[Bowers]

The partial pressure of oxygen in air is about 0.20 atm. (Air is 20 percent oxygen by volume.) In deep-sea diving, the composition of air the diver breathes must be changed to maintain this partial pressure. What must the oxygen content (in percent by volume) be when the total pressure exerted on the diver is 5.0 atm? (At constant temperature and pressure, the volume of a gas is directly proportional to the number of moles of gases.)

[Dan]

You must demonstrate that you have attempted the question.

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[Bowers]

Well I was thinking that if the partial pressure of of oxygen in air is about 0.20 atm then to find it in 5.0 atm could I use a proportion? - 0.20/1=X/5.0

[fledarmus]

That would tell you the partial pressure of the oxygen if you simply compressed the air from 1 atm to 5 atm. You actually want the partial pressure of the oxygen at 5 atm to be the same as it is at 1 atm (0.20 atm). What do you need to change?

[Bowers]

The percentage of oxygen in air? How do I figure out how much?

[fledarmus]

You use Dalton's law - http://en.wikipedia.org/wiki/Dalton%27s_law

[Bowers]

So the total pressure is 5.0 atm and the oxygen needs to be .20 atm so 5= .2 O2 + 4.8 other stuff? 

[Bowers]

0.2/5=0.04?

[Bowers]

4%? 

[Bowers]

Okay I entered in 4, but I forgot about sig digs and it was 4.0... so mad, but it has now changed to 3.0 atm instead of 5 would it be 6.0 now? or 7.0? It just has to be exact.

[Bowers]

Okay it's 0.2/3.0 and that is .06667 so it rounds to 6.6 percent and that was correct. Thanks a lot! and I like how this is setup so you don't just give people who need help the answer 

[fledarmus]

Well done