ΔTF = KF × b × i freezing point saltwater

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Gliese123
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Homework Statement



I think this is the right formula: ΔTF = KF × b × i
Freezing-point depression.
I have the amount of salt added to the water but how am I going to do to calculate the difference in temperature? Or with other words the freezing point of water saturated with salt (NaCl)? Or am I totally gone here? >_<
And KF is 1.853 K·kg/mol.

But b and i, what should they include? Be nice, I'm an amateur :p
 
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Borek said:
I guess you copied it from wikipedia (http://en.wikipedia.org/wiki/Freezing-point_depression#Calculation) - all symbols are explained there. Which one you have problems with?

Note that this equation works nicely for diluted solutions, but fails for concentrated ones. Saturated NaCl IS concentrated.

I don't really know the values to use. What should I use instead if concentrated ones doesn't work with that formula?
 
Gliese123 said:
I don't really know the values to use.

They should be calculated. Although in this paritcular case...

What should I use instead if concentrated ones doesn't work with that formula?

...freezing point tables (determined experimentally) sound like a better solution.
 
Okay. Now let's say I want to know when salt water freeze when I've 1.5 g NaCl in 0.25 L water.
ΔTF=1.853 * 0.025667 (mol NaCl) * 2 (ions Na+ & Cl-)
= gives me 0.0951219. Which is totally wrong I think. It's not that little in temperature difference if I had salt into water.. Please explain...
-___- ' I would be very glad :)
 
Borek said:
Number of moles of NaCl is calculated OK, but moles are not the concentration.

http://www.chembuddy.com/?left=concentration&right=molality

Now I think I got it:
E.g. Kf=1.867 * 0.25/ 0.01388 * 2 * 0.1368= ~9.13 °C => Freezing point is reduced to -9.13 °C
I think it should be right? If b is (2g/58.44)/0.25= 0.1368
Which is 0.25 L water & 2 grams of salt. xD
 
Last edited:
Gliese123 said:
Now I think I got it:
E.g. Kf=1.867 * 0.25/ 0.01388 * 2 * 0.1368= ~9.13 °C => Freezing point is reduced to -9.13 °C
I think it should be right? If b is (2g/58.44)/0.25= 0.1368
Which is 0.25 L water & 2 grams of salt. xD

I have no idea what is what of what in what you wrote - but it is wrong.

This is a simple plug and chug. The only thing you have to do is to correctly calculate concentration. You don't have to rearrange the equation to calculate Kf, as Kf is given, besides, Kf is not what you are looking for - ΔTF is.