You measure the concentration of O2 in seawater to be 5.0 mg O2/L H2O. Express this concentration as a percent. Question 2: The earths atmosphere contains 21% O2/L of air. How much more oxygen does the atmosphere hold than seawater from the previous question? So for question one I got 0.01% which I think is way off. For question 2, I was just going to subtract 21% from the answer from 1. I changed the milligrams to grams for question one.