Can anyone help me with this chemistry question?

Question

Here's the question:

"An unknown gas effuses at one half the speed of oxygen. What is the molar mass of the unknown? It is either HBr or HI. Which gas is it?"

If you know it, could you please show me how you got it, and what formula you used? Thanks.

Answer 1

Isn't there something called Grahams Law which states that the effusion rate is prorportional to the square root of molecular weight? In that case, the molecular weight of the unknown is 128, which is the MW of HI