[aaronhkg]

Question:

0.2g of a substance lowers the freezing point of 70g of its aqueous solution by 0.0673oC. Calculate the R.M.M. of the substance given the cryoscopic constant for water is 1.8K mol-1 1000g-1.

My answer:

Molality => -0.0673/-1.80 = 0.0362m

No. of mole => 0.1kg H2O X 0.0362m = 0.00362 mol of ??

Molar mass = 0.2/0.00362 mol = 55.2486g/ mol of ??

what did it do wrong? what's the use of the given figure 70g??

[rabolisk]

Your aqueous solution weighs 70 grams. This will be necessary in going from molality to moles.

[aaronhkg]

Thanks for your reply.

did you mean 70g/18 H2O = 3.89 mol? then what should I do?

[Borek]

0.1kg H2O

Where did you got 0.1 kg from?

[aaronhkg]

Oh!!! is it suppose to be 0.07kg X 0.0362m?

Now it make sense

Thx