Melting Points and Boiling Points, is their answer wrong?

[OrthoRehab33]

I just took a kaplan PS section test and am reviewing a question that I can't wrap my head around. It says Solution A= 1 molality of NaCl and Solution B= 1 molality of glucose and asks which solutions have a higher boiling point and melting point.

I understand that NaCl has the higher boiling point since it dissolves into 2 particles however, they claim that glucose has a higher melting point?? It was my impression that ionic forces have very high melting points compared to covalent bonds. They simply say that the higher number of particles = higher boiling point and lower melting point.
Is this true? If it was freezing point I'd understand based on the same principle but i think this is wrong considering melting point.

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[benjaminl1nus]

melting point and freezing point are the same thing. is this your question?

 

[OrthoRehab33]

I forgot about that, but no, my question is: which has a higher melting point NaCl or glucose? Kaplan says glucose. I dont think thats correct.

 

[ToldYouSo]

They are asking which SOLUTION has the higher melting point, not solute. NaCl has a much higher melting point than glucose, but a glucose solution has a higher melting point than a salt solution.

Boiling point elevation is matched by a freezing point depression. The higher boiling point of the NaCl solution corresponds to the lower freezing/melting point.

This is why you add salt to an ice/water mixture when making ice cream, the resulting slurry gets a lot colder and remains liquid because the freezing/melting point is lowered.

 

[jmbusclas]

I just took a kaplan PS section test and am reviewing a question that I can't wrap my head around. It says Solution A= 1 molality of NaCl and Solution B= 1 molality of glucose and asks which solutions have a higher boiling point and melting point.

I understand that NaCl has the higher boiling point since it dissolves into 2 particles however, they claim that glucose has a higher melting point?? It was my impression that ionic forces have very high melting points compared to covalent bonds. They simply say that the higher number of particles = higher boiling point and lower melting point.
Is this true? If it was freezing point I'd understand based on the same principle but i think this is wrong considering melting point.

This should be a simple colligative properties question.Colligative properties are solely dependent on the number of particles in solution. The type of the particle is completely irrelevant. Remember, colligative properties include boiling point elevation, freezing point depression, vapor pressure depression, and osmotic pressure.

The van't Hoff factor for NaCl is 2 while for glucose it is 1. Using our equations for boiling point elevation and freezing point depression (change in T = k*i*m) we will get the following [k(boiling) for water = 0.5 degrees and k(freezing) for water = 2 degrees]:

for the NaCl soln:
BP elevation = 0.5*2*1 = 1 degree
FP depression = 2*2*1 = 4 degrees

for the glucose soln:
BP elevation = 0.5*1*1 = 0.5 degrees
FP depression = 2*1*1 = 2 degrees

So, you can see that the freezing point of the NaCl soln is depressed more than the freezing point of the glucose soln. Since melting point and freezing point are equivalent, this equates to MP glucose soln > MP NaCl soln.

 

[OrthoRehab33]

Yeah, I get it now. Thanks for your help!