# Homework Help: Mean Free Path

1. Jan 13, 2005

### Kawrae

In a tank full of oxygen, how many molecular diameters d (on average) does an oxygen molecule travel (at 1.00 atm and 21.0°C) before colliding with another O2 molecule? (The diameter of the O2 molecule is approximately 3.60 10-10 m.)

>> I don't know why webassign is telling me this is wrong and I can't figure out what I'm doing wrong. I used this equation:

l=1/[(2^.5)(pi)(d^2)(nsubv)]

Since n subv is equal to N/V which is equal to P/kb*T, I got 2.49e25.

Then I just plugged everything in... and got 1.43e7 and divided by the diameter to see how many times it would cross that distance... and I got 3.98e16

What did I do wrong??

2. Jan 13, 2005

### dextercioby

The result is absurd.Check your numbers.
$$\lambda=\frac{1}{\sqrt{2}\pi n d^{2}}=\frac{kT}{\sqrt{2}\pi p d^{2}}$$

Plug the constants and the numbers in SI units correctly which means carefully.

Daniel.