Verified answer

start with...

PV = nRT

n = mass / mw

substitute and rearrange...

PV = (mass/mw) RT

mw = mass x RT / (PV)

solve...

mw = (0.481g) x (0.08206 Latm/moleK) x (295K) / ((767torr x 1atm/760torr) x 0.127L)

mw = 90.8 g/mole

Don't kill yourself!

Use PV = nRT, but make sure the P is in atmospheres, the V is in liters and the T is in Kelvins.

(767/760) (0.137 L) = (n) (0.08206) (295 K)

Solve for n, the moles of gas in your sample.

The, divide 0.481 g by the number of moles. That's your answer.

http://www.chemteam.info/GasLaw/Gas-Ideal.html

Be prepared. This is a very popular test question.