# PPM conversion

1. Oct 30, 2013

### Lars Bos

1. The problem statement, all variables and given/known data

A 185 m3 room must be filled with 4 ppm NO2 gas (Nitrogen dioxide). The gas comes in cylinder of 200 bar and 50 liters (20x50 = 10,000 liters) and 50 ppm. How many cylinders are needed?

2. Relevant equations

1 ppm = 1/1,000,000 (parts per million)
(can't think of more relevant equations)

3. The attempt at a solution

To be honest I do not know how to begin. The ppm part is completely new to me.

Last edited by a moderator: Oct 30, 2013
2. Oct 30, 2013

### Staff: Mentor

If ppm is 1/1,000,000, what mass of the NO2 is needed for 1 ppm if there are 1,000,000 kg of the air?

3. Oct 30, 2013

### Lars Bos

Molar mass of NO2 is 46 g/mol.

Does that mean we need 1/1,000,000th of the above?

4. Oct 30, 2013

### Staff: Mentor

Molar mass doesn't matter for now. Try to answer my question.

Note: question is incomplete, as it doesn't state if it is v/v or w/w ppm, these are two different cases (which will give slightly different answers).

5. Oct 30, 2013

### Lars Bos

What do you mean by v/v or w/w ppm?

Sorry, your question is not clear enough.

6. Oct 30, 2013

### Staff: Mentor

v/v - volume by volume (as in 1 L in 1,000,000 L)

w/w - weight by weight (as in 1 kg in 1,000,000 kg).

I am afraid my question is extremely basic and quite clear, if you have problems answering it you will have problems solving the original questions.

Let's try from the other end - if there are 10 kg of substance in 1,000,000 kg of the air, what is its ppm concentration?

7. Oct 30, 2013

### Lars Bos

That is 10/1,000,000 = 0.00001 or 10 ppm

Btw the original question is in v/v.

8. Oct 30, 2013

### Staff: Mentor

OK, v/v means it will be easier to solve.

10 L in 1,000,000 L is 10 ppm.

What gas volume is present in 1,000,000 L of air if the concentration is 1 ppm?

9. Oct 30, 2013

### Staff: Mentor

For gases, ppm usually refers to mole fraction x 106. What I can't figure out is how the mole fraction can increase from 4 ppm to 50 ppm, unless some reaction is occurring. Is there an equilibrium reaction involved?

10. Oct 31, 2013

### Lars Bos

That is 1 liter.

11. Oct 31, 2013

### Lars Bos

No, there is no chemical/equilibrium reaction involved.

12. Oct 31, 2013

### Staff: Mentor

So, can you calculate what volume of NO2 required, knowing the volume of the room?

I must admit I initially missed then 50 ppm part and I am not sure how to understand it. Perhaps it just means NO2 will be diluted.

13. Nov 4, 2013

### Lars Bos

They require a solution of 50 ppm NO2 with some unreactive gas in the cylinder.

14. Nov 4, 2013

### Staff: Mentor

So it is just dilution most likely.

How many liters of NO2 in 185 m3 at 4 ppm?

15. Nov 4, 2013

### Lars Bos

The amount is not given. All the given values are mentioned in the question.

I know this is quite tricky one.

16. Nov 4, 2013

### Staff: Mentor

There is nothing tricky about the problem.

You were already able to calculate volume of gas present in 1,000,000 L of air when the concentration is 1 ppm. Volume of NO2 in 185 m3 at 4 ppm is identical kind of a problem, just with different numbers. No magic here, simple application of exactly the same principle.

17. Nov 4, 2013

### Lars Bos

Are you trying to imply that since 185 m3 equals to 185,000 liters, the 4 ppm of NO2 amounts to 21.6 liters?

18. Nov 4, 2013

### Staff: Mentor

I am not "trying to imply" anything, I am trying to make you think.

No idea how you got 21.6 liters, and unfortunately it is not the correct number.

19. Nov 4, 2013

### Lars Bos

I understand that you are trying to help me out here and I appreciate it.

What I don't get is how to change the 185 m3 into ppm and then calculate how much 4 ppm amounts to.

20. Nov 4, 2013

### Staff: Mentor

Sorry, you have lost me. You don't change 185 m3 to ppm, one is volume, the other is concentration, you can't change one into another.

Let's try again:

In 1,000,000 liters 1 ppm means 1 L - that's what we already know.

In 1,000,000 liters 4 ppm means ..... L.

In 1,000 m3 4 ppm means ..... L.

In 100 m3 4 ppm means ..... L.

In 185 m3 4 ppm means ..... L.