# Help on amount of oxygen [dissolved] at equilibrium in an unknown sample

1. Nov 13, 2007

### ihatethem

Hi everyone. I need help on this. I am trying out this problem and trying to see if I am doing this right, but I can't quite figure it out. The original question is: "A water sample , at 25 C, Eh= 0.2 and pH=5.40. Assuming this system is at equilibrium, calculate the amt. of dissolved oxygen in ppm in the water sample.

What I know is that given KH = 0.0012589:

We know that O2 makes up about 21% of the atmosphere molecules, so its partial pressure is 0.21 atm (210,000 ppm)....Solving using Henrys Law

O2(aq) = PO2 x KH = 0.21 atm x 0.0012589 (M/atm) = 2.6 x 10-4 M...

given this M, we can convert from molarity to ppm...2.6 x 10-4 M = (2.6 x 10-4 mol) * 32 g = 0.0085 g/L = 8.5 mg/L

since mg/L is almost equal ppm. but it just doesn't seem to work. Teach is not taking my 8.5 answer. Anyone help me on what I'm doing wrong? Thanks.

Last edited: Nov 13, 2007