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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: hmrobinson09 on February 24, 2009, 08:16:37 PM

Title: Calculating molecular weight of unknown by fp depression?
Post by: hmrobinson09 on February 24, 2009, 08:16:37 PM
Can someone help me with this?
A solution of 2.24 x 10-1g of an organic compound of unknown molecular weight in 1.664g of camphor gave a freezing point of 176.44 Celsius. The freezing point of camphor is 179.80 Celsius, and the freezing point depression constant Kf 39.7 Celsius/m. Calculate the molecular weight of the unknown.
Title: Re: Calculating molecular weight of unknown by fp depression?
Post by: Borek on February 25, 2009, 02:32:57 AM
What is formula for calculation of freezing point depression?
Title: Re: Calculating molecular weight of unknown by fp depression?
Post by: sm2345 on February 27, 2009, 12:28:08 AM
Here is a formula which can be used to calculate the Molecular Weight of the solute:

ΔTf = (kfxW2x1000)/(M2xW1)

where ΔTf = Depression of freezing point
       
         kf  = Molal Depression Constant(Cryoscopic constant)
     
         W2 = Weight of the solute
 
         W1 = Weight of the solvent &

         M2 = Molecular Mass of the solute.
Title: Re: Calculating molecular weight of unknown by fp depression?
Post by: Borek on February 27, 2009, 03:34:41 AM
ΔTf = (kfxW2x1000)/(M2xW1)

While it is not wrong, it may be confusing, as it makes several conversions in one step, instead of concentrating on the general concept:

ΔTf = kf m

where m is molality.