Imagine that you have been assigned five substances for which you will observe the rate of diffusion. The only information that you have about your materials is the molecular weight of each substance, listed below:

Imagine that you have been assigned 5 substances for which you will observe the rate of diffusion. The only information that you have about your materials is the molecular weight of each substance, listed below:

   Substance #1: 212 mg

   Substance #2: 1918 mg

   Substance #3: 50 mg

   Substance #4: 4 mg

   Substance #5: 700 mg

If the substances are allowed to diffuse for one hour, what can you predict about the rate of diffusion?

The answer to the question is

That the lighter substances will diffuse more than the substances with larger molecular masses

That is Substance #4: 4 mg  which has the least molecular mass will diffuse the most

Explanation:

Graham's law of diffusion of a gas states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular mass

That is

i.e. r ∝ √(Molar Mass) and r × √(Molar Mass) = constant

Where r = Diffusion rate

Considering the listed gases we have

(Rate of subsatane 1)/ (Rte of substance 2) =

 =

≅ 3.0

Hence Substance 1 diffuses faster than substance 2

The same can be said of the rest where the rate of diffusion is inversely proportional to the molar mass

And so

R1:R2 = 3.00

R1:R3 = 0.48

R1:R4 = 0.14

R1:R5 = 1.82

Therefore the rate of diffusion of the gases in incresing order is

R2 → R5 → R1 → R3 → R4

Which is the same order as the decreasing order of their molar mass

Substance #2: 1918 mg , Substance #5: 700 mg, Substance #1: 212 mg, Substance #3: 50 mg   , Substance #4: 4 mg

where's the diagram?