Measured at 650C and 500 torr, the mass of 3.21 L of a gas is 3.5 g. The molecular weight (molar mass) of the gas is?

Pressure, P = 500/760 atm
Volume, V = 3.21 L
Mass, m = 3.5 g
Molar mass, M = ? g/mol
Gas constant, R = 0.08206 L atm / (mol K)
Temperature, T = (273 + 650) K = 923 K

PV = nRT  and  n = m/M
Then, PV = (m/M)RT
Hence, M = mRT/(PV)

Molar mass, M = 3.5 × 0.08206 × 923 / [(500/760) × 3.21] V = 126 g/mol

n = PV/ RT
500 torr = 0.658 atm
substituting in the universal gas equation we have 
number of moles of gas = 0.658 atm * 3.21L / 0.082057 L⋅atm⋅K−1⋅mol−1 * (650+273)
= 0.02788 moles
molar mass = 3.5 g / 0.027883215 mol = 126 g/ mol ???

I think your temperature  is wrong . 650 is too hot for a gas

you misread 65 degrees as 650 degrees 
when we plug in these values the answer then is 
molar mass 46 g/ mol 
not a gas that I recognise ?? maybe Na vapour as a dipole ? 

how would you start a solution to this problem?