The Colligative Properties - Video Tutorials & Practice Problems
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The Colligative Properties explain what happens to a pure solvent as it transitions to a solution.
The 4 Colligative Properties
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The Colligative Properties Concept 1
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Now, the 4 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 cognitive 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 collective properties will increase the more solute we add to our solvent. Conversely, we're going to say that our freezing point in vapor pressure, the more solute I add to my solvent the lower they go. Okay. So just remember these are the two effects of adding solute to a pure solvent. Now, let's discuss these four collater 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. Alright. So remember, we have our liquid becoming a gas, so vaporization, 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 gonna say this is measurable also again the whole idea of being at equilibrium. Finally, we have osmotic pressure. Osmotic pressure is just the 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 does this 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. Alright, so we'll go a little bit more in detail in terms of the mathematical applications of each of these collaterative 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.
Some Colligative Properties will increase and others will decrease with the addition of solute to a pure solvent.
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The Colligative Properties Example 1
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Benzene, which has a formula of c 6h6, has a boiling point of 80.1 degrees Celsius. 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 gonna say here we expect the boiling point to be a value that is now higher than 80.1 degrees Celsius. And if we look at all our choices, the only one that's above this original temperature of 80.1 degrees Celsius is option c. We'd expect our temperature, which is reasonable, to be 89.6 degrees. None of the other ones make any sense. Boiling point would not stay the same, and it definitely would not decrease. Adding solute to a pure solvent increases your boiling point.
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The Colligative Properties Concept 2
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Now the van't 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 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 3 ionic compounds. Each one, because they're ionic, can break up into ions. Here we'd have n a+ohminus. Here it breaks up into 2 ions. We just said that the van't Hoff factor is the number of ions produced when a soluble solute dissolves. So since there's 2 ions, I equals 2. Ammonium carbonate breaks up into 2 ammonium ions plus 1 carbonate ion, for a total of 3 ions. So here, I equals 3. Aluminum sulfate breaks up into 2 aluminum ions, and 3 sulfate ions, for a total of 5 ions, and because of that, I equals 5. Now covalent compounds are just compounds composed of only non metals together. Okay. We're gonna say here that they are because of this, they are non volatile and you're gonna say they are non ionizable or non electrolytes. Okay? So here, if they mentioned covalent solutes, if they mentioned that they're nonvolatile, if they mentioned 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 non electrolytes, which also means they don't break up into ions. Because of that, I equals 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 1. 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 1 in its value.
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The Colligative Properties Example 2
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Which of the following compounds will have the largest value for the Van't Hoff factor? Alright. So for the first one, we have aluminum chloride, which is which is an ionic compound, so it breaks up into ions. It's composed of 1 aluminum ion and 3 chloride ions. So in total, that's 4 ions, so I equals 4. For the next one, it's covalent because it's only nonmetals together, so I equals 1. For the next one it's ionic again because zinc is a metal, oxygen is a non metal. So it breaks up into zinc ion and oxide ion. That's 2 ions total, so I equals 2. Next we have ammonium which is NH 3. It's covalent because it's only nonmetals, so I equals 1. And then we have p 2 s 5, which is also covalent. It's only nonmetals together, So I equals 1. We can see the one with the greatest van't Hoff factor value is a, I equals 4 for aluminum chloride. So here the answer would be option a.
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The Colligative Properties Concept 3
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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 van't Hoff 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 osmolarity. So remember, osmolarity is ionic molarity, osmolarity is ionic molality. Alright. So for osmolarity solute formula, we're gonna 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, 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 will have the highest boiling point, or which one will have the lowest freezing point, etcetera. So keep this in mind. We use osmolarity and osmolality to determine the amount of solute that is
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The Colligative Properties Example 3
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What is the ionic molality of Potassium ions in 1.18 mobile solution, Potassium Phosphate? Alright. So we want the ionic molality of potassium ions. Alright. So we're gonna 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 3 potassium ions plus 1 phosphate ion. So how many potassium ions do we have? We have 3. So that'd be 3 times now the molality of the entire compound is 1.8 mol. So 3 times 1.18 moles gives us the ionic molality of potassium ions. So that would give us option e as our correct answer. So it'd be 3.54 molal of potassium ions.
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Problem
Problem
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
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Problem
Problem
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
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