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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: zxwut? on February 03, 2010, 09:16:34 PM

Title: finding molar mass from fp and mass
Post by: zxwut? on February 03, 2010, 09:16:34 PM

He asks:
A solution of 2.500g of BHT in 100.0g of benzene had a freezing point of 4.880 degrees Celcius. What is the molecular mass of BHT?

My first thought was to use MM of solute= [(Kb)(mass solute)(i)] / [(delta Tf)(kg solvent)] but I don't know the differential freezing point temperature. How do I go about finding that or am I totally off base here?

Title: Re: finding molar mass from fp and mass
Post by: Schrödinger on February 03, 2010, 10:26:02 PM

Your formula seems to be ok except for the fact that K_b has to replaced by K_f, since we are dealing with freezing point depression.

As for the (https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.forkosh.dreamhost.com%2Fmimetex.cgi%3F%7B+%5CDelta+T_f+%7D&hash=1b8bec431b8802cc1417909eb7606d0b61087272) part, you have the freezing point of the solution and you can find the freezing point of pure solvent (google/wikipedia-benzene)

Title: Re: finding molar mass from fp and mass
Post by: zxwut? on February 03, 2010, 11:09:32 PM

Oh, I see. Thank you very much, sir.

Title: Re: finding molar mass from fp and mass
Post by: Schrödinger on February 04, 2010, 12:30:39 AM

Dude, I'm just a 17 year old. 8)
You don't have to call me "sir"  :P.
Anyway, feel free to ask doubts