you need
  • For calculations take a pen, calculator, periodic table.
Guide
1
unit of molecular weight is 1/12 of the mass of an atom of carbon, which is conventionally taken as 12. The molecular weight is numerically equal to the sum of the relative atomic mass of all the atoms in the molecule, and it is very easy to calculate.
2
According to the law of Avogadro in equal volumes of gases at constant pressure and temperature will contain the same number of molecules.Because it was later withdrawn Mendeleev-Clapeyron equation.Now you need to use it, but it is valid only for gases!Put in a formula known to you the pre
ssure and temperature to yield molecular mass gas: M = (m ∙ R ∙ T) / (P ∙ V), where M - the desired molecular weight, m ​​- mass of substance, R - universal gasconstant (taken as 8.31 J / mol * K), T - temperature in Kelvin, P is the pressure in Pascals, V - the volume in cubic meters.
As you can see, this method requires a lot of data, but the accuracy of such calculations is minimal.
3
following method is much simpler.If you know only the weight of the substance and its chemical m number ν, then substitute the data into a formula: M = m / ν, where
m - mass of a substance (typically in grams), and ν - amount of substance in moles.
4
A is the easiest option if you know the chemical formula of the substance.Take periodic table, look at the molecular weight of each element, which is a part.For example, it is one of hydrogen, oxygen at - 16. A molecular weight to find all matter (take for example water, which consists of two molecules of hydrogen and one oxygen molecule) simply fold the mass of all of its constituent elements.Water: M (H2O) = 2M (H) + M (O) = 2 • 1 + 16 = 18 a.e. m.