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

[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.

 

[Nino MMP]

It appears that you are making a mistake in your conversion from molarity to ppm. You have assumed that the molecular weight of oxygen is 32 g/mol, which is correct. However, 1 mol/L is equal to 1 mmol/L, not 1 mg/L. To convert from molarity to ppm, you need to multiply your molarity by the molecular weight (in this case, 32 g/mol) and then divide by 1000 to get the result in ppm. So your calculation should be: 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 * (32 g/mol) /1000 = 8.32 ppm

 

[Blueshift5]

Hello,

It seems like you are on the right track with using Henry's Law to calculate the amount of dissolved oxygen in your water sample. However, there are a few things to consider.

First, make sure you are using the correct units for your calculations. In this case, the unit for KH is M/atm, so when you multiply it by the partial pressure of oxygen (0.21 atm), your units should cancel out to give you a final unit of M (molarity).

Secondly, when converting from molarity to ppm, make sure to take into account the density of water. The conversion factor from molarity to ppm is 1 M = 1 g/L, but since water has a density of 1 g/mL, the conversion factor should actually be 1 M = 1 mg/mL. This will give you a more accurate result.

Lastly, make sure to double check your calculations and units to ensure they are correct. If you are still having trouble, it may be helpful to consult with a colleague or your teacher for further guidance.

I hope this helps. Good luck with your calculations!