Hi,

Recall that the net flow of water requires a concentration gradient.

Here we have 100 nM of Na3PO4 and 200 nM of NaCl.

At first, this looks like a differential concentration gradient, but we have to keep in mind that these ions will dissociate which will account for how concentrated the solution is.

The dissociations are as follows:

Na3PO4 --> 3Na+ + PO43- (4 ions)

NaCl --> Na+ + Cl- (2 ions)

Therefore the effective concentrations are as follows:

100 nM x 4 ions = 400 nM

200 nM x 2 ions = 400 nM

Because these concentrations are the same, there will be no net water flow. The formula they have used for this account for this in the same way, I think it is covered at one point in the lessons. That being said, what I've done is essentially the same as the formula (i refers to the number of ion formed in solution, M is molarity and RT is a constant).

Please let me know if this is unclear.