you need
  • - periodic table of chemical elements;
  • - sealed bottle;
  • - scales;
  • - gauge;
  • - thermometer.
If you know the chemical formula of the substance, determine its molecular weight using the periodic table of chemical elements.To do this, identify the items that are included in the formula of the substance.Next, find the relative atomic masses, which are recorded in the table.If the atomic weight table provides a fractional number, round it to the nearest integer.If the chemical formula contains a number of atoms of a given element, multiply the weight of one atom to the number.These atomic mass
es fold and get a relative molecular weight substances.
example, to find the molecular weight of sulfuric acid H2SO4, find the relative atomic mass of elements that go into the formula, respectively, hydrogen, sulfur and oxygen, Ar (H) = 1, Ar (S) = 32,Ar (O) = 16.Given that the hydrogen atom in the molecule 2, and 4 oxygen atoms, the calculated molecular weight substance Mr (H2SO4) = 2 • 1 + 32 + 4 = 98 ∙ 16 atomic mass units.
In that case, if you know the amount of substance in moles ν and mass of the substance m, expressed in grams, determine its molar mass of this weight divide by the number of substances M = m / ν.It is numerically equal to its relative molecular mass.
If you know the number of molecules of the substance N, known mass m, find its molar mass.It will be equal to the molecular weight ratio of the mass found in grams to the number of molecules of the substance in the mass, and the result multiply by Avogadro constant NA = 6.022 * 23 January / mol (M = m ∙ N / NA).
to find the molecular weight of the unknown gas, find it a lot in a sealed bottle of known volume.To do this, pump out the gas from it, creating a vacuum there.Weigh the cylinder.Then upload your gas back and again find it a lot.The difference between the weight of the empty cylinder and injected, and will be equal to the mass of the gas.Measure the pressure in the cylinder using a pressure gauge in Pascals, and the temperature in Kelvin.To do this, measure the temperature of the ambient air, it will be equal to the temperature inside the container in degrees Celsius, to put it in Kelvin, add to the resulting value of 273.

Determine the molar mass of the gas by finding the product of the temperature T, the gas mass m, and the universal gas constant R (8.31).The resulting number to divide the pressure P and volume V, measured in cubic meters (M = m • 8,31 • T / (P • V)).This number will correspond to the molecular weight of the test gas.