If you have a sample of gas - say natural gas, and lets say it only contains CH4 and C2H6, and you know the mass of it, how do you calculate the percent of each gas in the sample?

Knowing only the mass is not sufficient. If you also know the volume (at a known pressure), then you can determine the ratio by solving two equations with two unknowns.

 

If you know the mass, then you have one equation:

 

x MW(CH4) + y MW(C2H6) = m

 

If you know the volume at a certain pressure, then you know the total number of moles of molecules (e.g. at standard pressure and 298K one mole of an ideal gas takes approximately 22.4 liters of volume).

 

Let's assume there are A moles of gas. Then you have a second equation:

 

x + y = A

 

With basic algebra, you can solve this set of equations, giving you x and y as number of moles of each compound.

What does the MW stand for, mass weight or molecular weight?

What does the MW stand for, mass weight or molecular weight?

Molecular weight:

 

Roughly speaking:

MW(CH4) = 12 + 4*1 = 16

MW(C2H6) = 2*12 + 6*1 = 30

That makes sense. Just look at the chemical formula to check your answer; does it make sense? In this case it does, so the answer should be correct, or at least basically so.