You find 7g of Nitrogen in 0.50 dm^3. Whats the mass?

Well, the mass is easy as you have specified it.....

If the dinitrogen is at room temperature, ##298*K##, you could also specify the pressure by means of the ideal gas equation:

##P=(nRT)/V=((7*g)/(28.02*g*mol^-1)xx0.0821*(L*atm)/(K*mol)*xx298*K)/(0.50*L)##

##~=12*atm##

Note that ##1*L-=1*dm^3##, and I can use these units interchangeably.1