A volume of 0.285 L of a gas at STP weighs 0.537 g. Calculate the molar mass of the gas?

PV=nRT where R = 0.0821. Therefore, (1)(0.285) = n(0.0821)(273).
i got 275.9
is this correct?

I don't think so.

Solve for n in your set up, then
n = grams/molar mass.
Substitute n and grams and solve for molar mass.