Question

A forensic chemist is given a white powder for analysis. She dissolved 0.5g in 8.0g of...

A forensic chemist is given a white powder for analysis. She dissolved 0.5g in 8.0g of benzene. The solution freezes at 3.9 degrees Celsius. Is the powder cocaine?
Cocaine is C17H21NO4. Benzene freezes at 5.5 degrees Celsius and its Kf is 5.13 kg K/mole.

Homework Answers

Answer #1

First lets calculate the molality given the Freezing point depression:

∆ Tf = Kf m

m = ∆ Tf / Kf

m= (5.5 - 3.9)/ 5.13

m = 0.31

Next, Calculate number of moles from molality (moles solute / Kg solvent)

Moles = molality x Kg solvent

Moles = 0.31 (moles/Kg) x (0.0080 Kg)

Moles = 2.5 x 10-3 White Powder

Is The Powder COCAINE (C17H21NO4) ?

Calculate Molecular Weight of White Powder

Moles = 2.5 x 10-3   Grams = 0.50

MW = m / mol = 0.50 /2.5 x 10-3 = 200

MW cocaine = 303,353 g/mol

Then it is not cocaine.

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