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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: punterkicker06 on September 12, 2005, 10:18:38 PM

Title: Whats the empirical formula of vanilin?
Post by: punterkicker06 on September 12, 2005, 10:18:38 PM

I have a homework question, that I know the answer to, but i need to show work, and i don't know how to attain the answer a legitimate way...

The quetions asks...

Vanilin, a flavoring agent, is made up of carbon, hydrogen, and oxygen, atoms.  When a sample of it weighing 2.500 g is burned in excess oxygen 5.79 g of Carbon dioxide and 1.18 grams of water are attained.  Whats the empirical formula of vanilin?

I know the answer is C₈H₈O₃

Title: Re:Hi , I need help w/ Empirical Formulae
Post by: mike on September 12, 2005, 11:10:36 PM

There are some good sites on the net for combustion analysis, try searching for "empirical formula from combustion analysis" or similar, I found this one which seems ok:

http://dbhs.wvusd.k12.ca.us/webdocs/Mole/CombustionAnalysis.html

Title: Re:Whats the empirical formula of vanilin?
Post by: Donaldson Tan on September 13, 2005, 12:27:14 PM

all carbon in the carbon dioxide comes from the vanilla sample

this tells u the number of carbon atoms in 2.5g of vanilla is the same as that in 5.79g of carbon dioxide.

all hydrogen in the water comes from the vanilla sample.

this tells u the number of hydrogen atoms in 2.5g of vanilla is the same as that in 1.18g of water.

given that u have found out the mass of hydrogen and carbon in the 2.5g of vanilla, what is the mass of oxygen in the sample?

next, find the molar ratio of carbon, hydrogen and oxygen in vanilla.

viola! u have deduced the emperial formula of vanilla (finally).

 :)