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  #1  
Old 06-04-2008, 03:44 PM
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Default ppm to mg/L

Hi all, I am trying to convert 200ppm by weight of dry hydrogen sulphide gas in air to mg/L?? I know many people say they are the same, but does the density of H2S or the fact that it is measured by weight make any difference???

Many thanks in advance
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  #2  
Old 06-05-2008, 02:33 AM
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Default Re: ppm to mg/L

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Originally Posted by Unregistered
Hi all, I am trying to convert 200ppm by weight of dry hydrogen sulphide gas in air to mg/L?? I know many people say they are the same, but does the density of H2S or the fact that it is measured by weight make any difference???

Many thanks in advance
Hi, this is quite an unusual way of expressing H2S content.

200ppm by weight means you have 200mg H2S per kilogram of air. (200mg H2S, 999,800mg air)

1 mole of ideal gas occupies about 22.4L at 0蚓, or about 24L at 25蚓.

One way to do this calculation is to work out how many moles you have in 1L of air, then work out what proportion will be H2S. If you would like to have a go at working this out yourself, you are most welcome to post your answer back, and we can check.

I think you can assume standard air, comprised of O2 and N2.
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  #3  
Old 06-05-2008, 04:16 AM
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Default Re: ppm to mg/L

I would be very suspicious that the 200 ppm figure is really by weight. Gas mixing ratios are almost always expressed as ppm by mole fraction.

Assuming it is by weight, you have to specify the temperature and pressure of the air. For 1 atm, 25 蚓, air weighs about 1.2 g/L. If it is 200 ppm w/w of H2S, that would be 0.24 mg/L.
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  #4  
Old 09-20-2010, 10:27 AM
roztal roztal is offline
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Default Re: ppm to mg/L

Hi,

I have an article which says that the concentration of acetone in human breath is around 4.4uM (micro molar). I would like to know how much it represents in ppm (parts per milion)

If I assume it is an ideal gas at 24/25 degrees, than the volume of 1 mol is 24L. Well, I have 4.4*10^-6 mol/L if I multiply it by 24L/mol I would get 105.6*10^-6.

Is this conversion correct? Is it ok to assume the breath to be an ideal gas or it is too far from it?

Thanks in advance
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  #5  
Old 09-20-2010, 10:54 AM
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Default Re: ppm to mg/L

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Originally Posted by roztal View Post
Hi,

I have an article which says that the concentration of acetone in human breath is around 4.4uM (micro molar). I would like to know how much it represents in ppm (parts per milion)

If I assume it is an ideal gas at 24/25 degrees, than the volume of 1 mol is 24L. Well, I have 4.4*10^-6 mol/L if I multiply it by 24L/mol I would get 105.6*10^-6.

Is this conversion correct? Is it ok to assume the breath to be an ideal gas or it is too far from it?

Thanks in advance
Just a note: human breath is closer to 36 or 37蚓. You can use a molar ratio for ppm for gases though, so 4.4 然 is 4.4 ppm, as i understand the question. (The reason you can use a molar ration for gases is because the volume for a mole is constant).
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  #6  
Old 09-20-2010, 01:20 PM
roztal roztal is offline
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Default Re: ppm to mg/L

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Originally Posted by Mrs X View Post
You can use a molar ratio for ppm for gases though, so 4.4 然 is 4.4 ppm, as i understand the question. (The reason you can use a molar ration for gases is because the volume for a mole is constant).
Why is the molar ratio equals ppm? The way I see it, ppm does not have a unit, so it could be 痢/g or 無/L. Molarity is given in mol/L so, 4.4 然 is 4.4痠ol/L.

I don't understand why is that equals to ppm. If it is an ideal gas, it's volume for moles is constant, but why does that make 然 equals to ppm?

Sorry if I am being too insistent, but I need to get this right for my work.

Thanks again
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Old 09-20-2010, 05:05 PM
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Default Re: ppm to mg/L

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Originally Posted by roztal View Post
Why is the molar ratio equals ppm? The way I see it, ppm does not have a unit, so it could be 痢/g or 無/L. Molarity is given in mol/L so, 4.4 然 is 4.4痠ol/L.

I don't understand why is that equals to ppm. If it is an ideal gas, it's volume for moles is constant, but why does that make 然 equals to ppm?

Sorry if I am being too insistent, but I need to get this right for my work.

