Lowest Boiling Point

[Jab1113]

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Which solution has the lowest boiling point? (GChem)

A. 0.5M FeCl3
B. 1.0M MgCl2
C. 0.5M CaCl2
D. 1.0M NaNO3
E. 1.5M NaCl


The answer is (D) but for some reason I still dont understand it. Might sound like a stupid question but I need clarification!

 

[Jab1113]

Which solution has the lowest boiling point? (GChem)

A. 0.5M FeCl3
B. 1.0M MgCl2
C. 0.5M CaCl2
D. 1.0M NaNO3
E. 1.5M NaCl


The answer is (D) but for some reason I still dont understand it. Might sound like a stupid question but I need clarification!

Ok from my understanding......we have to account that boiling point is a colligative property and requires us to account for the moles of solute particles not just number of moles

A) 0.5M * 4 (3Cl + 1Fe) = 2.0 M
B) 1.0M * 3 (2Cl + 1Mg) = 3.0 M
C) 0.5M * 3 (2Cl + 1Ca) = 1.5 M
D) 1.0M * 2 (1Na + 1NO3) = 2.0 M
E) 1.5M * 2 (1Na + 1Cl) = 3.0 M

The answer would be the one with the lowest Molarity as well as the lowest number of solute particles added which is (D).

Is there an easier way to approach this? wtf.

 

[rav4182]

well doesnt Fe+3 form a complex with water Fe(OH)3? in that case A could be cancelled out. D is the one with the least number of particles so it would be D then.

 

[woox]

Which solution has the lowest boiling point? (GChem)

A. 0.5M FeCl3
B. 1.0M MgCl2
C. 0.5M CaCl2
D. 1.0M NaNO3
E. 1.5M NaCl


The answer is (D) but for some reason I still dont understand it. Might sound like a stupid question but I need clarification!

I was able to answer it but from my Ochem knowledge.

I recall that the more "branched out" the molecule is the higher the boiling and melting point.

If you just looked at how much stuff is attached to each, you can see NaCl is a lot less branched than FeCl3...another way to think of it is FeCl3 has alot more surface area than NaCl...so it would take longer for it to break down hence FeCl3 has a higher boiling point than NaCl

Apply this logic to all the answer choices and NaCl is the "simplest"...I think the M is there to fool you

 

[yappy]

I dont think this logic is correct to understand this question. What you described is true for boiling organic solutions in the liquid phase. Such as hexane. And I believe that the more branched molecule will have a lower bp b/c less intermolecule forces (vanderr) are present.

This question deals with colligitive properties and the vonhoff factor.

When a solute is added to a pure solvent the boiling point is higher. Therefore, the solution with the least amount of solute (this is why they tell you the amount of moles) will have the lowest boiling point.

Because these molecules are salts that ionize in water you cannot simply look at the number of moles to determine the amount of solute. You must multiply the moles by the vanhoff factor. The vanhoff factor is simply the amount of ions that each molecule will dissassociate into after it is put into a pure liquid. ex: Nacl ---> Na+(aq) + Cl-(aq) = factor of 2.

I was able to answer it but from my Ochem knowledge.

I recall that the more "branched out" the molecule is the higher the boiling and melting point.

If you just looked at how much stuff is attached to each, you can see NaCl is a lot less branched than FeCl3...another way to think of it is FeCl3 has alot more surface area than NaCl...so it would take longer for it to break down hence FeCl3 has a higher boiling point than NaCl

Apply this logic to all the answer choices and NaCl is the "simplest"...I think the M is there to fool you

 

[ForeverKhaled]

Ok from my understanding......we have to account that boiling point is a colligative property and requires us to account for the moles of solute particles not just number of moles

A) 0.5M * 4 (3Cl + 1Fe) = 2.0 M
B) 1.0M * 3 (2Cl + 1Mg) = 3.0 M
C) 0.5M * 3 (2Cl + 1Ca) = 1.5 M
D) 1.0M * 2 (1Na + 1NO3) = 2.0 M
E) 1.5M * 2 (1Na + 1Cl) = 3.0 M

The answer would be the one with the lowest Molarity as well as the lowest number of solute particles added which is (D).

Is there an easier way to approach this? wtf.

Yappy's explanation is on point. This question tests the ability to recognize the colligative properties... Increasing the amount of solutes in a solvent increases the boiling point. The one that amounts to the least boiling point elevation is your answer. Your math is correct but the answer is wrong it should be (C). Unless its some very weird exception, which I highly doubt, I want to jump out on one limb and say that which ever source you got the answer from made a mistake.

If you find out anything else please let me know.

Thank you👍

 

[bharat008]

Answer should be "C". It has the lowest molality of 1.5m.
T(b) = im*boiling constant = so ignore the boiling constant since it would be same in all so the one that gives us the lowest "im" would have the lowest BP.

Does anyone agree with this??

 

[addis4ever]

answer should be "c". It has the lowest molality of 1.5m.
T(b) = im*boiling constant = so ignore the boiling constant since it would be same in all so the one that gives us the lowest "im" would have the lowest bp.

Does anyone agree with this??

+1

 

[ForeverKhaled]

yup exactly what i thought