Expressing concentrations as a percent

[Illyaria]

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 Earth's 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.

 

[Comeback City]

What was your attempt to solve #1?

 

[Illyaria]

What was your attempt to solve #1?

I converted the 1 5.0 mg to grams and got 0.005g. Then, I divided that by 50 which is the atomic msss for O2/L H2O I think and multiplied that by 100.

 

[Comeback City]

Atomic mass of Oxygen is 16.00 amu, so O₂ would be 32.00. Why did you divide by it though and then multiply by 100?