The molecule of substance is at the same time its minimum possible portion, and therefore its properties are defining for substance in general. This particle belongs to a microcosm therefore to consider and furthermore to weigh it, it is not possible. But the mass of one molecule can be calculated.
It is required to you
- - periodic table of chemical elements;
- - concept about the structure of a molecule and atom;
- - calculator.
Instruction
1. If chemical formula substances is known, determine its molar weight. For this purpose define atoms of which the molecule consists, and find their relative atomic masses in the Periodic Table. If one atom meets in molecule n of times, increase its weight by this number. Then put the found values and receive the molecular mass of this substance which is equal to its molar weight in g/mol. Find the mass of one molecule, having divided the molar mass of substance M into constant Avogadro of NA =6,022∙10^23 1/mol, m0=M/NA.
2. An example Find the mass of one molecule of water. The molecule of water (H2O) consists of two atoms of hydrogen and one atom of oxygen. The relative atomic mass of hydrogen is equal 1, for two atoms we will receive number 2, and the relative atomic mass of oxygen is equal to 16. Then the molar mass of water will be equal to 2+16=18 g/mol. Determine the mass of one molecule: m0=18 / (6.022^23) ≈3∙10^ (-23).
3. The mass of a molecule can be calculated if the quantity of molecules in this substance is known. For this purpose divide substance m lump into quantity of particles of N (m0=m/N). For example, if it is known what in 240 g of substance contains 6∙10^24 molecules, then the mass of one molecule will be m0=240 / (6∙10^24) =4∙10^ (-23).
4. Determine the mass of one molecule of substance with a sufficient accuracy, having learned quantity of protons and neutrons which are a part of its atomic nuclei of which it consists. The mass of an electron shell and a mass defect in this case should be neglected. Take the mass of a proton and neutron equal 1.67∙10^ (-24). For example, if it is known if the molecule consists of two atoms of oxygen, what its weight? The atomic nucleus of oxygen incorporates 8 protons and 8 neutrons. Total number of nucleons 8+8=16. Then the mass of atom is equal to 16∙1.67∙10^ (-24) =2.672∙10^ (-23). As the molecule consists of two atoms, its weight is equal to 2∙2.672∙10^ (-23) =5.344∙10^ (-23).