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## Homework Statement

I do need to calculate the mean free path for an ion with a radius of 9 x 10‐8 cm in pressures of 10‐8 Torr, 10‐6 Torr, 10‐4 Torr and 10‐2 Torr

## Homework Equations

There are different equations I found but the most one is λ= 1/Nσ

N gas number density which I do not know how to calculate based on radius and pressure.

And σ which is collision cross section σ =πd2

Another equation I found = 7×10−6/ r coll2 ×P(torr)

## The Attempt at a Solution

I did convert radius to nm (0.9 nm) and applied the second equation

mean free path= 7×10−6/ 0.81 ×P(10-8) and so on for the different pressure

but I am not sure if this attempt is wright

These calculations are to use in the ion free path (ion mobility) in mass spectrometer instruments.