chemistry
posted by lelia .
5.0 moles of a gas is put into a container of 2.0L. More gas is added to the flask s othat there is now 15 moles of the gas present. What must the new volume be if temperature and pressure are to remain constant?

Probably there are easier ways to do this but I would approach it this way.
Use PV = nRT for the first scenario. You know n, V, R and T (T is not listed but since it doesn't change, make up a number for it300 sounds ok to me). Solve for pressure.
Now use PV = nRT for the second scenario. You know n, R, use 300 for T, and use pressure you calculated above. Solve for the new volume.(I think the answer should come out to about 6 L but check my thinking). I can show you the easier (shorter but perhaps a little harder to understand) way if you are interested. 
what if the question doesnt say that the temperature and pressure stay constant. for example..3.0L of a gas has a pressure of 12.0 atm. What is the new pressure if the gas is expanded to 17.0L?....would you assume that the temperature would stay the same or is there another way to solve this?

The procedure doesn't change.
Use PV = nRT for the first scenario. You know n, R, T, and V. Calculate P.
Then the second scenario, calculate V for PV = nRT using whatever the new T, P, and n values are. 
If that is a new question you posted, the procedure I wrote will not solve that one BUT the procedure is still the same. It would be easier to use P1V2 = P2V2.