI unit of measurement, it is necessary to multiply the rounded number by.
To find the molar masses of complex substances, you just need to add the molar masses of the elements that make up the molecule of this substance.
Introducing the concept of molar mass, we can now obtain another formula for determining the amount of substance:
Here: - the mass of a portion of the substance; is the molar mass of the substance.
Starting from the next lesson, we begin to study the gases. In the next lesson, we introduce a physical gas model, which we will work with in the future, list the parameters describing the gas, and formulate an equation relating these parameters.
Bibliography
- Myakishev G.Ya., Sinyakov A.Z. Molecular physics. Thermodynamics. - M.: Bustard, 2010.
- Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M .: Ileksa, 2005.
- Kasyanov V.A. Physics 10th grade. - M.: Bustard, 2010.
- Narod.ru ().
- Techemy.com ().
- Encyclopaedia Around the World ().
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