Homework Help: stoichiometry

Posted by escape on Saturday, October 4, 2008 at 11:00am.

A confiscated white substance, suspected of being cocaine, was purified by a forensic chemist and subjected to elemental analysis. Combustion of a 50.86-mg sample yielded 150.0 mg CO2 and 46.05 mg H2O. Analysis for nitrogen showed that the compound contained 9.39% N by mass. The formula of cocaine is C17H21NO4. Can the forensic chemist conclude that the suspected compound is cocaine?

stoichiometry - SraJMcGin, Saturday, October 4, 2008 at 11:43am
Thank you for using the Jiskha Homework Help Forum. Here is what I found on testing:

1. http://www.ez-test.com.au/ezwhite.htm

2. http://www.labtestsonline.org.uk/news/drug050222.html

3. (slide down to field tests): http://www.odvinc.com/newsletters/sp97-2.html

Sra
stoichiometry - Damon, Saturday, October 4, 2008 at 6:19pm
In the cocaine formula for every N (14 g/mole) atom here are 17 C (12g) atoms and 2 H (1g) atoms
Therefore thee ratios of masses of those three elements must be:
N -- 14
C --17*12 = 204
H --2*1 =2

Now how many moles of C in 150 g CO2?
(I am going to use grams instead of milligrams for everything because we are only interested in ratios in the end)
mole of CO2 = 12 +32 = 44 g/mole
so we have 150/44 = 3.41 moles CO2
and therefore 3.41 moles of C
which is 3.41 * 12 = 40.9 grams of C

Now how many moles of H in 46.05 g of H2O?
mole of H2O = 2+16 = 18 g/mole
so we have 46.05/8 = 2.56 moles H2O
and thus 2*2.56 = 5.12 moles of H

Now check ratios of either moles or grams
moles C/moles H = 3.41/5.12 = .666
but we know that we need 17 moles C to every 2 moles H if it is cocaine!
check my arithmetic and do more similar analysis for Nitrogen as well
typo - Damon, Saturday, October 4, 2008 at 6:22pm
46/18 not 46/8
The calculation is right above but I typed 8 instead of 18

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