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I am facing problem with the following question:

Predict which solution will have the highest boiling point in increasing order?

  1. $1~\%$ solution of $\ce{C6H12O6}$

  2. $1~\%$ solution of $\ce{NaCl}$

  3. $1~\%$ solution of $\ce{ZnSO4}$

  4. $1~\%$ solution of $\ce{(NH2)2CO}$

I know that more is the van ’t Hoff factor more will be the elevation in boiling point and hence the solution will have more boiling point. Hence 1. should have the lowest boiling point followed by 4. But I am not sure about options 2. and 3.

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  • $\begingroup$ What do colligative properties depend on? $\endgroup$
    – getafix
    Oct 6, 2016 at 23:18
  • $\begingroup$ @getafix It depends only on the number of solute particles and not on the nature of solute particles $\endgroup$
    – Pink
    Oct 6, 2016 at 23:26
  • $\begingroup$ You were right that you need to look at the Van 't Hoff factors. Think about which of 2. or 3. is more soluble in water. Also, between 1. and 4., think about whether any association of molecules can occur via H-bonding and how would that affect the Van t' Hoff factor. $\endgroup$
    – Vlad
    Oct 6, 2016 at 23:45
  • $\begingroup$ I don't think the solubility is the most important factor here. The most important question to ask is what the question means by 1%? I assume this is a mass-to-mass percentage... From there you can apply what you know about colligative properties. $\endgroup$
    – Zhe
    Oct 7, 2016 at 1:13
  • $\begingroup$ @Zhe yes it I'd mass to mass percentage.but how it could affect if it were some other percentage. $\endgroup$
    – Pink
    Oct 7, 2016 at 1:17

1 Answer 1

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The first aspect to consider is the van't Hoff factor. Thus #1 is first and #4 is last. Since both #2 and #3 have the same factor of 2, we must consider other properties.

A major part of BP is how many molecules of solvent are exposed on the surface layer. Therefore, the ions with the greater diameter should take up more surface area, decreasing the number of solvent molecules available to 'escape'.. decreasing the vapor pressure and increasing the BP elevation. My guess is that the polyatomic ion is quite large in comparison and the deciding factor.

from lowest BP to highest #1,#2, #3, #4

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  • $\begingroup$ More is the can't Hoff factor more should be the elevation in boiling point so in on way 4 can have the highest boiling point so I think you are wrong.in my view correct increasing order should be 3>2>1>4 $\endgroup$
    – Pink
    Oct 7, 2016 at 23:09
  • $\begingroup$ Let's not throw around wild accusations, shall we? $\endgroup$
    – Martin - マーチン ♦
    Oct 8, 2016 at 14:13

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