# The Colligative Properties

The 4 Colligative Properties help to explain what happens to a pure solvent as solute is added to it.

## Properties of Solutions

Concept: The Colligative Properties.

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##### Video Transcript

Welcome back, guys! In this new video, we’re going to take a look at the properties of solutions. Now we’re going to say that the four colligative properties help to explain what happens to a pure solvent as we add solute to it.
Let’s say that this right here on our left represents pure water, a pure solvent. And let’s say I decide to drop some NaCl in there, some regular table salt. So now we know that table salt will break up into ions. It will break up into Na+ and Cl-. They’re going to be much smaller relatively speaking, so they’re going to form – they’re going to be the solute within our solvent. So here we’re going to have solute plus solvent so we’re going to have here a solution.
Basically, the four colligative properties explains what happens to boiling point, freezing point, osmotic pressure and vapor pressure as we add solute. Basically, they explain what changes are going to happen when my pure solvent becomes a solution.

The four colligative properties of Boiling Point, Freezing Point, Vapor Pressure and Osmotic Pressure will either increase or decrease with the addition of solute to a pure solvent.

Example: Explain what happens to each of the following properties as solute is added to a pure solvent.

a.  Boiling Point              b.  Freezing Point

c. Osmotic Pressure        d. Vapor Pressure

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Example: Which of the following compounds will have the highest boiling point?

a) 0.10 m sucrose

b) 0.10 m CsBrO­­3

c) 0.35 m CH3OH

d) 0.15 m SrBr2

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Example: Pure water boils at 100oC. What is the expected boiling point of water after the addition of 13.12 g calcium bromide, CaBr2, to 325 g water. Kb = 0.512 oC/m. (MW of CaBr2 is 199.88 g/mol)

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Example: The vapor pressure of water at 100.0oC is 0.630 atm.  Determine the amount (in grams) of aluminum fluoride, AlF3, (in grams) needed to reduce its vapor pressure to 0.550 atm. (MW of AlF3 is 83.98 g/mol)

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Problem: Beta-carotene is the most important of the A vitamins. Calculate the molar mass of Beta-carotene if 25.0 mL of a solution containing 9.88 mg of Beta-carotene has an osmotic pressure of 56.16 mmHg at 30 degrees of Celsius.

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## Vapor Pressure

Vapor Pressure is achieved by the equilibrium rate of vaporization equaling the rate of condensation.

Concept: Defining Vapor Pressure.

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##### Video Transcript

Hey guys! In this new video, we’re going to take a look at the liquid state and in particular, the effect it has in vapor pressure. We’re going to say that vapor pressure is defined as the partial pressure of paper molecules above the surface of the liquid under the dynamic equilibrium conditions of vaporization and condensation.
What does this mean? All this means is that vapor pressure is when the rates of condensation and vaporization are equal to one another. Think about it. In practical terms, let's say you’re boiling a pot of water and it has a lid. I know I’m not a great drawer but I'm trying here. In this pot we have liquid. What do we do to it? We're going to heat it up. Once we start heating up a pot of water, what's going to happen? Slowly but surely, the water's going to start to boil. Once it starts to boil, this liquid is going to get vaporized into gas. We’re going to actually have gas molecules exiting the liquid.
Now eventually, those gas molecules will reach to the top of the lid, which is considerably cooler than the actual boiling water. Once these gas molecules touch the lid, what's going to happen? They’re going to condense back into a liquid. What's going to happen now is they’re going to start dripping down back into the boiling water only to be vaporized once again into gas, go back up, condense again, drop back down, vaporize again, go back up. This is a continuous process.
We’re saying vapor pressure is when the rate vaporization or evaporation equals the rate of condensation. Remember, vaporization we're going from liquid to gas and then condensation, we're going from gas back down to liquid. Vapor pressure is basically when these things are in equilibrium with each other.

Example: The vapor pressure of pure liquid A is 550 torr and the vapor pressure of pure liquid B is 320 torr at room temperature. If the vapor pressure of a solution containing A and B is 465 torr, what is the mole fraction of A in the solution?

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Example: Determine the vapor pressure lowering associated with 1.32 m C6H12O6 solution (MW: 180.156 g/mol) at 25oC.

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Problem: The following boiling points belong to one of the following compounds: 117°C, 78°C, 34.5°C and 23°C.

CH3-O-CH3

CH3CH2OH

CH3CH2-O-CH2CH3

CH3CH2CH2CH2OH

a)  Which boiling point goes with what compound?

b) If each of the following substances were placed in separate sealed clear bottles at room temperature, could you identify one of the substances right away?

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