Oxygen (O2), 80% Helium (He), and 16% Nitrogen (N2). Under normal circumstances this gas would be pretty light. It certainly is compared to air. A mole of air at 21% O2, 78% N2 and 1% Argon (Ar) weighs 0.21x2x16 + 0.78x2x14.01 + .01×39.95 = 28.98 grams. The mix he was breathing