Posted by Kim on .
A cylinder with a movable piston contains 2.00g of helium,He, at room temperature. More helium was added to the cylinder and the volume was adjusted so that the gas pressure remained the same. How many grams of helium were added to the cylinder if the volume was chanded from 2.00L to 2.70L? The temperature was held constant.

General Chemistry 
DrBob222,
The easy way to explain but the long way to work it is the following:
Use PV = nRT. You know V = 2.00L, you can assume any value for T, you know n = 0.500(2.00 g/molar mass He). Solve for P.
Then for the 2.7O L volume, use PV = nRT, substitute P from the first calculation, T is the same as in the first, and solve for n. Multiply by molar mass to obtain grams and subtract from the original 2.00 to find how much additional He was added.