Arrange the following solutions in order of decreasing osmotic pressure:

(a) 0.10 M urea 
(b) 0.06 M NaCl 
(c) 0.05 M Ba(NO3)2 
(d) 0.06 M sucrose.

The forumla for this is Osmotic Pressure = (Concentration of
ions)*R(molar gas constant)*T (in K)
since we are doing a comparison we can ignore R because it is a
constant and the same with T since we have to assume a constant
temperature.

Therefore the only diffrence is in the ion concentrations. Urea and
sucrose form only one ion. NaCl fragments into Na+ and Cl- so it forms
two ions. Ba(NO3)2 form Ba2+ and 2 NO3- so it forms 3 ions.
We use the number of ions that it forms as a multiplication factor to
the concentration to get the ion concentration.

So urea is .10M *1 - meaning it has an ion concentration of 0.10M
NaCl is  0.06M *2 - meaning it has an ion concentration of 0.12M
Ba(NO3)2 is 0.05M *3 - meaning it has an ion concentration of 0.15M
Sucrose is 0.06M *1 - meaning it has an ion concentration of 0.06M

Therefore the order is Ba(NO3)2 > NaCl > urea > sucrose
or c,b,a,d

thank you very much, i undertsand it now...just in time for my test tonight!!