# Water Chemistry Problem

1. Sep 17, 2010

### sdoug041

The question:

Nitrate concentrations exceeding 44.3 mg NO3-/L are a concern in drinking water due to the infant disease, methemoglobinemia.

Nitrate concentrations near three rural wells were reported as 0.01 mg NO3- N/L, 1.3 mg NO3- N/L, and 20.0mg NO3- N/L. Do any of these three wells exceed the 44.3 ppm level?

The problem:

I have no problem with conversions, it's just the units in this case. I've asked peers about the NO3- N/L unit and they said the N stands for nitrogen. If this is so, I don't know what the implications are. Whats the difference between the NO3-/L and NO3- N/L unit?

2. Sep 17, 2010

### Staff: Mentor

Doesn't make sense to me.

I can easily imagine NO3- reported as 0.01 mg N/L, but the way you have it written it is - to say the least - strange.

3. Sep 17, 2010

### alxm

Hmm, I've https://www.physicsforums.com/showpost.php?p=2162757&postcount=2"..

But to elaborate a bit more; you have the whole nitrogen cycle, so nitrogen in water (with living stuff in it) is being converted between nitrate, nitrite, ammonia, etc.
Therefore they distinguish between "N/L" nitrogen per liter (the mass of N from all compounds) "NO3- N/L", nitrate nitrogen per liter (just the nitrate nitrogen) and nitrate/L (the mass of nitrate, including oxygen).

Last edited by a moderator: Apr 25, 2017
4. Sep 17, 2010

### sdoug041

But for every mole of NO3-, there is 1 mole of N... So is there no conversions to be made here? I can just simply look at the values and if they are less than 44.3, they don't exceed the limit?

5. Sep 17, 2010

### Staff: Mentor

ppm is usually weight/weight, so you need some conversion.

6. Sep 17, 2010

### sdoug041

But a mg/L is a ppm, at least concerning aqueous solutions.

there is 10^6 mg of water in a litre.

I think I'm still missing something though. It can't be as easy as just looking at the given values...

Last edited: Sep 17, 2010
7. Sep 17, 2010

### alxm

Your units are in mg/L, not mol/L. One mole of N doesn't weigh the same as 1 mole of NO3-

8. Sep 18, 2010

### sdoug041

Ah that is a good point, it's starting to become a little clearer.

So:

(0.01 mg NO3- N/L) x (1 mol N / 14000 mg N) x (1 mol NO3- / 1 mol N) x (62000 mg NO3- / 1 mol NO3-)

= 0.04 mg NO3-/L

Am I understanding this concept correctly?