Question

A scuba tank contains heliox, a special breathing gas composed of 5.0% oxygen and the remainder helium, for use by divers that spend extended amounts of time at depth underwater. The volume of the tank is 20 L and the pressure within it is 1500 lb/in2. Calculate the mass in grams of each gas in the tank and their partial pressures at 20 degrees celsius.

(Well I have the molar mass of HeO2 at 35 g/mol but do I have to go through the process of turning grams to moles to moles to grams? And I am not entirely sure about the last part whether I am to find the partial pressure or the partial pressure is 20 degrees celsius. Am I overthinking this?)

Answer 1

Perhaps. Partial pressure is what you are to find and you should understand that partial pressure is NOT measured in degrees C but in atm or mm Hg or some other pressure unit. What you want to do is to convert 1500 lb/in^2 to atm Then use PV = nRT, substitute with the conditions listed and solve for n = number of mols (total) of the mixture. 5% times that will tell you how much He is there and the difference (or 95%) = mols O2. After you know mols you can go one of two ways.
1. Resubstitute individual mols and the other conditions into PV = nRT and solve for pHe and the same thing with mols O2 to find pO2.

2. OR you may convert mols He and mols O2 to mole fractions.
XO2 = mols O2/total mols.
XHe = mols He/total mols.
Then pO2 = XO2*Ptotal
and pHe = XHe*Ptotal