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