The molar mass of substance is a mass of one asking, that is its such quantity in which is 6.022*10^23 elementary particles – atoms, ions or molecules. Unit of its measurement – gram/mol.
Instruction
1. To calculate molar weight, you will need only Mendeleyev's table, elementary knowledge of chemistry and ability to make calculations, of course. For example, widely known substance – sulfuric acid. It finds so broad application in various industries that the name "chemistry blood" by right carries. What its molecular weight?
2. Write an exact formula of sulfuric acid: H2SO4. And now take Mendeleyev's table and look what atomic masses of all elements which are its part. These elements three – hydrogen, sulfur and oxygen. Atomic the mass of hydrogen it is equal 1, are gray – 32, oxygen – 16. Therefore, the total molecular mass of sulfuric acid, taking into account indexes, is equal: 1*2 + 32 + 16*4 = 98 and. e. m (atomic units of mass).
3. And now let's remember one more definition asking: this amount of substance which mass is in number equal in grams to its weight expressed in atomic units. Thus, it turns out that 1 mol of sulfuric acid weighs 98 grams. Here such is its molar weight. The task is solved.
4. Let's assume, to you such conditions are set: there are 800 milliliters 0.2 molar solutions (0.2M) of some salt, and it is known that this salt in a dry form weighs 25 grams. It is required to calculate its molar weight.
5. For a start remember definition 1-molar (1M) solutions. It is solution which 1 liter contains 1 mol of any substance. Respectively, 1 liter 0.2M of solution would contain 0.2 moles of a substance. But you have not 1 liter, but 0.8 liters. Therefore, actually you have 0.8*0.2 = 0.16 moles of a substance.
6. And everything becomes farther already as easy as shelling pears. If 25 grams of salt under the terms of a task make 0.16 asking what quantity is equal to one I ask? Having made calculation in one action, you will find: 25/0.16 = 156.25 grams. The molar mass of salt is 156.25 grams/mol. The task is solved.
7. In calculations you used the rounded sizes of atomic scales of hydrogen, sulfur and oxygen. If it is required to make calculations with a big accuracy, rounding is inadmissible.