physics
posted by sweety .
Oxygen gas having a volume of 784 cm3 at 23.3°C and 1.03 x 105 Pa expands until its volume is 1170 cm3 and its pressure is 1.07 x 105 Pa. Find (a) the number of moles of oxygen present and (b) the final temperature of the sample. (Unit kelvin, K, for part (b).)

Use the perfect gas law to get the number of moles, n. You can use initial condition values of p, V and T . The inital T is 296.5 K, V is 0.784 l and p = 1.017 atm. R = 0.08205 latm/mole
n = PV/RT = 0.0328 moles
The temperature after expansion depends upon the type of expansion. Usually an isentropic expansion is assumed, with the gas doing work while it pushes against an external boundary or gas. If it expands into a larger volume that had been previously empty, the gas temperature will not change.
For an isentropic expansion of a diatomic gas,
T^(5/2) * V = constant
The 5/2 exponent equals 1/[gamma  1].
gamma = 1.4 for diatomics
(T'/296.5)^2.5 = 784/1170 = 0.670
T'/296.5 = 0.852
t' = 252.6 K = 20.6 C 
thank u so much for the help

T' is supposed to be the final temperature