rate of diffusion

Jan 2010
In this problem, you are to find out how much faster helium diffuses than chlorine. To find the rate of diffusion, To find out, you put the square root of the density of the second one over the density of the first one. The density of chlorine is .003214. The density of helium is .00017847. When my son and I did this problem, we put the density of helium over the density of chlorine and took the square root of both of them. When he worked it out on his calculator, we got the answer 2. However, the book says the answer is 4.2. The formula for the rate of diffusion is rate1/rate2
= the square root of density2/density1. Were we supposed to divide them and then take the square root?


CHF Hall of Fame
Apr 2009
When I calculated it with your numbers and that formula I got 4.2

Like your previous question, I suggest you learn how to work equations to put them into a calculator (dont miss brackets or anything!)
Jan 2019

In the above question the similar answer is
According to Graham's law of diffusion,
rNH₃/rHCl = √MHCl/√MNH₃

Here molecular weight of ammonia (NH₃) = 17 gm mol⁻¹ and molecular weight of hydrogen chloride (M₂) = 36.5 gm mol⁻¹
∴ rNH₃ = rHCl × (√MHCl/√MNH₃) = rHCl × (√36.5/√17) = 1.46 rHCl

Thus ammonia will diffuse 1.46 times faseter then hydrogen chloride gas.