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Chemistry: Colligative Properties: Ionic Solutions

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  • Type: Video Tutorial
  • Length: 10:47
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 116 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Physical Properties of Solutions (14 lessons, $22.77)
Chemistry: Colligative Properties (5 lessons, $9.90)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Thinkwell
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We've been talking about how a solute dissolved in a solvent gives rise to various physical property changes, boiling point, freezing point, and so on. What about a solute, that when we added to the solution, broke apart into more than one particle? In other words, just schematically, suppose that we had a solute molecule of some kind that once we put it in solution, let's say, split into two pieces. Well, the number of particles in solution then would actually be twice as much as we would be counting on based on what we started with. And remember that colligative properties depend on the number of things in solution not the chemical nature of whatever the things are in solution. This particular situation comes up most commonly with ions and so what we're going to do now is talk about colligative properties, but of ionic materials rather than molecular materials.
When we have solutions of electrolytes, our formulas take on very, very similar meaning, but they have one additional term. If we look again, as an example, at boiling point elevation or freezing point depression, like before, they are equal to the boiling point elevation constant of the solvent times the molal concentration but there is an additional term here now, in this case, new. Now this sometimes will show up in textbooks as "i" or other letters but this extra constant here, called the van't Hoff factor, will be one if we get a one-to-one correspondence between the amount of moles of stuff we put in, and the amount of moles of stuff that is dissolved. But, for the situation I just described, where a molecule, once is it in solution, breaks into pieces, this can take on a different integer value. For example, suppose that we are thinking about sodium chloride. For every equivalent amount of sodium chloride, or I should say, for every one molecule, if you will, of sodium chloride, even though we know it is not in discreet molecules, that I put in, I get two ions, a sodium ion and a chloride ion. So one mole of sodium chloride as measured out will actually give me one mole of sodium and one mole of chloride, or two moles of ions. So, in that case, this factor here, new, would actually be two. It is simply a fudge factor that corrects for the fact that I get more particles of solution than I started with in a sense. If I look at this ionic material, ferric bromide, or FeBr, I'm going to end up with a total for every one mole of iron three bromide, I'll end up with a total of four particles, or four moles rather, of ions. One iron and three bromides. Well, what about acetic acid? Why would it have a new of 1? We've written a description for acetic acid, that acetic acid goes to acetate ion plus H+, so why shouldn't new be 2? After all, we can write that equation down? The reason is, if you recall, acetic acid is a very weak acid, meaning that it dissociates only to a very small percentage in water and I'll remind you we looked at an example of that and we probed that using an electrical conductivity apparatus. Acetic acid, which is vinegar, starts out at neutral and when I pour it into the water, we see just a tiny bit of glow indicating that there is a very small amount of ionization but for the most part, there is virtually almost entirely acetic acid in there still, not acetate and H+. So new is very close to 1 because acetic acid acts very much like a neutral molecule. Again, 99.99% of it, in fact, the neutral form of acetic acid. That's in great contrast to sodium chloride, which we said had a new of 2, because in this case we get complete conversion into the ions that make it up. And so of course the light burns very brightly now because we have high amounts of ions.
So, what is this good for? Well one thing certainly it's good for is for correcting all of our colligative properties. I mean this would work for osmotic pressures well, it would work for Raoult's Law as well, for correcting those equations to also include things that will ionize in solution. In fact, you might even ask what about things that when you dissolve them, they dimerize instead of split in half? So, suppose it's the exact opposite situation. Suppose you throw in, for instance, acetic acid in solution in very dilute concentration in a different solvent than water. Acetic acid tends to dimerize, to form discreet units of two as they connect to each other and so we actually in that case have half as many particles as we're expecting to have. So, in that case, new would be a half instead of two. So, new gives us the information about whether particles are coming together and sticking more than we're expecting or whether they are splitting apart into pieces.
So let's use this to try to analyze what's going on in solution. Suppose, here's a problem. Here again, suppose that we are trying to figure out how potassium ferrous cyanide splits apart into solution. So that's our formula but how does it ionize? We're pretty sure that the potassiums break off because we're used to potassium plus ions. Does it in fact form ? Or is there some other combination that we're going to find? If it indeed split into those ions, we'd have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 different ions for every 1 mole of the potassium ferrous cyanide. So again one mole of this salt gives us ten ions if indeed it broke apart the way I just described. Well, does it do that or is there some other connection or way that this thing separates? So, to probe that, what we're going to do is take a little under a half of a gram here, point four nine grams, of the salt, dissolve it in one hundred grams of water. We know the freezing point depression, that's found to be experimentally point eleven degrees Celsius. So I know I'm going to use, again, freezing point depression this time to tell me something about what's going on in solution. I know my freezing point depression. And I want to know how many ions are actually in solution for this particular thing. Here's my freezing point constant. I know my molecular weight for the ferrocyanide, the potassium ferrocyanide, so my problem is going to look this. Here's my thought process. I know what the freezing point change is. My temperature change in the freezing point for pure versus solution. I can relate that to moles, in particular concentration molal, moles per kilogram of solvent. And then I can compare that with what I actually put in. I have a pure substance that I know the molecular weight for. The only thing I don't know in this case is new freezing point. I know my change in temperature, I know my molal concentration, I know my constant. The only unknown is new. And that is going to tell me about what the salt is doing once it is dissolved. So, here we go then. We're just going to solve for new. I just said in English what I wrote down here. I need to solve for molal concentration. That, remember, is going to be, in this case, moles divided by kilograms solvent, so moles is going to be grams divided by grams per mole and that is going to give me moles of the stuff that I put in, divided by kilograms of solvent, in this case point one kilograms or one hundred grams. And that gives me a value for my molal. So, point zero one five moles of solute per kilogram. Now, not moles of ions, moles of solute in this case. Now, the only unknown is new and so new is just going to be coming back to this equation here; I've just rearranged this, new is the change of temperature divided by our constant divided by our molal of the ionic salt that we put in and what I find is that there is a four to one ratio between these three numbers. In other words, if new was one, I wouldn't need this anymore and this times this would equal this. But because new is not one, I know that I am getting ions once I dissolve this material. In particular, I'm getting almost four. This is essentially experimental error. I've got about four ions for every one potassium ferrocyanide that I dissolve and what that tells me at the end of the day is that this does not in fact break into potassium and iron and cyanide, but rather it converts into three potassium ions plus ferrocyanide, which is . So it's telling us that those cyanides are staying tightly bound to that iron. This is an example of what we refer to as a complex ion and we'll see more examples of this later where transition metals in particular very commonly will do this, where they have a lot of other groups attached to them still in solution. So, once again, what I got this time by taking advantage of the fact that this constant new shows up in colligative properties when we ionize materials, is colligative properties tell me, once again, how much stuff I've got dissolved. If I know how much stuff I've got dissolved, and I know how much mass I put in, I can make that connection between how many moles of ions do I have for the amount of stuff that I put in? And in this case, again, what we found was some information about the molecular level. How this compound breaks apart into pieces, in this case, specifically that the ferrocyanide unit sticks together in solution.
Physical Properties of Solutions
Colligative Properties
Colligative Properties of Ionic Solutions Page [2 of 2]

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