Hi, I have been struggling a little with this question. Scuba divers breathe a mixture of O2(g) and He(g) to avoid "the bends, a condition caused by nitrogen in the blood. If 65.0g O2(g) and 2.00g He(g) are placed in a 5.0L tank at 25oC, calculate: If the average human takes 15 breaths per minute, and breathes in 0.50L at 1.00 atm, calculate how long the gas in the tank will last? This is what I've done: Pressure in the tank: n(He) = (2.00 g He)/(4.00 g/mol) = 0.500 mol He n(O2) = (65.0 g)/(32 .00 g/mol) = 2.03125 mol O2 n(total) = n(He) + n(O2) = 2.53125 mol P(total) = (n(total)RT)/V = (2.53125)(0.082057)(298)/5.0 P(total) = 12.379 atm time to empty: P1V1 = P2V2 (1.00 atm)(0.50 L) = (12 atm)x, where x is the volume breathed in one breath x = 0.0416667 L in one min: Vbreathed = 15x = 0.625 L (1 min)/(0.625 L) = t/(5.0 L) Therefore t = 8.0 min. Is all that right or instead of using the total pressure in the tank I should use the partial pressure of O2? The answer should be 7.8 min which I'm not getting. But I have also tried it with O2 partial volume and I don't get the answer. Am I missing something? Any help is appreciated. Thanks.