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#1
12-27-2010, 06:03 AM
 prakash Junior Member Join Date: Oct 2010 Posts: 10 Rep Power: 0
vapor density from liquid density

I have a question... is it possible to obtain vapor density of a given organic compound (say acetone) from its liquid density? if so please explain the procedure or if there are any tables available, please direct me to the same. thanks in advance guys.
#2
12-27-2010, 07:53 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: vapor density from liquid density

Quote:
 Originally Posted by prakash I have a question... is it possible to obtain vapor density of a given organic compound (say acetone) from its liquid density? if so please explain the procedure or if there are any tables available, please direct me to the same. thanks in advance guys.
It is not possible and the vapor density will be a MUCH stronger function of temperature than the liquid density is. The CRC Handbook of Chemistry and Physics has a few tables. I am sure there must be better references but I don't know what they are.
#3
12-27-2010, 05:59 PM
 prakash Junior Member Join Date: Oct 2010 Posts: 10 Rep Power: 0
Re: vapor density from liquid density

thanks for the answer.... i already checked the crc handbook and all data are given for liquids and not gases...

in the MSDS relative densities are given, like if air has density of 1 then acetone has 3.... my question is can we directly multiply the density of air at a given temperature by this number to get the density of acetone or whatever gas?
#4
12-28-2010, 02:06 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: vapor density from liquid density

Quote:
 Originally Posted by prakash thanks for the answer.... i already checked the crc handbook and all data are given for liquids and not gases... in the MSDS relative densities are given, like if air has density of 1 then acetone has 3.... my question is can we directly multiply the density of air at a given temperature by this number to get the density of acetone or whatever gas?
You can use the density of air at the same temperature and pressure. The vapor pressure of the liquid is a strong function of temperature.
#5
12-28-2010, 05:52 AM
 prakash Junior Member Join Date: Oct 2010 Posts: 10 Rep Power: 0
Re: vapor density from liquid density

thanks again... so if the density of air at 28 C is 1.166 kg?m3, it can be multiplied by 3 so long as acetone is at the same temperature, right?
#6
12-28-2010, 06:12 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: vapor density from liquid density

Quote:
 Originally Posted by prakash thanks again... so if the density of air at 28 C is 1.166 kg?m3, it can be multiplied by 3 so long as acetone is at the same temperature, right?
No. You would need to know the vapor pressure of acetone at that temperature, and whether you have saturated vapor pressure. You would then need to know the density of air at that temperature and pressure.

I note it also gives Van Der Waals constants.

I would ignore the specific gravity and figure density directly from the Ideal Gas Equation (using molecular weight, temperature, and vapor pressure) or for better accuracy, Van Der Waal's equation (and the given constants).

(Van Der Waal's equation has two real gas corrections vs Ideal Gas Law)

Last edited by JohnS; 12-28-2010 at 06:24 AM.
#7
12-28-2010, 07:46 PM
 prakash Junior Member Join Date: Oct 2010 Posts: 10 Rep Power: 0
Re: vapor density from liquid density

Quote:
 Originally Posted by JohnS No. You would need to know the vapor pressure of acetone at that temperature, and whether you have saturated vapor pressure. You would then need to know the density of air at that temperature and pressure. Edit: This page gives saturated vapor pressure vs temperature. I note it also gives Van Der Waals constants. I would ignore the specific gravity and figure density directly from the Ideal Gas Equation (using molecular weight, temperature, and vapor pressure) or for better accuracy, Van Der Waal's equation (and the given constants). (Van Der Waal's equation has two real gas corrections vs Ideal Gas Law)

i am not able to see the link for the page (saturated vapor pressure vs temperature... can you upload it again?

can i use the ideal gas equation itself? cause i cant calculate the container volume as its shape is a bit complex... what will be the error percentage approximately?

and can i use the equations for vapor pressure given in wikipedia for the respective gases? are they the equations for saturated vapor pressures? i think i have to apply saturated vapor pressures because all the vapors are produced through bubbling air into the respective liquids... so the vapors must be saturated i think.

Last edited by prakash; 12-29-2010 at 01:05 AM.
#8
12-29-2010, 02:20 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: vapor density from liquid density

Quote:
 Originally Posted by prakash i am not able to see the link for the page (saturated vapor pressure vs temperature... can you upload it again? can i use the ideal gas equation itself? cause i cant calculate the container volume as its shape is a bit complex... what will be the error percentage approximately? and can i use the equations for vapor pressure given in wikipedia for the respective gases? are they the equations for saturated vapor pressures? i think i have to apply saturated vapor pressures because all the vapors are produced through bubbling air into the respective liquids... so the vapors must be saturated i think.
It looks like the link wasn't in my note:
http://en.wikipedia.org/wiki/Acetone_(data_page)

Yes, ideal gas, PV = nRT
From mass and molecular weight n = m/Mw
substituing, m/V = P*Mw/(R*T), m/V is density

Your "vapor" will be a combination of air and acetone vapor. The ambient air pressure will be reduced by the vapor pressure of the acetone. You may use Dalton's Law of Partial Pressure to independently calculate the air density and acetone density, and add them for the mixture.

Last edited by JohnS; 12-29-2010 at 02:27 AM.
#9
12-29-2010, 07:01 AM
 prakash Junior Member Join Date: Oct 2010 Posts: 10 Rep Power: 0
Re: vapor density from liquid density

Quote:
 Originally Posted by JohnS It looks like the link wasn't in my note: Yes, ideal gas, PV = nRT From mass and molecular weight n = m/Mw substituing, m/V = P*Mw/(R*T), m/V is density Your "vapor" will be a combination of air and acetone vapor. The ambient air pressure will be reduced by the vapor pressure of the acetone. You may use Dalton's Law of Partial Pressure to independently calculate the air density and acetone density, and add them for the mixture.

thanks so much.... what i have to do is determine the ppm content of a volatile gas in air... so i have divided two quantities, each of which is a ratio of the product of mass flow rate and density to the molar mass. so dividing the two quantities (gas and air respectively), i get the ppm after multiplying by a million.

i know the temperature of the mixture of air and gas at the exit, so i got the density of air from standard tables at that temperature, and now since i got the density of the gas too from the ideal gas law, i can proceed further..

i only hope i got the formulas right? can u just check it up and tell me?

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