You will need
  • periodic table of chemical elements;
  • - the concept of the structure of the molecule and atom;
  • calculator.
If you know the chemical formula of the substance, determine its molar mass. To do this, define the atoms that comprise the molecule, and find their relative atomic masses in the periodic system of chemical elements. If one atom occurs in the molecule n times, multiply its mass by this number. Then put the values and get the molecular weight of this substance, which is equal to its molar mass in g/mol. Find the mass of one moleculesby dividing the molar mass of the substance M on Avogadro's constant PA=6,022∙10^23 1/mol, m0=M/ PA.
Example Find the mass of one molecule of water. The water molecule (H2O) consists of two hydrogen atoms and one oxygen atom. Relative atomic mass of hydrogen is 1, for two atoms to get the number 2 and the relative atomic mass of oxygen is 16. Then the molar mass of water is 2+16=18 g/mol. Determine the mass of one molecule: m0=18/(6,022^23)≈3∙10^(-23) G.
The mass of molecules can be calculated if we know the number of molecules in a given substance. To do this, divide the total mass of the substance m on the number of particles N (m0=m/N). For example, if you know that 240 g of the substance contains 6∙10^24 molecules, the mass of one molecule will be m0=240/(6∙10^24)=4∙10^(-23) G.
Determine the mass of one molecule of a substance with sufficient accuracy, knowing the number of protons and neutrons, which are part of its nuclei, of which it is composed. The mass of the electron shell and the mass defect in this case should be neglected. The mass of the proton and neutron get equal 1,67∙10^(-24) g. for Example, if you know if a molecule consists of two atoms of oxygen, what is its mass? The nucleus of the atom of oxygen is composed of 8 protons and 8 neutrons. The total number of nucleons 8+8=16. Then the mass of the atom is equal 16∙1,67∙10^(-24)=2,672∙10^(-23) g. Since a molecule consists of two atoms, its mass is equal 2∙2,672∙10^(-23)=5,344∙10^(-23) G.