Colligative Properties

[MedPR]

I don't understand them at all.


Colligative properties = properties that depend on the # of solute particles, but not on the type.

So BP and FP are colligative properties because dissolving some solute in a solvent will change them. But density is not a colligative property. Why not? The density of pure water in a beaker will be less than the density after I dissolve NaCl in it. The NaCl is what causes the BP to go up and the FP to go down just like it would cause the density to go up.

???

---

Members don't see this ad.

 

[chiddler]

after googling, I found this:

Intensive properties (such as density and concentration) are characteristic properties of the substance; they do not depend on the size of the sample being studied. This section introduces a third category that is a subset of the intensive properties of a system. This third category, known as colligative properties, can only be applied to solutions.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/colligative.php

 

[milski]

That's because of the second part of the definition. While dissolving NaCl will change the density, that change depends on what you are dissolving. If you dissolved something heavier, the density will change differently but the change in BP/MP will be the same.

 

[MT Headed]

The density will not be affected by the number of resultant particles in solution. If the NaCl didn't break up into Na+ ions and Cl- ions, then the density would not be affected, but the MP and BP would be affected because the number of particles per liter would now be different.

 

[MedPR]

The density will not be affected by the number of resultant particles in solution. If the NaCl didn't break up into Na+ ions and Cl- ions, then the density would not be affected, but the MP and BP would be affected because the number of particles per liter would now be different.

Huh?

If the NaCl didn't dissociate, the MP and BP wouldn't be affected either. If you put a bunch of solid on the bottom of a beaker filled with water, the BP is the same as water in another beaker without solid at the bottom.


The person above you said that if BP/MP are affected by the number of particles only, and not by the molar mass of the particles. The density, however, is affected by the molar mass of the particles. If that's the definition of colligative property, then I understand.

 

[MT Headed]

The issue is that if you put 100 particles of a substance into solution and it dissolves into 100 particles, then you would have 100 particles and your colligative property would be affected by a certain amount.

But if you threw 100 particles of NaCl into water, it would dissociage into 100 particles of Na+ and 100 particles of Cl- for a grand total of 200 particles and the colligative property would be affected by twice as much. The boiling poinnt would go up by twice as much. In my previous post, I meant to say that if the NaCl dissolved and broke up into individual NaCl's but didn't ionize.

99% of chemistry seems to be affected by 'moles of molecules'. The thing about colligative properties is that it is always measured in 'moles of particles' so you need to throw in an extra conversion factor, something like (2 mol particles/1 mol NaCl).

 

[MedPR]

The issue is that if you put 100 particles of a substance into solution and it dissolves into 100 particles, then you would have 100 particles and your colligative property would be affected by a certain amount.

But if you threw 100 particles of NaCl into water, it would dissociage into 100 particles of Na+ and 100 particles of Cl- for a grand total of 200 particles and the colligative property would be affected by twice as much. The boiling poinnt would go up by twice as much. In my previous post, I meant to say that if the NaCl dissolved and broke up into individual NaCl's but didn't ionize.

99% of chemistry seems to be affected by 'moles of molecules'. The thing about colligative properties is that it is always measured in 'moles of particles' so you need to throw in an extra conversion factor, something like (2 mol particles/1 mol NaCl).

Yea, I understand that part. I guess I wasn't clear in the OP. I'm not confused about why/how solutes affect BP/FP/Conductivity. I am confused by the term "colligative property" and why Density is not a colligative property.

I guess my question is something like this. Colligative properties (BP, FP, conductivity) are affected by the number of solute particles (2 for dissociated NaCl, etc). Density is also affected by the number of solute particles, so why isn't density a colligative property?

But someone already explained that it's not colligative because the mass of the particles being dissolved matters in density, but not in the colligative properties.

 

[milski]

Yea, I understand that part. I guess I wasn't clear in the OP. I'm not confused about why/how solutes affect BP/FP/Conductivity. I am confused by the term "colligative property" and why Density is not a colligative property.

I guess my question is something like this. Colligative properties (BP, FP, conductivity) are affected by the number of solute particles (2 for dissociated NaCl, etc). Density is also affected by the number of solute particles, so why isn't density a colligative property?

Because colligative properties are affected only by the number of particles, not the type. From MT Headed's example: It matters for BP/FP if the particles dissociate or not but it does not matter if you are dissociating NaCl or KCl. On the other hand for something like density, it does not matter if it will dissociate but it matters if you're adding NaCl or KCl.

The density will not be affected by the number of resultant particles in solution. If the NaCl didn't break up into Na+ ions and Cl- ions, then the density would not be affected, but the MP and BP would be affected because the number of particles per liter would now be different.

I think he actually meant that the density will be the same between the case where NaCl dissociates or does not, not between pure water and solution of NaCl.

 

[MedPR]

Because colligative properties are affected only by the number of particles, not the type. From MT Headed's example: It matters for BP/FP if the particles dissociate or not but it does not matter if you are dissociating NaCl or KCl. On the other hand for something like density, it does not matter if it will dissociate but it matters if you're adding NaCl or KCl.



I think he actually meant that the density will be the same between the case where NaCl dissociates or does not, not between pure water and solution of NaCl.

It seems like you guys are both saying that. If NaCl or KCl didn't dissociate (for whatever reason), how would they impact the density? If they just precipitate to the bottom of the beaker, isn't the density of the liquid exactly the same as before you added the insoluble NaCl or KCl?

I understand the second part, the part about the molar mass since, and how it affects density but not FP or BP.

 

[milski]

It seems like you guys are both saying that. If NaCl or KCl didn't dissociate (for whatever reason), how would they impact the density? If they just precipitate to the bottom of the beaker, isn't the density of the liquid exactly the same as before you added the insoluble NaCl or KCl?

I understand the second part, the part about the molar mass since, and how it affects density but not FP or BP.

We are talking about the case where the 'magic' insoluble NaCl has not precipitated yet to the bottom but is suspended in the fluid. Yes, it's somewhat made up example, but the idea is that for some properties only the number of particles matters while for others it is the number and the type of particles that matter.

The number of particles is always going to matter, so another way to think about this is if the type matters or not. If some property is the same, regardless of the type of particles that you're adding, it is colligative.

 

[MedPR]

We are talking about the case where the 'magic' insoluble NaCl has not precipitated yet to the bottom but is suspended in the fluid. Yes, it's somewhat made up example, but the idea is that for some properties only the number of particles matters while for others it is the number and the type of particles that matter.

The number of particles is always going to matter, so another way to think about this is if the type matters or not. If some property is the same, regardless of the type of particles that you're adding, it is colligative.

Oh I see. So the magical NaCl could be glucose and the magical KCl could be fructose. Both dissolve as only 1 particle in water, but have different molar masses so they will have the same affect on BP and FP, but different affect on density?

 

[milski]

Oh I see. So the magical NaCl could be glucose and the magical KCl could be fructose. Both dissolve as only 1 particle in water, but have different molar masses so they will have the same affect on BP and FP, but different affect on density?

Yes, that's it.

And if you managed to make something with the same molecular weight as say glucose but which dissociates to two particles, it will have the same effect on density as glucose but different effect on BP/FP.

 

[MedPR]

Yes, that's it.

And if you managed to make something with the same molecular weight as say glucose but which dissociates to two particles, it will have the same effect on density as glucose but different effect on BP/FP.

Gotcha, thank you!