A forensic scientist was given a baggie of unlabled white powder by an investiga

Question

A forensic scientist was given a baggie of unlabled white powder by an investigator. based on the documents found at a crime scene, the powder had to be one of the four drugs (shown on the list below)

Molar Mass

(g/mol)

To determine the idenity of the substance in question she weighed out 0.3167 grams and combusted the powder in excess oxygen, capturing the gases released with no residual solid remaining. the combusted analysis produced 0.7668 g of CO2, 0.2415 g of H2O, and 0.1233 g of NO2 gas.

(a) Determine the mystery drug using the data from the combustion analysis and compare your results to the list provided (after showing all work) Hint: the molecule will only contain [C, H, N, and/or O]

(b) Consider the following hypothetical scenario: At the end of the analysis some solid resuidue had actually remained in the combustion chamber, indicating incomplete combustion. Based on your discussions of error, suggest how it would have affected the final results? Hint: O2 is always in excess in combustion analysis [explian in detail what else is affected]

Potential Drug

Molar Mass

(g/mol)

Caffine 194.19 Novocaine 236.31 Cocaine 303.35 Oxycodone 315.36

Explanation / Answer

(a) Mystery drug

moles of C = 0.7668 g/44 g/mol = 0.017 mol

moles of H = 2 x 0.2415 g/18 g/mol = 0.027 mol

moles of N = 0.1233 g/46 g/mol = 0.0027 mol

mass of C = 0.017 x 12 = 0.204 g

mass of H = 0.027 x 1 = 0.027 g

mass of N = 0.0027 x 14 = 0.038 g

mass of O = 0.3167 - mass(C + H + N) = 0.0477 g

moles of O = 0.0477/16 = 0.0026 mol

Divide by smallest factor,

C = 0.017/0.0026 = 6

H = 0.027/0.0026 = 10

N = 0.0027/0.0026 = 1

O = 0.0026/0.0026 = 1

Empirical formula of drug = C6H10NO

empircial formula mass = 112 g

Multiplied with 2 = 224 which is closest to 234.

Identity of drug possibly = Novocaine

(b) If not all of the material has been combusted. The net weight of CO2, H2O and NO2 would be lower than actual value. Thus lower value of C, H, N and O would be calculated from it. The resulting empirical formula would have lower values for all. Thus the molecular formula predicted will also be much lower and incorrect drug analysis would be performed.

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