Gas Problems: How to Tell Which Kind You Have

What to Look For	What to Do
one set of conditions (P,V,n,T)	use the ideal gas law PV=nRT
two sets of conditions (“before” & “after”)	start with the combined gas law: and cancel anything that doesn’t change.
mixture of gases or gas collected over water (which means a mixture of gases, one of which is water vapor)	use Dalton’s Law: Ptotal = P1 + P2 + P3+… and then do a separate PV=nRT problem for each gas
the problem involves a manometer	convert the height in mm Hg to the appropriate pressure units for the problem
the problem involves effusion, diffusion, or the speeds of molecules	use Graham’s Law: or