How Is Oxygen Partial Pressure Maintained in Deep Sea Diving?

[clipperdude21]

1. A deep sea diver has descended a distance of 50m below the ocean surface. Asummer that he is supplied with air from the survace.
a) what is oxygen partial pressure in the lungs of the diver at this depth.
b) suppose we want the oxygen partial pressure in the lungs of the diver at this depth to be the same as that at the surface. This can be done by providing the diver with a mixutre of helium and air. Use Dalton;s Law to compute the proper mixing ratio in moles between helium and air and explain the mechanism.

Homework Equations

3. (a) I got this right I am pretty sure. I just did P=patm + pgh and got 605,700. Then multiplied it by 0.21 since oxygen is 21% of air and got 127,197 Pa.
(b) I couldn't do this one!

 

[Astronuc]

It would be helpful if one showed the steps and units.

for part b. at the surface the pressure of air is 1 atm, 14.7 psia, or 101325 Pa, of which ~0.21 is oxygen.

What is the mixture of He/O2 such that the oxygen partial pressure is the same as the surface pressure when the diver is 50 m below the surface?