Question

"what is the osmotic role of plasma albumin, and how would a low plasma albumin level promote edema"

Answer 1

Plasma albumins in the blood plasma are osmotically active solutes. They do not pass through the capillary walls to tissues as part of tissue fluid. Therefore they ensure that in the efferent capillaries, the tissue fluid flows back into the capillaries by osmosis. This is due to the fact that the blood plasma will be hypertonic to the tissue fluid.
Low plasma albumins levels will cause retention of tissue fluid, and it's excessive accumulation, in the tissues hence result to edema.

Answer 2

what is the osmotic role of plasma albumin, and how would a low plasma albumin level promote edema

Plasma albumin is very essential for regulating the osmotic balance between –  

a) Fluid with in the tissues

b) Blood

Plasma albumin in unable to cross the cell membrane and thus it only exists in the blood capillaries and never enter into the tissue fluids. The condition of low plasma albumin level arises when the fluid level increases in tissue thereby causing osmotic imbalance.  

A region having high solute concentration (i.e. plasma) draws water across a permeable membrane from a region having high water concentration. Thus low albumin levels increases the water concentrations in the plasma thereby leaking it out of the bloodstream and into the interstitial spaces. This leads to edema