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In graham's law of diffusion most numericals I have seen assumes the formula

$$\frac{r_1}{r_2}=\frac{P_1}{P_2}\sqrt{ \frac {M_2}{M_1}}$$

Where they say $P_1,P_2$ are the partial pressures of the two gases receptively inside the container.But my question is should'nt it be the difference in partial pressures inside and outside the orifice?

Because say if the pressure outside equals pressure inside then the rate of diffusion becomes $0$ .Then graham's law fails in such a case.

Please tell me if I am going wrong.

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    $\begingroup$ Well, yes, it should be the difference of pressures inside and outside; it's just that we assume the gas is escaping into a kind of void, so the outside pressure can be ignored. $\endgroup$ Feb 5, 2016 at 10:37

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