# Comparing rates of effusion between species I'm struggling with (i) and (ii). I understand how to do (iii) and (iv) as it is just an application of Graham's Law of Effusion twice and n times (solve for n) respectively. I am starting with

rate of effusion = -dN/dt = (1/2)(N/V)SQRT(2kT/pi*m)*dA

where N/V is the number density, k is boltzmann's constant, T is temperature, m is the mass of the molecule and dA is the area of the aperture.

When I do the ratio of rates for the 235:238 isotopes, I get:

(rate for 235)/(rate for 238) = (N_235/N_238)SQRT(m_238/m_235)

where N_235 and N_238 is the number of molecules of the 235UF6 and 238UF6 isotopes respectively and m is the mass of the molecules (or relative molecular mass).

I don't know how to get from where i am to the equation or have I gone completely wrong from the getgo?

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