You need to use the ideal gas equation:
[tex]PV = nRT[/tex]
P is pressure (in atm or bar)
V is volume (in L)
n is the number of moles (in mol)
R is an experimentally determined ratio of about 0.82
T is the temperature (in Kelvin)
Note: For all gases, you need to convert Celsius or Fahrenheit to Kelvin.
[tex] 25C + 273.15 = 298.15K [/tex]
Get V alone.
[tex] V = \frac{nRT}{P} [/tex]
Plug in the values you have.
[tex] V = \frac{2.50 mol * 0.82 * 298.15 K}{1.49 atm}[/tex]
you get:
[tex] V = 611.2075L [/tex]
A fun fact: the gas used to fill up a balloon is Helium.
