[kainess]

Hello! I have this question:
A mixture of He and O₂ gases is used by deep-sea divers. The pressure of the oxygen gas in the tank must be the same as what the diver would ordinarily breathe. If the total pressure of the tank is 8.0 atm, what percent of the mixture should be O₂.

Are we supposed to know what pressure the diver ordinarily breathes? I know I would just calculate what that is over 8.0 atm. But I’m not sure where we’re supposed to know, sorry if this is supposed to be common information. I’m genuinely confused, is there a way to find that through the problem?

[Borek]

What is the pressure of the air at sea level? What percentage of that is oxygen? What is its partial pressure?

(And yes, it is a common knowledge, such questions assume you know these things).

[Corribus]

As a piece of advice - when you encounter questions like this that assume a bit of "common knowledge", and you do not have this knowledge, the best way forward is to state the limit of your knowledge and then assume an estimated value and work through the problem with that estimated value. That way, at least, you are demonstrating that you know how to solve the problem. In this instance, for example, you may not know that oxygen is around 20% of air at sea level (even this is just an approximation), but you probably know that oxygen is not the major component of air. So you could just assume, say, 10% (or whatever) and work through the problem with that value. Maybe you lose a sliver of credit for getting "the wrong answer" but I believe most teachers would award the majority of credit for showing mastery of the fundamental concepts being tested (partial pressures, etc.).

That is, of course, unless you were told the correct value in your lectures and were expected to know it but weren't paying attention.