Tuesday

July 29, 2014

July 29, 2014

Posted by **manny** on Friday, September 7, 2007 at 7:48pm.

When a 20.0 gram sample of an unknown compound is dissolved in 500 grams of benzene, the freezing pointof the resulting solution was observed to be 3.77 degrees C. If the freezing point of pure benzene is 5.48 degrees C and the Kf fro benzene is 5.12 degrees C/m, calculate the molar mass of the unknown compound.

ÄTf = KfM

Kf for benzene = 5.12 degrees C/m

the normal freezing point = 5.480C

ÄT = (5.48-3.77) = 1.71

1.71 degrees = (5.12 degrees C/m)(20.0g/MW)(5kg of solvent)

MW = 20.48g/mol

Is this correct?

- chem- molar weight -
**DrBob222**, Friday, September 7, 2007 at 8:03pmdelta T is correct at 1.71 C.

Then delta T = Kf*m and

m = molality = 1.71/5.12 = 0.334 mols/kg

mols = g/MW

0.334 = 20/MW/0.5 kg or

0.334 = (20/MW)*(1/0.5kg)

MW = ??

I have something like 120 or so. Check my thinking. Check my arithmetic.

- chem- molar weight -
**manny**, Friday, September 7, 2007 at 8:13pmok got it!!!!

Thanks for clearing it up for me!

**Related Questions**

Math - The density if a sample is 5.8g/cm^3. the sample fills a cylindrical ...

Chem Problem - I originally asked you the first question and I got the answer 7....

Chem Problem - I originally asked you the first question and I got the answer 7....

chem - A 3.48 gram sample of sodium sulfate hydrate lost 1.35 gram of water when...

Stats: Joint Density Function - Let the joint density function of X and Ybe ...

chemistry - A 2.91 gram sample of a gaseous compound that contains only boron ...

Math - 1 2/5 divided by 21 Which is correct? A. 21/1 x 5/7 = 105/7 =15 or B. 1/...

Chemistry - Specifications for lactated Ringer's solution, which is used for ...

ethics - A coworker calls you at 9 a.m. at work and asks for a favor. He is ...

Chemistry - Figuring uncertainty - Our group had to create a procedure that ...