Pressure of oxygen

Question

If the partial pressure of oxygen is the same at the top of Mount Everest (elevation above sea level 8848m) as Toronto (elevation above sea level 76m), why is it so much harder (and dangerous) to climb Mount Everest?

Answer 1

Dalton's Law:

Even though the partial pressure of O2 may be the same at both altitudes, the TOTAL pressure is much less atop Mt. Everest. Since the partial pressure x Total pressure = pressure of the gas, and is also equal to the mole fraction of that gas, there is much less oxygen gas in the atmosphere (in moles) atop Mt.Everest compared to in Toronto.