# I have cation concentrations. How to figure out corresponding H and O concentrations?

Hello all:

I would like to test some software, to find out whether it works correctly. To do this, I want to reproduce a calculation that someone did using a different method.

This calculation involves chemistry, and my chemistry skills are not great. I would really appreciate if someone could help me out with what is probably a rookie question!

I have data for cation/anion concentrations. But for some reason, although the cations are within a fluid made up of H and O, the H and O data are not given. Given the cation concentrations, is it possible to figure out the corresponding amounts of H and O? If so, how?

millimoles/liter
Na + = 110.0
Mg2+ = 5.0
Ca2+ = 0.966
Cl- = 121

Thanks a lot for any help!

Borek
Mentor

For very precise calculations you would need solution density. For approximate calculations, you can safely assume concentration of water to be 1000/18 mol/L.

But I have a feeling you might be trying to do something unnecessary, something that has a source in your confusion.

Thanks a lot Borek. You are probably right that I don't properly understand this...

What is the significance of the 1000/18? I just need the concentration of H and O that would go along with the cation concentrations (if that is possible). I can then find out the solution density. If it helps, this is all at 25 C.

Borek
Mentor

You can't find solution density after - this is a thing that has to be measured, so you need to know it before being able to calculate anything. Solution is rather diluted, so it will be quite close to 1.00 g/mL - could be that's accurate enough for your needs.

1000/18 is just a molar concentration of pure water - assuming 1.00g/mL, 1L mass is 1000g, molar mass of water is 18g/mol. Knowing molar concentration of water you can calculate amount of H and O (although it doesn't make much sense to me - it could make sense as a part of the proper mass balance for the system, but I doubt you need it).

Thanks Borek, this helps a lot.

I got the data from a paper published by people who tested their code. I don't understand why they don't specify H and O if it isn't clear what H and O must be! Anyway, maybe they used the assumption you describe, so I'll try that.

Much appreciated.

Borek
Mentor

Concentration of water is usually considered to be constant, so - depending on what you do - you can completely ignore its presence. Perhaps that's the case.