Look:
The a gas component's partial pressure is the whole gas sample's pressure and the mole fraction component:
Pi = P * x
x= n(gas)/n(whole gas sample)
n means how many mole.
So because we have got ONLY ratios, we can assume how many moles were the sample, bacause in the end we will get again just relative thing.
So I assume we have 1 mole gas sample. (If we had two mole, would not be the problem. (1mol*0.201)/1mol equals (2mol*0.201)/2mol. It is the same.)
then x is 0.201/1 so 0.201 and we have the x.
So the O2-s partial pressure is x times the whole gas samples pressure, so 0.201*762= 153.16 mmHg
Anyway I hope this helped. Remember if nothing solid and concrete thing is mentioned in the task and you can't solve it, assume something, and you will still get the same relative solutions. This does not work if you have to tell how many gramms is the thing, because it's not relative.
And I hope it is an okay solution, I don't really trust my skills in physics.
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