Empirical/Molecular Formula Review Question

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Nicole Gamboa 1G
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Joined: Fri Sep 25, 2015 3:00 am

Empirical/Molecular Formula Review Question

Postby Nicole Gamboa 1G » Sun Nov 01, 2015 11:45 pm

Hello! Can someone please help me with this problem? Thank you so much!

A combustion analysis was conducted on an known molecule. After 28.4 grams of the unknown molecule was combusted, 43.82 grams of carbon dioxide and 16.44 grams of water were produced. The molecular mass of the molecule is 342.3 g/mol. What is the empirical formula of the unknown molecule? What is the molecular formula of the unknown molecule?

Chem_Mod
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Re: Empirical/Molecular Formula Review Question

Postby Chem_Mod » Mon Nov 02, 2015 11:42 pm

Find the moles of carbon atoms, convert to mass, divide by the sample weight to get %C.
Do the same thing for hydrogen to get %H.
Subtract from 100% to get %O.
Now it's just a straightforward empirical formula problem (assume 100g, convert to moles, find ratios, etc)

mayasinha1B
Posts: 70
Joined: Fri Sep 29, 2017 7:04 am

Re: Empirical/Molecular Formula Review Question

Postby mayasinha1B » Tue Oct 03, 2017 10:02 am

You can calculate the grams of carbon using the given mass amount of CO2. You convert the mass of CO2 into moles using moles = mass/molar mass of CO2. Then, since CO2 and the compound are in a 1:1 ratio, you can use that same amount of moles to calculate the mass of Carbon using the equation moles=mass/molar mass of C. You do the same procedure for Hydrogen, except the ratio for Hydrogen to water is 2:1 moles. If the compound contains oxygen (if masses don't add up to original given mass), you then subtract masses from the original mass. This is the mass of oxygen. Then you calculate empirical or molecular formula with these masses as in a normal example.


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