Chemistry problem...

[Armueller2001]

Convert each of the following air concentrations to ppm if the temperature at the time of sampling is 28C and the atmospheric pressure is 720 mm Hg.

28 mg/m3 toluene


Apparently ppm is a volume measurement in air, so I need to find out how many liters 28mg of toluene occupies.

T = 301.15 K
Pressure = .9474 atm

1 mol of Toluene is 92.1384g so 28mg is .00030389 mol

PV=nRT

V = (.00030389)(.0821)(301.15)/(.9474)

V = .0079L so in 1 cubic meter, that's 7.9ppm correct?

Thanks!

[MJ5150]

Oh bummer, I thought this would be another thread about a failed relationship. This crap is way over my head, sorry.

-Mike

[username n/a]

So, I clicked on this thread on accident. I'm just gonna leave now.
*Walks away*

[ehque]

While your answer may be correct (i didn't check), calculating ppm from the ratio of partial pressures makes more sense than from ratio of "volumes", as the toluene will diffuse to occupy the full space of the container.

So, the volume should be fixed at 1 m^3, then calculate the partial pressure of the toluene at 28C (using PV=nRT), then compare that to the given atmospheric pressure as a ratio or percentage. You should arrive at the same answer, but the working should make more sense if your chem tutor is anal.

I hope i'm clear?

[bongomania]

Ppm is just grams of solute per (divided by) grams of solution. So convert everything to grams, divide, and you're done. Temperature and pressure are not relevant to the actual problem, since they are just given in that problem as a sort of context for the numbers.

Edit: I just double-checked, and there's a magnitude issue I forgot before: ppm is micrograms solute per mL of solution. Still just a question of conversion to grams and dividing, but take note of the micro and milli magnitudes.

[ehque]

Originally Posted by bongomania

Ppm is just grams of solute per (divided by) grams of solution. So convert everything to grams, divide, and you're done. Temperature and pressure are not relevant to the actual problem, since they are just given in that problem as a sort of context for the numbers.

Depends if you're talking about gases or liquids. While ppm is easy to calculate mass-wise for liquid solvents, ppm in gases is usually calculated in partial pressures, since you don't know the density of "air". Which is precisely why temperature and pressure was given. If i am not wrong. It has been a while.

[Armueller2001]

The teacher's slides say that ppm with gases are a volume/volume ratio.

[bongomania]

Originally Posted by ehque

ppm in gases is usually calculated in partial pressures, since you don't know the density of "air". Which is precisely why temperature and pressure was given.

You're absolutely right about partial pressures, I had forgotten about that. However I think the nature of the problem needs to be clarified. Ppm is not a volume measurement, so you'll need to focus on those partial pressures. (Yes I'm admitting it's not as simple as I thought .)

Another edit: I see where you say it's a vol/vol relationship. I guess that just wasn't how it was taught to me, but I don't doubt your teacher is right.

[ehque]

Originally Posted by Armueller2001

The teacher's slides say that ppm with gases are a volume/volume ratio.

While both calculations will come to the same answer, note that vol/vol ratios have no meaning in real life. The toluene is not taking up a particular volume in the container, and air taking up the rest. It clearly permeates the the whole container.

Partial pressure, on the other hand, is a very real concept. Just ask any oil-rig engineer, pilot or diver.