Possible molecular formula for a gas and placed into a container?

[federox]

What is a possible molecular formula for a 4.35g sample of gas that has a density of 1.45g/L when placed in a container at 21.0C and 1.25 atm?

Note: R = 0.0821 L atm mol^-1 K^-1

please show work! thanks so much!

[dalyningkenk]

Given the Ideal Gas Law
PV = nRT
n = mass of gas (m) / molecular mass (M):
PV = (m/M)RT
Density is mass per volume, so, rearranging,
PM = (m/V)RT, or
PM = dRT, where d = density.
M = dRT/P; substituting,
M = (4.35g ) (R = 0.0821 L atm mol^-1 K^-1) (294.15 K) / (1.25 atm)
M = 84 g/mole
The permanent gas closest to this molecular mass is Krypton (Kr), MW = 83.9 g/mole.