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14. RELATIVE ATOMIC AND MOLECULAR MASSES. MOL. AVOGADRO'S NUMBER. MOLAR WEIGHT. MOLAR VOLUME.
In this section we will get acquainted with sizes which you will often use at problem solving, performance of exercises.
Modern methods of research allow to determine extremely small mass of atoms with big accuracy. So, for example, the mass of carbon atom is equal to 1,993·10-26 kg. It is very a bit size. Therefore in chemistry not absolute values of atomic masses, but relative are used. The atomic mass unit equal to 1/12 part of mass of carbon atom is taken for unit of atomic mass.
The relative atomic mass of chemical element is called the size showing in how many time the mass of this atom it is more than 1/12 mass of carbon atom. It is designated by the letter Ar. Relative atomic masses are specified in the periodic table. For example Ar (H)=1, Ar (P)=31. We round atomic masses to entire quantities, excepting chlorine atom - Ar (Cl) =35,5.
The relative molecular mass of substance is called size, the mass of molecule, pokayvayushchy in how many time, is more than 1/12 mass of carbon atom. It is designated by Mr. You know that molecules consist of atoms therefore relative molecular mass consists of the sum of atomic masses of the atoms making molecule taking into account number of atoms. For example Мr(H2SO4)=1·2+32+16 · 4=98.
Let's enter one more size - amount of substance which is measured in moths.
The mol is the amount of substance containing so many base units (atoms, molecules, ions) how many atoms contain in 12 g of carbon. It is designated by letter ν (nu)
Knowing the mass of carbon atom of 1,993·10-26 kg it is possible to calculate number of atoms in 0,012 kg of carbon:NA=0,012/1,993·10-26=6,02·1023
This number is called constant Avogodro and NA, dimension 1/mol or mol-1 is designated, and shows number of base units in pier of any substance. I.e. 1 mol of any substance contains identical number of base units 6,02 · 1023. Using this size to mozhnoopredelit amount of substance on formula:
It is difficult to work with number of particles therefore the concept the molar mass which shows what is entered the weight of 1 mol of specific substance is equal. Molar mass is equal to the attitude of mass of substance towards amount of substance, is designated by letter M.
Molar mass is calculated as well as relative molecular mass, but in difference from it has dimension of g/mol. For example M(H2SO4)=1·2+32+16 · 4=98g/mol. It means that the weight of 1 mol of sulphuric acid is equal to 98 g.
Still interesting the fact that one mol of any gas occupies the volume of 22,4 liters. This consistent pattern has been determined by the Italian scientist Amadeo Avrgadro. For gases it is possible to use size the molar volume which is designated by the mol/l letter Vm, Vm=22.4.
where gas V-volume in l.
We will use these formulas at the solution of different tasks. We will get acquainted with some of types of tasks on the following page.
