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I am doing some air emissions calculations and I really just have a bad feeling about my results. By definition, molecular weights are given in $\pu{g/gmol}$. For the substance I have, I was told by a technical contact at Citgo that this material has a molecular weight of $\pu{140 g/mol}$ (which should be $\pu{g/gmol}$). Whenever I convert this $\pu{lb/lbmol}$ (which is required for this particular equation), I'm getting a value of $\pu{0.3086 lb/lbmol}$. There are 453 grams in a pound. Is it not correct to divide 140 grams by 453 grams to get the total number of pounds?

My final result for annual emissions is pretty low.

Gaurang Tandon
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Ryan_C
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3 Answers3

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A gram-mole ($\pu{gmol}$),often just called a mole ($\pu{mol}$), is the amount of a material whose mass in grams equal to the numerical value of the atomic or formula mass. For example, a gram-mole of water has a mass of 18.017 grams, this number being the formula mass for water.

A pound-mole ($\pu{lbmol}$) is the amount of a substance whose mass in pounds-mass equals the atomic or formula mass. For water, one pound mole has a mass of $18.017$ lb-mass.

The pound-mole is bigger than the gram-mole by the same factor as a pound is bigger than a gram - so pounds per pound-mole equals grams per gram-mole. You do not divide by anything.

Gaurang Tandon
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Oscar Lanzi
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If the amount of weight you are measuring it in is pounds then use pound moles if the amount your measuring in is grams use gram moles.

140g/g-mol = 140lb/lb-mol

that explanation is confusing to people the simplest way i have seen it explained is that 453.592gmol = 1 lbm-mol

so if you are given the weight of a substance say you have 1000lbm of hydrogen and you want to know the amount of moles.

simply do 1000lbm * lbmol/2lbm = (500lbmol)*453.592gmol/1lbm-mol = 226,796 gmol. If you want to convert the other way do the exact opposite.

btw lbm is lbm just means pounds weight not lbf pounds force.

seth boyd
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https://www.calculand.com/unit-converter/?gruppe=Molar+Mass&einheit=Pound+per+pound-mole+%5Blb%2Flbmol%5D

1 lb / lb-mol is exactly 1 g / mol, because 1 lb-mol is 453,59 mol, which is the same ratio that there is between lb and g. The link above is a very useful converter.

Alexandre Oliveira
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