Partial Pressure and Altitude Question from Berkeley Review

[msdaisy]

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Why does oxygen's partial pressure increase at low altitudes with low ambient temperatures (compared to normal air) but the mole fraction stays the same? I know that lower temperatures and lower altitudes make gas denser. But why would oxygen's partial pressure increase? Isn't partial pressure relative to mol fraction?

 

[whiteshadodw]

Why does oxygen's partial pressure increase at low altitudes with low ambient temperatures (compared to normal air) but the mole fraction stays the same? I know that lower temperatures and lower altitudes make gas denser. But why would oxygen's partial pressure increase? Isn't partial pressure relative to mol fraction?

because the total pressure of all the gases increases as altitude decreases.

PO2 = mole fraction*total pressure

PN2 = mole fraction* total pressure

Total P = PN2 + PO2 = XO2*P total + XN2*P total = (XO2 + XN2)*P total = 1 * Ptotal

XO2+XN2 = 1 for air assuming its 75% N2 and 25% O2. that's totally idealistic, but it gives you an idea. as P total increases, everything else has to increase, but since mole fraction cannot be changed, it means PN2 and PO2 have to change.

total pressure increases so both PN2 and PO2 increase as well.