[photogrl]

Which of the following solutions would have the lowest freezing point?

1M MgCl₂

1M NaCl

1M C₆H₁₂O₆

1M Ca₃N₂


I believe the correct answer is 1M C₆H₁₂O₆, because doesn't the freezing point of a solution have to do with the number of molecules of said solution? If so, then the glucose would be the right answer.

Or am I absolutely dead wrong? Help? Please?

Thank you.

[Yggdrasil]

If I remember correctly, freezing point depression is bases on molality, not molarity.  Take a look at the definition of the two, and see if you can come up with an answer based on that.  Feel free to post your guess, and we'll check whether it's right.

[photogrl]

OK.....my guess is now 1M MgCl₂...

*wrings hands*

Is that right? I'm not completely understanding the freezing point concept.  

[Hgrigsby]

The only way I could solve that is if I had more info, the #kg in the solution (to solve for Molality) not to mention the mols of the solute.  If you have that, then I can happily give you an equation. But that is as much as I can help.

[photogrl]

Sorry, but the question is written verbatim. I don't have any more info to give you.  

[xiankai]

from my point of view i'll think it in terms of covalent/ionic bonds in general. since covalent bonded substances usually have low freezing points compared to the ionic bonded substances, i'll pick glucose.

[Yggdrasil]

The answer has nothing to do with the properties of the solute.  Rather, it just depends on the molality of your solition.  Since molality is mass of solute/mass of solvent, it stands to reason that the solute with the highest greatest mass will have the greatest molality (since all the solutions are at 1.0M, meaning they all have equal numbers of solute molecules per unit volume).  Comparing the molecular weights of all the solutes, the sugar (C₆H₁₂O₆) has the highest molecular weight, and therefore the sugar solution has the highest molality.  Therefore, the sugar solution has the lowest freezing point.

So your original guess (and xiankai's) were correct, but for the wrong reasons

[Borek]

The answer has something to do with the properties of the solute. Think about effects of dissociation - in which solution molality of ALL dissolved ions/particles will be the highest?

Glucose solution is NOT the correct answer.

[photogrl]

In as close as you can get to layman's terms....please tell me why the glucose is not the right answer?

Thank you.

[Borek]

Freezing point depression is proportional to number of entities (be it ions or particles) swimming freely in the solution. Think in which solution number of such entities (for the same molal concentration) will be the highest.

[AWK]

1M Ca3N2

This part of problem is very odd. Ca3N2 is insoluble in almost all solvents, in water it reacts to form almost insoluble Ca(OH)2 and ammonia.

[xiankai]

in what context does this question happen to be in?

[Borek]

This part of problem is very odd. Ca3N2 is insoluble in almost all solvents, in water it reacts to form almost insoluble Ca(OH)2 and ammonia.

Yeah, I didn't touch that as there were (are?) problems with the much more basic stuff.

Seeing such things I always wonder if it is a question that is meant be tricky or is it stupid teacher.

Taking things literally this solution should have the lowest freezing point - 3NH₃ plus some dissolved Ca(OH)2 gives more then MgCl₂  

[Yggdrasil]

The answer has something to do with the properties of the solute. Think about effects of dissociation - in which solution molality of ALL dissolved ions/particles will be the highest?

Glucose solution is NOT the correct answer.

Ah, reversed the definition of molality in my head.  Thanks for the correction.

[photogrl]

Thanks for all the help...I found out the answer to that question was the Ca₃N₂. I had a lot of people tell me it was MgCl₃, and even explain WHY it was MGCl₂...and I got it wrong anyway. Oh well.