Question

Estimate how much the brain would swell due to osmosis if the concentration of the cerebrospinal fluid fell to 280mM.

in other words find what the new volume of the brain would be once equilibrium was reestablish.
assume that (i) the cerebrospinal won't change; (ii) only water is passing across the cell membranes into the brain cells (not solutes); and (iii) the volume of the brain is mostly water

Answer 1

The environment would become hypotonic, causing water to move into the cell and the cell would swell. Estimate how much the brain would swell due to osmosis if the concentration of the cerebrospinal fluid fell to 280 mM. In other words, find what the new volume of the brain would be once equilibrium was reestablished.

If that's what you mean.