This is just a direct application of the Ideal Gas Law.
P V = n R T
n = P V / R T
Remember, the temperature T is always the absolute temperature, measured in kelvins.
T = 25oC = (25 + 273) K= 298 K
n = (1.0 atm) (1.3 L) / [ (0.082 L-atm / mol-K) (298 K)]
n = 0.053 moles
(c) 2002, Doug Davis; all rights reserved.