GC question - vapor pressure (related destroyer #18)

[sminj85]

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Guys..

I want someone to explain this clearly..

As far as I know, vapor pressure is decreased when a NONVOLATILE NONELECTROLYTE is dissolved in a liquid. Correct?


If you look at Destroyer GC #18, it is asking to find the lowest freezing point. (which is asking the lowest vapor pressure as well)

The answere is c) Na2SO4 but isn't it an ELECTROLYTE? Doesn't this disobey the definition above saying vapor pressure decreases if NONELECTROLYTE is dissolved?

I am wondering if we can apply the collagative theory as long as a solute is NONVOLATILE( no matter if it is Electolyte or Nonelectrolyte ..)


I am so confused!! Can someone exaplin this clearly? Thanks!😀

 

[doc toothache]

It would help if you wrote the entire question.

 

[sminj85]

It would help if you wrote the entire question.

I am not specifically asking about this question. but yes, I will write the entire questions.

#18. WHich solution has the lowst freezing point?

a) 0.2 M KCL
b) 1.2 M C6H12O6
c) 1.4 M Na2SO4
d) 2.4 M C2H5OH
e) 1.8 M NaNO3

Again, I know that b) and d) are nonelectrolytes and the rest are electrolytes. But I am wodering if vapor pressure is decreased when ELECTOLYTES are dissolved. (from above statement, Vapor Pressure lowers only if Nonvolatile Nonelectrolyte solutes are dissolved.)

 

[Badgerfaithful]

The vapor pressure of the liquid solvent is decreased when a solute is added to the solvent. Since this is a colligative property, it doesn't matter what the nature of the solute is.

It becomes important to consider electrolytes when determining if the solute will completely dissociate in the solution. The answer to the question is C because Na2SO4 will dissociate into ions and make the solution greater osmolarity than will the other solutes. Hope this helps.

 

[oreoeater82]

Freezing point depression is a colligative property 👍, that is, a property that depends on the number of particles, not their identities. The van't hoff factor takes into account how many particles are formed when a substance dissolves in water. One mole of Na2SO4 will produce 2 moles of Na ions and 1 mole of sulfate ions. This creates a solution that is essentially 4.2M, using the concentrations from above. All other solutions will have a lower concentration of solute particles.

KCL ~ 0.4M
Glucose ~ 1.2 M as it is nonelectrolytic
C2H5OH ~ 2.5 M for the same reason as glucose
NaNO3 ~ 3.6 M for it dissociates into Na and NO3

Good luck

 

[sminj85]

The vapor pressure of the liquid solvent is decreased when a solute is added to the solvent. Since this is a colligative property, it doesn't matter what the nature of the solute is.

It becomes important to consider electrolytes when determining if the solute will completely dissociate in the solution. The answer to the question is C because Na2SO4 will dissociate into ions and make the solution greater osmolarity than will the other solutes. Hope this helps.

Freezing point depression is a colligative property 👍, that is, a property that depends on the number of particles, not their identities. The van't hoff factor takes into account how many particles are formed when a substance dissolves in water. One mole of Na2SO4 will produce 2 moles of Na ions and 1 mole of sulfate ions. This creates a solution that is essentially 4.2M, using the concentrations from above. All other solutions will have a lower concentration of solute particles.

KCL ~ 0.4M
Glucose ~ 1.2 M as it is nonelectrolytic
C2H5OH ~ 2.5 M for the same reason as glucose
NaNO3 ~ 3.6 M for it dissociates into Na and NO3

Good luck

Thanks guys.. now I understand !🙂