How To Calculate the Molecular Weight of Gases and Vapors

If you are a chemistry student, you probably will be expected to learn how to find the molecular weight of various gases and vapors. This is more than an academic or mathematical exercise; if you plan to go into a scientific career such as medical practice or research, you will need to have a strong understanding of molecular weight. Researchers often use their knowledge of a substance's molecular weight in order to make predictions about its molecular makeup and characteristics, and many medical treatments may be based on molecular weight.  It is more difficult to get an accurate molecular weight for a gaseous substance than for a liquid or solid, as you cannot easily measure its reactions to various changes such as changes in temperature. You can, however, accurately predict a gaseous substance's molecular weight by using Avogadro's Law, otherwise known as the Ideal Gas Law.

The Ideal Gas Law defines the relationship between certain variables in every gas, one of which is the gas' molecular weight. If you know the value of the rest of the variables, you can easily find the molecular weight.

  1. So your first step should be to write out the formula for the Ideal Gas Law and manipulate it to help you search for the molecular weight. The formula states that PV = M/MW x RT, so you can easily rewrite the law to state that MW =MRT/PV.
  2. Next, look up the value of R in a table or calculate it yourself. R is a constant, but its value is dependent upon your measurement system. Plug R into the formula.
  3. Plug all other known variables into the formula once you have determined a value for R. At this point, you should have neatly written out on your paper a formula containing your entire calculation. It is important to write this out and not just plug the numbers directly into a calculator to cut down on the possibility of errors and see clearly what you are doing.
  4. Finally, calculate the molecular weight of the gas using a calculator. You can do this step by hand if you wish, but the calculations are complex, and the potential for arithmetic errors is great.

Once you have finished this calculation, you should have a molecular weight. You may wish to look at a table of typical molecular weights if you are doing a school assignment so that you can make sure your answer makes sense. In the real world, there will be no table for the specific gas you are studying, but you will have an idea of whether your weight is within the realm of possibility because of your knowledge of typical weights for various gases and vapors.


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