The mole fraction of O_2 in air is about 0.21 making the partial pressure of O_2 P_o_2 = (0.21)(1 atm) = 0.21 atm. A descends to a depth of 100 ft and pressure at that d is 3.9 atm. If the diver is breathing a 79 % N_2 and 21% O_2 mixture at 3.9 atm, what is the partial pressure of O_2 at that depth? 3.1 atm 0.21 atm 0.82 atm 0 39 atm