The Colligative Properties: Videos & Practice Problems

As solutes are introduced into a pure solvent, transforming it into a solution, the colligative properties—boiling point, freezing point, vapor pressure, and osmotic pressure—are impacted. Boiling point and osmotic pressure increase with the addition of solute, while freezing point and vapor pressure decrease. The Van't Hoff factor (I) is crucial in quantifying these changes, representing the number of particles into which a solute dissociates in solution. Ionic compounds dissociate into multiple ions, thus having a Van't Hoff factor greater than one, whereas covalent compounds, being non-electrolytes, do not dissociate and have an I value of one. The colligative properties are further calculated using osmolarity and osmolality, which incorporate the Van't Hoff factor and the molarity or molality of the solution, respectively. These concepts are essential for predicting how a solution will behave compared to its pure solvent and for calculating the extent of colligative property changes.

Colligative Properties

Now the four colligative properties discuss what happens to a pure solvent as a solute is added to it. So they're discussing what happens as our pure solvent transitions into a solution. Because remember, when we add solute to a solvent, it becomes a solution. We're going to say as solute is added to a solvent, some colligative properties will increase while others will decrease.

Now here when we take a look at boiling point and osmotic pressure, we're going to say that these two colligative properties will increase the more solute we add to our solvent. Conversely, we're going to say that our freezing point and vapor pressure, the more solute I add to my solvent, the lower they go. OK, so just remember these are the two effects of adding solute to a pure solvent.

Now let's discuss these four colligative properties a little bit more closely. So boiling point, remember boiling point is just the temperature where we're going to have an equilibrium between our liquid and gas phases, right? So remember we have our liquid becoming a gas or vaporization and then we have our gas condensing down back into our liquid. Boiling point is when there's an equilibrium between these two changes.

Freezing point, well freezing point is where there's an equilibrium between our solid phase and our liquid phase. So going from solid to liquid, we have melting or fusion occurring. And then going from liquid to solid, we have freezing occurring. Freezing point is when both of these phase change properties or processes are happening at the same time, so they're at equilibrium with one another.

Now vapor pressure, we're going to say here that vapor pressure is basically the pressure exerted by a gas at the surface of a liquid and we're going to say this is measurable also. Again, the whole idea of being at equilibrium. Finally, we have osmotic pressure. Osmotic pressure is just a force that drives osmosis. Remember the movement of water from an area of low concentration to an area of high concentration.

So here we have an illustration where this side is more concentrated and so water would rush towards this side. Here, what effect does this have? Well, the right side the water level will be lower and the left side, the water has increased because again, water moved from an area of low concentration to an area of high concentration, right. So we'll go a little bit more in detail in terms of the mathematical applications of each of these colligative properties, but for now, realize that they have to do with our transition from a pure solvent to a solution through the addition of some type of solute.

Boiling Point Elevation Example

Benzene, which has a formula of C6H6, has a boiling point of 80.1°C. What is a possible new boiling point once an unknown amount of glucose is added to the benzene solvent? So remember we discussed this earlier on. We say the more solute you add to a pure solvent then the higher the boiling point would be.

So we're going to say here we expect the boiling point to be a value that is now higher than 80.1°C. And if we look at all our choices, the only one that's above this original temperature of 80.1°C is option C. We'd expect our temperature, which is reasonable, to be 89.6°C.

None of the other ones make any sense. Boiling point would not stay the same, and it definitely would not decrease. Adding solute to our pure solvent increases your boiling.

Van't Hoff Factor

Now the van Hoff factor which uses the variable I equals the number of ions produced from dissolving a soluble solute. Now when we talk about solutes, we group them as being either ionic or covalent in nature. Now remember ionic compounds are composed of a positive ion connected to a negative ion. That positive ion can be in the form of the ammonium ion or a metal, and then that negative ion will be in the form of nonmetal.

So here, if we take a look, we have sodium hydroxide, ammonium carbonate, and aluminum sulfate as our three ionic compounds. Each one, because they are ionic, can break up into ions. Here we'd have Na+ OH-. Here it breaks up into two ions. We just said that the Van Hoff factor is a number of ions produced when a soluble solute dissolves. So since there's two ions I = 2, ammonium carbonate breaks up into two ammonium ions plus one carbonate ion, for a total of 3 ions. So here I = 3. Aluminum sulfate breaks up into two aluminum ions and three sulfate ions, for a total of 5 ions, and because of that I = 5.

Now, covalent compounds are just compounds composed of only nonmetals together. OK, we're going to say here that they are. Because of this they are non volatile and you're going to say they are non ionizable or non electrolytes. OK, so here if they mention covalent solutes, if they mention that they're non volatile, if they mention that they're non electrolytes, we group them all under covalent solutes. So here we have glucose, we have chlorine, we have methanol, and here we have something called urea. So urine, that's one of the main components of it.

