[themedicinebudha]

Q: Place the following solutions in order of decreasing osmotic pressure.

     I. 0.011M C12H12O11 (sucrose)   II. 0.0095M C6H12O6 (glucose)  III. 0.0060M C3H8O3 (glycerin)

Here is the answer I have been given, but I don't understand why it's correct (if it is):

A:  I > III > II

I know solutions that contain the highest number of ionic particles generally have the highest osmotic pressure, but I don't understand why 0.0060M glycerin has a higher osmotic pressure than 0.0095M glucose?

[BetaAmyloid]

It is correct.

Remember, Π(osmotic pressure) = iMRT.

Assuming that all have the same temperature, and we know that all have the same R-constant, then we could look at it as Π = iM.

Ionic particles correlate with the van't hoff factor i, which is the number of theoretical dissociations of ions of a chemical.

For example, NaCl Na⁺ + Cl^-. Here, NaCl dissociates into 2 ions, therefore it's van't hoff factor is 2. But all of these substances listed in your question are organic, and therefore do not theoretically dissociate. So all of the van't hoff factors here are 1. Now we can simply say Π = M. All of the listed molarities are equal to the osmotic pressures.

Sucrose .011 Π > C₃H₈O₃ .0060 Π > C₆H₁₂O₆ .0095 Π.

Higher van't hoff factors do not always mean that a certain substances have a higher osmotic pressure, it is also correlated to the amount of the substance (as well as temperature, which is not listed here). Since molarity equals n/V, and we assume all the volumes are the same here, then we see that sucrose has a higher concentration than C₃H₈O₃, and C₃H₈O₃ has a higher concentration than C₆H₁₂O₆!

Hope this helps!

[Borek]

.0060 Π > .0095 Π

So, you are saying that 60 is larger than 95?

What about 1 being larger than 2?

[BetaAmyloid]

.0060 Π > .0095 Π

So, you are saying that 60 is larger than 95?

Wow. I re-read that like 3 times to make sure it was right. My bad...

Clarify the rest to him then, if you don't mind!