I saw the ask about the tonicity post, followed the link... Great job on it btw. I was just wondering if you knew of a good way to explain osmolarity and its relation to tonicity? my teacher droned on about it and I just got really confused.
This Khan Academy video may help.
A quick overview from me: both terms describe the concentration of solute on either side of a membrane, but tonicity only describes what happens to the solutes that can't penetrate the membrane. I.e., a cell's tonicity is how the concentration of impermeant solute drives the movement of water in or out of the cell.
On the other hand, osmolarity considers the total concentration of solutes: not just the impermeant ones, but the permeant solutes too (which can pass through the cell's membrane). So, you have to consider the fact that permeant solutes and water will move through the cell's membrane before it reaches equilibrium with its surroundings.