In an experiment, an unknown gas effuses at one-half the speed of oxygen gas, which has a molar mass of 32 g/mol. which might be the unknown gas?

If we assume the rate of diffusion of oxygen is 1 , then that of the unknown gas is 1/2.
From the Grahams law of diffusion;
R1/R2= √mm2/√mm1
 1/0.5 = √mm2/√32
   4 = mm2/32
mm2 = 128
Therefore the molecular mass of the unknown gas i 128 g/mol
I therefore think the gas is Hydrogen iodide
since, H=1, I= 127 , thus HI = 128 g/mol