A selectively permeable membrane separates solution a and

b. the concentration of water molecules in solution b is higher than in solution

a. describe how the water molecules will work

The water molecules will flow from b to a due to osmosis.

Osmosis is where water molecules will flow from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane.

When the water molecule concentration is higher, it has a higher water potential top. Water potential is the tendency for them to flow to a lower region.

The net movement will stop until both sides of the solution has a same water potential.