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Chemistry: Introducing Inorganic, Organic and Physical Chemistry 3rd

The atmospheric pressure and temperature both fall as the altitude increases. The total pressure at a height of $20 \mathrm{km}$ is about $5 \%$ of that at the Earth's surface (i.e. at sea level) and the temperature is about $-55^{\circ} \mathrm{C}$. Estimate the collision frequency and the mean free path of oxygen molecules under these conditions. (For oxygen at this temperature, the mean speed, $\bar{c}=380 \mathrm{ms}^{-1}$, (Section 8.5)