Thanks again
I agree, you need to get it right.

"ppm" means parts per million. For a gas that means volume or mol ratios. They are the same thing for an ideal gas, as 1mole of ideal gas ocupies 24L at room temp.

Think about 1L of breath, with 4.4然 acetone. There are 1,000,000無 of breath. 4.4無 are acetone, 999,995.6無 are not.

Hope this helps, but it seems as though you need to do some reading around this subject to understand it better.
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Old 09-21-2010, 09:14 AM
roztal roztal is offline
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Default Re: ppm to mg/L

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Originally Posted by Mrs X View Post
Think about 1L of breath, with 4.4然 acetone. There are 1,000,000無 of breath. 4.4無 are acetone, 999,995.6無 are not.
OK, there is still something I don't agree (or I don't understand).

Acording to wekepedia: "Molarity (in units of mol/L, molar, or M) or molar concentration denotes the number of moles of a given substance per liter of solution. A capital letter M is used to abbreviate units of mol/L

So, if I have 4.4然 of acetone, in 1L of breath or 1,000,000無 I will have 4.4 moles instead of 4.4無 as you said, isn't that right?

So 4.4然 is not equals 4.4ppm... If, as you said, 1 mol of an ideal gas at room temp. has 24Liters, than I could use the ratio 24L/mol and multiply it by the concentration ratio 4.4*10^-6 mol/L => 24L/mol*4.4*10^-6 mol/L.. In this case all the units would cancel out and I would find the ppm value??

Ok, that was my theory, but if what you said before is right and my thought is not, than I would ask you to give some complementary reading because I am really getting the whole thing wrong.

Thanks again!!

Last edited by roztal; 09-21-2010 at 09:25 AM.
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  #9  
Old 09-21-2010, 10:47 AM
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Default Re: ppm to mg/L

Quote:
Originally Posted by roztal View Post
OK, there is still something I don't agree (or I don't understand).

Acording to wekepedia: "Molarity (in units of mol/L, molar, or M) or molar concentration denotes the number of moles of a given substance per liter of solution. A capital letter M is used to abbreviate units of mol/L

So, if I have 4.4然 of acetone, in 1L of breath or 1,000,000無 I will have 4.4 moles instead of 4.4無 as you said, isn't that right?

So 4.4然 is not equals 4.4ppm... If, as you said, 1 mol of an ideal gas at room temp. has 24Liters, than I could use the ratio 24L/mol and multiply it by the concentration ratio 4.4*10^-6 mol/L => 24L/mol*4.4*10^-6 mol/L.. In this case all the units would cancel out and I would find the ppm value??

Ok, that was my theory, but if what you said before is right and my thought is not, than I would ask you to give some complementary reading because I am really getting the whole thing wrong.

Thanks again!!
Wikipaedia is talking about a solution, which means a solid dissolved in a solvent.

You however, are asking about a gas, which is different. For an ideal gas, 然 means 無/L, because of the volume taken up by 1 mole of any gas obeying the ideal gas law is the same. The weight of the gas doesn't matter.

Are you at a university? - Suggest you go to the library and get the text used in the introductory chem classes, or applied physics classes to help you.
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  #10  
Old 09-21-2010, 11:03 AM
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Default Re: ppm to mg/L

This is VERY confusing. I have never seen the term molarity applied to gases, only liquid solutions. If you imagine a control volume around a body of gas, a measure in grams (of some pollutant) per liter of total mixture is a very poor measure, as the volume of the control volume changes with temperature and pressure, but the grams of pollutant doesn't. Gases are normally specified by molar ratio, that is moles of A per mole of mixture.

However, since "acetone breath" usually relates to diabetes, I attempted to Google the concept, and I did find some research in which the concentration of acetone in exhaled breath was stated in nanomoles per liter. (By the way, the figure was not in excess of 1 痠ol/L and the subjects were in significant ketoacidosis. I question the value asserted as "normal").

I suppose one can use the molar volume for exhaled breath at some reasonable temperature and pressure (1 atm, 37 蚓??) to convert to moles and get a molar ratio expressable as ppm. I have no idea how they account for volume change as the "bag of breath" collected cools towards room temperature or the excess humidity condenses out (and acetone is miscible in water). I would think the protocols of sample collection are important and influence results. (Note: at 37 蚓, 1 atm, molar volume is 25.4 L)
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