Boiling Point Elevation

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G1SG2

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Okay, so we can calculate boiling point elevation using deltaT=kb*m*i, where kb is the constant, m is the molality, and i is the van't hoff factor. Now, suppose we are comparing NaCl with CaO. Theoretically, the van't Hoff factor for both NaCl and CaO will be 2, as both will dissociate into two ions. However, in reality, the CaO will dissociate less than NaCl, as unlike the ions Na+ and Cl-, the ions in CaO will have a 2+ charge (Ca2+, O2-). Thus, Ca2+ and O2- will attract each other more strongly and won't dissociate AS much, and will have a real van't Hoff factor less than 2.

I know on the MCAT we are always supposed to assume ideal conditions. However, what if I'm faced with a question that asks if the boiling point elevation/vapor pressure of a solution is dependent on the charge of the ions AND the molality? Should I ignore the charge factor, since we are supposed to assume ideal conditions? Or is that suggesting to me that the conditions are non-ideal?

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Okay, so we can calculate boiling point elevation using deltaT=kb*m*i, where kb is the constant, m is the molality, and i is the van't hoff factor. Now, suppose we are comparing NaCl with CaO. Theoretically, the van't Hoff factor for both NaCl and CaO will be 2, as both will dissociate into two ions. However, in reality, the CaO will dissociate less than NaCl, as unlike the ions Na+ and Cl-, the ions in CaO will have a 2+ charge (Ca2+, O2-). Thus, Ca2+ and O2- will attract each other more strongly and won't dissociate AS much, and will have a real van't Hoff factor less than 2.

I know on the MCAT we are always supposed to assume ideal conditions. However, what if I'm faced with a question that asks if the boiling point elevation/vapor pressure of a solution is dependent on the charge of the ions AND the molality? Should I ignore the charge factor, since we are supposed to assume ideal conditions? Or is that suggesting to me that the conditions are non-ideal?

Unless otherwise stated conditions are non-ideal, I would assume conditions are ideal and go with 2 as the van't hoff factor for both NaCl and CaO.
 
Unless otherwise stated conditions are non-ideal, I would assume conditions are ideal and go with 2 as the van't hoff factor for both NaCl and CaO.

Even though it mentions the magnitude of charge, I should still assume ideal conditions and go with just concentration of solute?
 
Even though it mentions the magnitude of charge, I should still assume ideal conditions and go with just concentration of solute?

Yeah, I think so unless the question is specifically asking how having a higher charge magnitude effects the van't hoff factor of CaO. Remember estimation is, almost always, as good as an answer on the MCAT.:diebanana: Banana = MCAT.
 
Yeah, I think so unless the question is specifically asking how having a higher charge magnitude effects the van't hoff factor of CaO. Remember estimation is, almost always, as good as an answer on the MCAT.:diebanana: Banana = MCAT.
:laugh:
 
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