Problem: A sample of an organic compound (a nonelectrolyte) weighing 1.35 g lowered the freezing point of 10.0 g of benzene by 3.66 °C. Calculate the molar mass of the compound.

🤓 Based on our data, we think this question is relevant for Professor Brack's class at HOFSTRA.

FREE Expert Solution

Change in freezing point (ΔT­f) is given by:


ΔTf=Tf, pure solvent-Tf, solution


The change in freezing point is also related to the molality of the solution:


ΔTf=imKf


where: 

i = van’t Hoff factor

m = molality of the solution (in m or mol/kg)

Kf = freezing point depression constant (in ˚C/m)


Recall that the molality of a solution is given by:


molality=moles solutekg solvent




Step 1:

ΔTf = 3.66˚C            i = 1 (benzene is a non-electrolyte)

m = ??                     Kf = 1.86 ˚C • kg/mol (can be found in textbooks or online)



Solving for molality:


ΔTf=imKf


3.66°C=1m5.12 °Cm


m=3.66 °C5.12 °Cm = 0.7148 mol compound / kg benzene



Step 3: 

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Problem Details

A sample of an organic compound (a nonelectrolyte) weighing 1.35 g lowered the freezing point of 10.0 g of benzene by 3.66 °C. Calculate the molar mass of the compound.

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Our tutors have indicated that to solve this problem you will need to apply the The Colligative Properties concept. You can view video lessons to learn The Colligative Properties. Or if you need more The Colligative Properties practice, you can also practice The Colligative Properties practice problems.

What professor is this problem relevant for?

Based on our data, we think this problem is relevant for Professor Brack's class at HOFSTRA.

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Our data indicates that this problem or a close variation was asked in Chemistry - OpenStax 2015th Edition. You can also practice Chemistry - OpenStax 2015th Edition practice problems.