# Homework Help: Question on the Boiling point of NaCl solution.

1. Jan 22, 2010

### Choronzon

The experimental boiling point of the NaCl solution is lower than that calculated, assuming that NaCl is completely dissociated in solution. Why is this the case?

I'm thinking that this is because some of the Na+ and Cl- ions reassociate for a short time, thus causing the solution to contain somewhat less than two times the original concentration of NaCl. The phrase in the question "assuming that NaCl is completely dissociated in solution." is making me doubt this answer. Does anyone have any suggestions?

2. Jan 22, 2010

### Staff: Mentor

We usually discuss boiling point change in terms of concentration of dissolved substance, but it is lowered solvent concentration (molar fraction) that really matters... When you dissolve salt its molar faction goes down - first of all because total numer fo moles goes up, but that's not the only reason.

Last edited by a moderator: Aug 13, 2013
3. Jan 22, 2010

### Choronzon

How would that make a difference between the calculated and experimental values? I understand that the boiling point changes--but why the discrepancy between the experimental results and the calculations?

4. Jan 22, 2010

### Staff: Mentor

Calculated value doesn't take everything happening in the solution into account.

Last edited by a moderator: Aug 13, 2013
5. Jan 22, 2010

### Gokul43201

Staff Emeritus
I think you have it right, but it comes down to correctly interpreting the intent of an ambiguously worded question. I think the intent of the question is to tell you that the calculation assumes complete dissociation, whereas in reality, as you guessed, there is some undissociated NaCl, and this makes the actual value of the number of particles in solution lower than the calculated value.

In fact, a more accurate calculation will be based on the measured dissociation constant of NaCl, rather than assuming this is 100%.

6. Jan 22, 2010

### Staff: Mentor

Obviously it wasn't just my ears that froze today at -18 deg C, but also parts of my brain :grumpy: All the time I was thinking about temperature being higher, not lower...

7. Jan 24, 2010

### Choronzon

Thanks for the help guys—I was pretty sure about my answer, but the question throws me off a bit.