Michael B. answered • 03/26/13
Seasoned and experienced tutor with extensive science background
In essence, it accounts for the number of particles that can occur in "ideal" solutions. The typical definition is that it is the ratio of the actual concentration of particles in solution to the concentration of the solute as calculated by its mass.
So for non-ionic compounds in solution, like glucose (C6H12O6) , the van't Hoff factor is 1.
For ions with a one to one ratio, like NaCl, this dissociates into ions of Na+1 and Cl-1 and the van't Hoff factor is then 2.
For other ions it follows suit, so Magnesium Chloride MgCl2 dissociates to Mg2+, Cl-1, and another Cl-1 for an ideal van't Hoff factor of 3.
For something like Calcium Phosphate, Ca3(PO4)2 you would have 3 Ca2+ and 2 PO43- for a van't Hoff factor of 5. Using this example, you can see that for every 1 mole of Calcium Phosphate you have 3 moles of Calcium ions and 2 moles of Phosphate ions. Theoretically 5 moles of ions for every one mole of compound we dissolve in solution.
The reason I used "ideal" when describing those examples is because it relies on the assumption of total dissociation in order to hold true. The real scenario is going to have pairs of ions that will form in solution so that the actual ratio observed is less than the theoretical ratio we calculate.
Dick B.
You've got the right idea, but this doesn't quite get it.
The van't Hoff factor is an experimentally determined value. In an infinitely dilute solution (this is the "ideal" solution mentioned above), the van't Hoff factor would be the number of particles (ions or molecules) when one formula unit of a substance dissolves. Molecular compounds don't generally break apart, so each unit of these stays as one particle. A formula unit of NaCl would break apart into a sodium ion and a chloride ion, so would have an ideal van't Hoff factor of 2.
Now, this "ideal" makes certain assumptions - the main one being that the separate particles do not interact with each other. Of course, oppositely-charged particles are attracted to each other. However, in an infinitely dilute solution, the particles are infinitely far away from each other, so feel zero attraction, and will never come into contact. In a very dilute solution, the particles are generally far enough apart that the attraction is very small. Every time a positive ion and a negative ion contact, they will tend to stick together for a short period of time, forming an ion pair, and acting as a single particle. The more concentrated the solution becomes, the closer the ions get to each other, and the likelihood that they will come into contact with each other increases.
Picture it this way. If you dissolve 10 units of NaCl, you will have 10 sodium ions and 10 chloride ions. If they are infinitely far away, you have 20 particles present. Now imagine that one sodium ion and one chloride ion come into contact and stay together. Now you have nine sodium ions, nine chloride ions and one ion pair, for a total of 19 particles. In this case, 10 units give rise to 19 particles, so there is a 1.9:1 ratio of particles to original formula units. Thus, the van't Hoff factor for this solution would be 1.9.
08/10/13