ΔTF = KF × b × i freezing point saltwater

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Discussion Overview

The discussion revolves around the calculation of freezing point depression in saltwater using the formula ΔTF = KF × b × i. Participants explore how to apply this formula, particularly in the context of saturated solutions and the specific case of NaCl in water.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents the formula for freezing point depression and seeks guidance on calculating the temperature difference when salt is added to water.
  • Another participant notes that the formula is suitable for diluted solutions but may not apply to concentrated solutions like saturated NaCl.
  • There is uncertainty about the appropriate values to use for the calculation, particularly for concentrated solutions.
  • A participant suggests using freezing point tables as a better alternative for concentrated solutions.
  • One participant calculates the number of moles of NaCl but expresses confusion about the resulting temperature difference, questioning the validity of their calculation.
  • Another participant points out that the number of moles does not equate to concentration and provides a link for clarification on molality.
  • One participant attempts a recalculation using a different approach but is met with skepticism from another, who claims the method is incorrect and emphasizes the need for accurate concentration calculations.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the freezing point depression formula for concentrated solutions, and there is no consensus on the correct approach to calculating the freezing point of saltwater. The discussion remains unresolved regarding the proper methodology and values to use.

Contextual Notes

Participants highlight limitations in their understanding of concentration versus moles, as well as the challenges of applying the freezing point depression formula to concentrated solutions. There are unresolved mathematical steps and assumptions regarding the calculations presented.

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.
 

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