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Finding Molar Mass Through Dumas Method

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We are trying to determine the molar mass of an unknown liquid through Dumas Method: measure density (mass/volume) of its vapor at a known temperature and pressure. I am having trouble answering question 3 and question 4 as attached.

1) I did the calculations for question 3 but am really lost on whether I plugged in the right numbers eg whether I use the # of moles calculated from the ideal gas equation in question C. If I do that, wouldn't the response make no sense? We're trying to determine the difference between the known between an ideal or non-ideal gas but if the moles of the gas is derived from the ideal gas equation to plug into non-ideal gas equation, wouldn't the results be wrong? Also, it says you can determine the actual volume of the gas molecules from the volume of the liquid, because in a liquid, the molecules are compressed about as close together as possible - what does this mean? I had no idea how to solve this so I just used the volume of a liquid - is this wrong? Are my calculations correct?
2) I'm having trouble with question 4: part a&b, have to eplain why chemically (not just b/c of mathematics as I already did. Also, question c&d - please advise.

thank you!

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Solution Preview

Hi there,

I've attached my workings as a word document :)

In answer to your direct queries above...

1) You have, in my opinion used the right numbers and your calculations look good! The question has told you to use your previously obtained molar mass (which comes from your previously obtained calculated moles..). The point of the van der waals correction part to this question is to show you the general deviation in comparison to the ideal equation. Even if the number of moles in this case is not perfect, the outcome would show the same trend in ...

Solution Summary

The expert finds molar mass through Dumas Methods.