You would have to use the ideal gas law: PV = nRT where,
P = pressure
V = volume
n = moles
R = universal gas constant for a specific pressure (in this case it's 62.364 torr)
T = temperature in Kelvin
First, convert Celsius to kelvin by adding 273 to 25, which gives you 298K
Now plug in your variables to find n:
(800 torr)(1.25 L) = n(62.364)(298K)
1000 = 18584.472n
Now divide 1000 by 18584.472 to get n:
1000/18584.472 = 0.054 moles
