Chemistry question about volume?

Question

A resting adult requires about 240 mL of pure oxygen per minute. If inhaled air contains 20 percent oxygen by volume and exhaled air 16 percent, what is the volume of air per breath?

(assuming that the volume of air exhaled and inhaled are the same)

ahh. im about to pull my hair out!!

Answer 1

Assuming 1 breath = volume of air inhaled = volume of air exhaled.  

Adults breathe somewhere between 12-20 times a minutes on average - let's say 16 times a minute.  Since you require 240 mL 02/minute, that means you'd need 15 mL of 02 per breath.  (240/16 = 15)

It sounds like you're using an amount of oxygen equal to 4% of V each time you breathe (since the oxygen level drops from 20% of V to 16% of V).  So, that 15 mL of 02 is equal to 4% of V.

15 = 0.04 V, so V = 375 mL.

In fact, I think tidal volume (the technical name for the volume of 1 normal breath) is usually around 500 mL.  You'd get that as your answer if you used a respiratory rate of 12 breaths/minute.