Colligative properties

[BestDoctorEver]

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According to TBR (if I remember correctly), colligative properties are: BP elevation, Freezing depression, Osmotic pressure and conductivity. I just had a question on my first TPR full length that says:


Which of the following are examples of colligative properties?

• vapor pressure
• electrical conductivity
• osmotic pressure

The answer was vapor pressure and osmotic pressure...Is vapor pressure a colligative property?

 

[MedPR]

According to TBR (if I remember correctly), colligative properties are: BP elevation, Freezing depression, Osmotic pressure and conductivity. I just had a question on my first TPR full length that says:

Which of the following are examples of colligative properties?

• vapor pressure
• electrical conductivity
• osmotic pressure

A.
I and II only

B.
I and III only

Correct Answer

C.
II and III only

D.
I, II, and III



Is vapor pressure a colligative property?

Why isn't it D? Osmotic pressure is a colligative property as well. If boiling point is a colligative property, how can vapor pressure not be a colligative property?

 

[pfaction]

I'm going to hemorrhage information as I am literally doing TPR's colligative property section.

1) Boiling point (related to vapor pressure).
With increased molality, boiling point rises. the heat needed to get to 760 torr is increased. evaporation still occurs at 760 torr, but more heat (analogous to temperature) is needed. why is this? solvent acts to INCREASE ADHERENCE of molecules to each other, more energy needed to break off. thus, boiling point rises, condensation point rises.

2) vapor pressure depression (related to boiling point, after all, the reason we measure vapor pressure is for boiling):
vapor pressure is determined by: temperature, intermol bonds, and solute concentration. the weaker intermol bonds, the less temp needed to boil and thus the higher the vapor pressure is. water vs methane. methane has vdw, water has h-bonds. vpressure of methane is more. When you introduce solvents, just like bp elevation, it "anchors" the molecules->increased stability due to intermol bonds->DEPR vapor pressure.

3) decr freezing (analogous to melting point)
similarly, solutes will mess up intermol bonding with to freeze, so it takes "more" energy (lower temps) for things to freeze/solidify. vice versa: when you're melting something, the thing with more solutes melts at a lower temperature.


I was thinking incr molality with electrolytes would increase conductivity. I would have chosen D.

 

[BestDoctorEver]

Why isn't it D? Osmotic pressure is a colligative property as well. If boiling point is a colligative property, how can vapor pressure not be a colligative property?

The correct answer was B...Conductivity is not included. I see why vapor pressure is a colligative property.

 

[pfaction]

I don't understand how that's not a colligative property. It would make sense that 1M or 1m NaCl conducts better than 2M or 2m NaCl...more ions to let the electrons flow?!

 

[MedPR]

According to TBR (if I remember correctly), colligative properties are: BP elevation, Freezing depression, Osmotic pressure and conductivity. I just had a question on my first TPR full length that says:


Which of the following are examples of colligative properties?

• vapor pressure
• electrical conductivity
• osmotic pressure

The answer was vapor pressure and osmotic pressure...Is vapor pressure a colligative property?

The correct answer was B...Conductivity is not included. I see why vapor pressure is a colligative property.

Then why are you asking if it's a colligative?

 

[MedPR]

.

 

[pfaction]

It should, since deltaTmelt (melting is the same point as freezing remember) = kf(i)m. 1 molal of glucose will depress less than 1 molal NaCl, due to (1) vs (2). So when going reverse, NaCl should melt first because it's freezing point would be, say, -2 vs -1 for glucose in water.

 

[MedPR]

It should, since deltaTmelt (melting is the same point as freezing remember) = kf(i)m. 1 molal of glucose will depress less than 1 molal NaCl, due to (1) vs (2). So when going reverse, NaCl should melt first because it's freezing point would be, say, -2 vs -1 for glucose in water.

Yea that's why I took out my post. If you look at it from that point of view the melting point is depressed, but the term FP depression is comparing the pure substance to an impure substance, not two completely different substances as you are doing.

 

[BestDoctorEver]

Then why are you asking if it's a colligative?

I meant I got if after you explained it..My question is why conductivity is not included in the answer? I remember BR said conductivity is colligative property.

 

[MedPR]

I meant I got if after you explained it..My question is why conductivity is not included in the answer? I remember BR said conductivity is colligative property.

Oh, yea that's my question too 🙁

 

[sweet d]

A colligative property depends on the number of solute particles but cannot depend on the type of solute.

Conductivity is related to the number of dissolved particles, but they must be ions. Increasing the concentration of NaCl would affect conductivity but increasing the number of glucose molecules would not. For this reason I don't think it can be considered a colligative property.