Answer:
64 amu
Step-by-step explanation:
An unknown gas (X) diffuses 0.25 times as fast as He. Mathematically, we can say that the rate of diffusion of X (rX) is 0.25 that of He.
rX = 0.25 rHe
rX/rHe = 0.25
Given the molecular mass of He (MHe) is 4.00 amu, we can calculate the molecular mass of X using Graham's law of diffusión.
rX/rHe = √(MHe/MX)
0.25 = √(4.00 amu/MX)
MX = 64 amu