Homework Help: chem

Posted by jessie on Sunday, September 26, 2010 at 5:39pm.

What is the freezing point (°C) of a solution prepared by dissolving 11.3 g of Ca(NO3)2
(formula weight = 164 g/mol) in 115 g of water? The molal freezing point depression constant for
water is 1.86 °C/m.

whats wrong with my answer:

molal: 0.069 moles Ca(NO3)2/0.115kg= 0.6
0.6x1.86=1.116
but its wrong, the answer is supposed to be -3.34

chem - DrBob222, Sunday, September 26, 2010 at 6:00pm
You forgot that Ca(NO3)2 ionizes into three ions so the van't Hoff factor is 3.
delta T = i*Kf*m
So multiply your answer by 3 and you'll have it.

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