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### Topic: Molecular weight determination  (Read 1729 times)

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#### shafaifer

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##### Molecular weight determination
« on: January 03, 2015, 08:31:17 AM »
Solution of 100 g of a substance in 750 grams of the solvent (CCl4) depresses the freezing point of the solvent by 10.5 K. The freezing point constant for the solvent is 30 K/(mol kg-1). Determine the molecular weight of the unknown substance.

My thinking of this exercise is the following:

1. The particle concentration is determined: ΔT/Kf, this gives a molality. To turn it into a molarity I devided it by the density of the solvent. So now I have the molarity of the unknown compound, right?.

2. With both the density and the molar mass of solvent known I can calculate how many moles of CCl4 there are present and the volume and further the concentration of CCl4.

3. Now I should be able to calculate the volume of the unknown substance by c1V1=c2V2 <=> V1 = (c2V2)/c1.

4. Thus I can now calculate how many moles of the substance present and hereby,

5: Calculate molcecular weight by: M = m/n

I am not sure about if some of these steps are correct, but I hope some of you can tell this to me ,

sincerely,

MVS

#### unsu

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##### Re: Molecular weight determination
« Reply #1 on: January 03, 2015, 12:42:37 PM »
Hi,

You do not need to convert the molality to molarity. You also do not need to use the dilution equation.
The only equation you use is ΔT = Kf·m

Assume M = molar mass, then write an equation with one variable (M) and find M.