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wpjo (Mechanical)
27 Jun 06 20:15
I have a specification for LP-Gas given in weight percent for each constituent, such as propane C3H8 (95%), ethane C2H6 (1.2%), butane C4H10 (1.75%) and so on. I must now designate the volume percent of each constituent. I am looking for the method, a calculator or a spreadsheet to perform this conversion for various mixtures. Thanks.
Helpful Member!(3)  mbeychok (Chemical)
27 Jun 06 22:20
wpjo:

Here's how to do it:

-- Make a vertical column (Column 1) and list your various gas components (as C4H10, C3H8, C2H6, etc.) from top to bottom of list.

-- Make Column 2 (just to the right of Column 1) and using your weight percentages, list the mass of each component (as 1.75 for butane, 95.00 for propane, 1.20 for ethane, etc.). This column must add up to 100.00 when done.

-- Make Column 3 (just to the right of Column 2) and divide the mass of each component by the molecular weight of that component (1.75 divided by 58 for butane, 95.00 divided by 44 for propane, etc.) which gives you the mols of each component and list those moles from top to bottom of that column. Add up that column and enter the total at the bottom.

-- For a gas, mole percent is equal to volume percent. So now, make Column 4 (just to the right of Column 3) and calculate the mole percent of each component.  Column 4 must add up to 100.00. If not, you made an arithmetic error somewhere.

You can set it up on a spreadsheet ... but that will take you longer to do, than just going ahead with paper and pencil and working it out. It's very simple!

Keep in mind, mole percent = volume percent only for gases.  Don't try the above method with liquids.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

Helpful Member!(2)  25362 (Chemical)
29 Jun 06 0:02

The reason for that is found in Avogadro's principle which states that at a given temperature and pressure the volume of a gas sample is proportional to the amount of molecules present, V prop n.

This principle, together with Boyle's law: V prop 1/P and Charles'law:
V prop T, are incorporated in the famous expression known as the ideal gas law: PV = nRT. Where R is the proportionality constant, having the same value for every gas and called the gas constant. One of its values is 0.08206 L.atm/(K.mol).
Real gases that obey this law are called ideal gases.

The volumes occupied by one mol of various kinds of molecules are thus approximately the same at "normal" pressures, and approach the same value as the pressure is lowered.

At the (internationally) accepted standard temperature and pressure (STP) defined as 0oC (273.16 K) and 1 atm, the molar volume of most gases is found to be approximately 22.4 L.

The physical reason why gases behave nearly ideally under normal conditions (room temperature and 1 atm) is that their molecules move largely independently and -because of the relatively large distances between them- interact only weakly with one another except during collisions.

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