Molality

[ADH]

Hello,

 

Suppose we dissolved 1 mol of NaCl in 1 kg of water; would the molality of the resulting solution be 1 m or 2 m?

According to Wiki and EK, it would be a 1 m solution, since they calculate molality based on the number of moles of the original substance added, before dissociation occurs. So their equation for boiling point elevation is

deltaT = kim (where k is a constant, and i is the van Hoff factor, e.g. i = 2 for our example above).

BUT, on the AAMC list of Physical Sciences topics, it lists the equation for boiling point elevation as deltaT = km. Since there is no van Hoff factor in this equation, it makes me think that AAMC, like Kaplan, expects the calculation of molality to include dissociation, so the above example would have a molality of 2 m.

 

If I were asked the above question on MCAT, would it be safest to choose

2 m?

 

Thanks!

[Bretzky]

The molality is 1m still. There is only 1 mol of NaCl, and 1 kg of Water. The rest is insignificant for molality.

I'm not sure if I completely understand, but I think your reasoning is a little off. The aamc formula deltaT = km is the general formula. It works for let's say, glucose. The other formula with the vant hoff factor or whatever takes into consideration ionic solutions, because of dissociation that tales place, and the fact that both species can play a role in the freezing/ boiling point depression. I think the vant hoff factor's definition is something along the lines of "the number of particles a substance dissociates into once in a solution."

 

But it really doesn't have anything to do with molality per se. So definitely put 1m.

[ameltingbanana]

The molality is 1m still. There is only 1 mol of NaCl, and 1 kg of Water. The rest is insignificant for molality.

I'm not sure if I completely understand, but I think your reasoning is a little off. The aamc formula deltaT = km is the general formula. It works for let's say, glucose. The other formula with the vant hoff factor or whatever takes into consideration ionic solutions, because of dissociation that tales place, and the fact that both species can play a role in the freezing/ boiling point depression. I think the vant hoff factor's definition is something along the lines of "the number of particles a substance dissociates into once in a solution."

 

But it really doesn't have anything to do with molality per se. So definitely put 1m.

 

What the above poster posted about his glucose example is right, but the van hoff't factor does come into play here, as NaCl can and will dissociate into 2 different ions upon addition to solvent. It's been a while since I've done stuff like this, but I believe that 1 mol NaCl added to 1kg of water results in a 2 molal solution (as molality doesn't take into account the identity of the ions, just the number of the ions present). Hope that helps - if I'm wrong, someone please correct me.

[thehumanmacbook]

molality should be one. I'm not sure why you're bringing in the boiling point elevation or van hoff factor into this...it's simply one. Molality is the moles of solute divided by solvent. You don't have "double" the moles that you think you have because the ion dissociates....you only have 1 mol NaCl.

 

It's true that you have 1 mol of Na and 1 mol of Cl after dissociation in the solvent....but you don't add them together and say you have 2 mol NaCl. That makes absolutely no sense.

[ameltingbanana]

Ah, yes, both of the other posters are right. I got confused and started thinking about osmoles, essentially. 1 molal NaCl would be the right answer.

[Let'sGo1990]

Ah, yes, both of the other posters are right. I got confused and started thinking about osmoles, essentially. 1 molal NaCl would be the right answer.

 

Heh. I made the same mistake before I read the other posts.

[ADH]

molality should be one. I'm not sure why you're bringing in the boiling point elevation or van hoff factor into this...it's simply one. Molality is the moles of solute divided by solvent. You don't have "double" the moles that you think you have because the ion dissociates....you only have 1 mol NaCl.

 

It's true that you have 1 mol of Na and 1 mol of Cl after dissociation in the solvent....but you don't add them together and say you have 2 mol NaCl. That makes absolutely no sense.

 

I mentioned the example of the equation for boiling point elevation because the form of the equation given my AAMC could only work if the molality calculated in the given example was 2 m (or if the factor of 2 was included with the constant k). But I agree with your conclusion that the answer is 1m. Thanks for the help!