The extent of absolute majority of molecules is so small that they do not manage to be seen even in the most powerful microscope. Respectively, the mass of one molecule is unimaginably small. Therefore for the ignorant person the thought that it is possible to weigh the one and only molecule, will seem absurd. Nevertheless, it is quite real.
1. Let's assume, set the task for you: there are 100 liters of some gas weighing 143 grams at a normal temperature and pressure. How many does one molecule of this gas weigh?
2. It is known that under normal conditions one mol (that is such amount of substance in which contains 6.022*10^23 its elementary particles) any gas occupies the volume, approximately equal 22.4 liters. Therefore, 100 liters of gas make 100/22.4 = 4.464 asking. Or in round figures 4.46 asking.
3. Now consider how many molecules contain in such number of moles of this gas. Make calculation: 4.46*6.022*10^23 = 2.686*10^24, or, in round figures, 2.69*10^24.
4. Knowing the mass of gas (under the terms of a task) and the calculated number of molecules, find the mass of one molecule: 143/2.69*10^24 = 53.16*10^-24, or 5.316*10^-23 gram. Here how many the molecule of this gas weighs.
5. Consider other task. Let's assume, you have squared iron bar which length of 15 centimeters, width – 4 centimeters, and height – 3 centimeters. It is necessary to find the mass of one molecule of iron.
6. Calculate bar volume. Having multiplied sizes of length, width and height, you receive: 15*4*3 = 180 cubic centimeters. By means of any suitable reference book (physical, chemical or technical) find iron density size: 7.874 граммов/см^3. For simplification of calculations round to 7.87. Having multiplied sizes of volume and density, find the mass of iron bar: 180*7.87 = 1416.6 grams.
7. Now, having looked in Mendeleyev's table, determine the molar mass of iron which is in number equal to molecular weight, but is expressed in other sizes. The molecular mass of iron is equal 55.847 and. e. m (atomic units of mass). Therefore, its molar weight is equal 55.847 grams/mol. Or, in round figures, one mol of iron weighs 55.85 grams.
8. Calculate how many moths of iron contain in bar weighing 1416.6 grams. 1416.6/55.85 = 25.36. Now, knowing how many elementary particles contain in one pier of any substance, you will easily find total number of molecules of iron: 25.36 * 6.022х*10^23 = 1.527*10^25. And last action: 1416.6/1.527*10^25 = 9.277*10^-23граммов.