Molecular Formula Question
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Molecular Formula Question

[From: ] [author: ] [Date: 12-08-13] [Hit: ]
75 mg of carbon dioxide. If the molar mass of amphetamine is less than 160 g/mole, what is its molecular formula?My friend needs help on this AP Chem packet question.I only went up to Honours, so Im not sure about this.......
The stimulant amphetamine contains only carbon, nitrogen, and hydrogen. Combustion analysis of a 42.92 mg sample of amphetamine gives 37.187 mg water and 125.75 mg of carbon dioxide. If the molar mass of amphetamine is less than 160 g/mole, what is its molecular formula?

My friend needs help on this AP Chem packet question.
I only went up to Honours, so I'm not sure about this.

Please help,
X

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mass of H = (2H/H2O) x 37.187g = 0.1111 x 37.187 = 4.132g. %H = (4.132/42.92) x 100 = 9.63%
mass of C = (C/CO2) x 125.75 = 0.2727 x 125.75 = 34.295g %C = (34.295/42.92) x 100 = 79.90%
Hence % N = 100 - (9.63 + 79.90) = 10.47%
H= 9.63/1 = 9.63, C = 79.9/12 = 6.66 and N = 10.47/14 = 0.75
Mole proportion: H = 9.63/0.75 = 13 , C = 6.66/0.75 = 9 and N = 0.75/0.75 = 1
Empirical formular = C9H13N
To determine the molecular formula, (C9H13N)x = 160 i,e 135x = 160 hence x = 1.2 = 1
So the molecular formula is C9H13N
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