1. total gas pressure - P
2. volume - V
3. gas temperature - T
4. vapor pressure - p
5. molar mass of the substance - MW
Molar mass? I assume this is the molecular weight?
Your first problem is to work out the number of moles of the gas. From what you've given me, it looks like you should use the ideal gas law:
R is the "universal gas constant". It's available in many, many units, and you can always convert from the available units to units you want: Gas constant - Wikipedia, the free encyclopedia. Click here for more information
In the above equation, you have P, V, and T. You can look up R (from the link, above), so you can calculate "n", the number of moles of the substance. The molecular weight is in grams per mole so all you have to do is multiply the number of moles by the molecular weight and viola
, you have your mass.
Now, I may not have answered your question because I didn't use the vapor pressure. You may not have given me all of the information to solve the question. The vapor pressure of a substance is the pressure of a substance above a liquid (at a given temperature).