All of these are covalent in nature and therefore they are non volatile, which means they don't break up into ions and they're not electrolytes, which also means they don't break up into ions. Because of that, I = 1. Now technically 0 ions are formed, so they just stay in the form that they're in, so that still counts. They stay in the form that they're in, so we just count them as one. So I for them would equal to 1. So remember ionic compounds break up into ions. It's important you get the number correct to write to find the right number for I. For covalent solutes they don't break up into ions, their I will always just be one in its value.

Van't Hoff Factor Example

Which of the following compounds will have the largest value for the vanthu factor? All right, so for the first one, we have aluminum chloride, which is an ionic compound, so it breaks up into ions. It's composed of 1 aluminum ion and three chloride ions. So in total that's four ions. So I=4.

For the next one, it's covalent because it's only nonmetals together, so I=1. For the next one, it's ionic again because zinc is a metal. Oxygen is a nonmetal, so it breaks up into zinc ion and oxide ion. That's two ions total, so I=2.

Next we have ammonium, which is NH₃. It's covalent because it's only nonmetals, so I=1 and then we have P₂S₅ which is also covalent. It's only nonmetals together, so I=1. We can see the one with the greatest Van Hoff factor value is I=4 for aluminum chloride. So here the answer would be option A.

Colligative Properties

So remember for the colligative properties, the more solute you add, the more they can be affected. Boiling point and osmotic pressure will keep going up freezing point and vapor pressure will go down. Now the solute amount added equals the number of ions of that solute.

Well remember that the Vanhoff factor, so that's I times the concentration of that compound or solute. Now this concentration could represent either molarity or molality. And if we're incorporating ions with molarity or molality, then they become osmolarity and osmolality. So remember, osmolarity is ionic molarity, Osmolarity is ionic molality.

All right. So for osmolarity solute formula, we're going to say osmolarity, which is the amount of solute equals I the number of ions for the solute times the molarity of the compound as a whole. Osmolality is the same kind of idea. It equals I the number of solute on ions that we have times the molality of the compound.

These two formulas will help us determine the amount of solute that a solute given to us represents, and that'll help us determine which one I have the highest boiling point or which one have the lowest freezing point, et cetera. So keep this in mind. We use osmolarity and osmolality to determine the amount of salute that is added.

Ionic Molality Example

What is the ionic molality of potassium ions in 1.18 molal solution? Potassium phosphate. All right, so we want the ionic molality of potassium ions. All right, So we're going to say ionic molality is just osmolality. So here it will equal the ions of potassium ion times the molality of the compound.

If we break this up, it breaks up into three potassium ions plus one phosphate ion. So how many potassium ions do we have? We have three, so that would be three times. Now the molality of the entire compound is 1.18 mol, so 3×1.18 mol  gives us the ionic molality of potassium ions.

So that will give us option E as our correct answer. So it would be 3.54 molal of potassium ions.

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

• A. 0.10 M sucrose
• B. 0.10 M AgCl
• C. 0.25 M NH₄NO₃
• D. 0.45 M pure water

Answer: (C)

Which of the following compound will have the highest vapor pressure?

• A. 0.45 m dinitrogen pentoxide
• B. 0.10 m aluminum chloride
• C. 0.50 m iron (III) perchlorate
• D. 0.15 m calcium phosphate
• E. 0.30 m Potassium sulfide

Answer: (B)

Here’s what students ask on this topic:

What are the colligative properties?

Colligative properties are characteristics of solutions that depend on the number of dissolved particles in the solution, rather than the identity of the solute molecules. These properties arise because the presence of solute particles affects the properties of the solvent. There are four main colligative properties:

1. Vapor Pressure Lowering: The presence of a non-volatile solute reduces the vapor pressure of the solvent. This happens because solute particles occupy space at the surface of the liquid, making it harder for solvent molecules to escape into the gas phase.
2. Boiling Point Elevation: The boiling point of a solvent will be higher when a solute is dissolved in it. This occurs because the lower vapor pressure requires more heat to reach the boiling point.
3. Freezing Point Depression: The freezing point of a solvent is lowered when a solute is dissolved in it. This is because the solute particles disrupt the formation of the solid structure of the solvent, requiring a lower temperature to solidify.
4. Osmotic Pressure: This is the pressure required to stop the flow of the solvent into the solution through a semipermeable membrane. It is directly proportional to the concentration of the solute particles in the solution.

Understanding these properties is important in fields like chemistry, biology, and engineering, as they influence various processes and applications.