Crime scene investigators keep a wide variety of compounds on hand to help with

Question

Crime scene investigators keep a wide variety of compounds on hand to help with identifying unknown substances they find in the course of their duties. One such investigator, while reorganizing their shelves, has mixed up several small vials and is unsure about the identity of a certain powder. Elemental analysis of the compound reveals that it is 63.57% carbon, 6.000% hydrogen, 9.267% nitrogen, 21.17% oxygen by mass. Which of the following compounds could the powder be?

C3H6NO3 = hexamethylene triperoxide diamine (HMTD), commonly used explosive

C17H19NO3 = morphine, analgesic

C7H5N3O6 = 2,4,6-trinitrotoluene (TNT), common used explosive

C11H15NO2 = 3,4-methylenedioxymethamphetamine (MDMA), illicit drug

C4H5N2O = caffeine, stimulant

C21H23NO5 = heroin, illicit drug

C8H9NO2 = acetaminophen, analgesic

C10H15N = methamphetamine, stimulant

Explanation / Answer

MW of C = 12

MW of H = 1

MW of O = 16

MW of N = 14

assume a basis of 100 grams

then

63.57 is C, 6 is H, 9.26 is N and 21.2 O

Divide all by its molar weight to find the moles

is C = 63.57/4 = 5.3

H = 6/1 = 6

N = 9.26 /14 = 1.5

O = 21.2/16 = 1.33

Find ratios between substances

C:N ratio --> 5.3/1.5 = 3.5X or 7C:2N

C:H ratio --> 5.3/6 = 0.88

C:O ratio --> 5.3/1.33 = 3.98 or 4

All of the following CANT be

Therefore, only

C4H5N2O

C21H23NO5

C8H9NO2

Fit the profile (C:O ratio)

Find N:O ratio -->1.5/1.33 = 1.12 or almost 1:1

C21H23NO5 cant be since ratio is to different

Stick with

C4H5N2O

C8H9NO2

Find H:N ratio --> 6/1.5 = 4H:1N

Nearest answer is

C4H5N2O --> caffeine

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