Question 2 of 4 Map General Chemistry University Science Books 4th Edition prese

Question

Question 2 of 4 Map General Chemistry University Science Books 4th Edition presented by Sapling Learning Donald McQuarrie. Peter A. Rock Ethan Gallogly Quinine is a natural product extracted from the bark of the cinchona tree, which is native to South America. Quinine is used as an antimalarial agent. When 1.78 g of quinine is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 4.58 °C. The freezing point and Kf constant for cyclohexane can be found here Calculate the molar mass of quinine. Number g mol o Previous Check Answer o Next Exit Hint

Explanation / Answer

The extent of freezing point depression depends only on the solute concentration that can be estimated by a simple linear relationship with the cryoscopic constant:

Tf = m kf i

kf is the freezing point constant
i is the Van't Hoff factor which in this case is 1
m is molality (moles of solute/kg of solvent)
Tf is temperature change

Tf is -4.58 °C

Kf = -20.2 °CKg/mol

And    i = 1

We can get molality   m = Tf /kfi

Molality m = -4.58 °C /(-20.2 °CKg/mol)(1) = 0.227 mol/Kg

Molality = moles of solute / Kg of solvent

So we can calculate moles of solute: moles = (molality)(Kg of solvent)

moles = 0.227 (mol/Kg)(25.0 x 10-3 Kg) = 0.00567 moles

In addition….    moles = mass/molar mass

molar mass = mass/moles = 1.78g/0.00567moles

Molar mass of Quinine = 313.9 g/mol

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