So you need to know the osmolarity of your tissue so you can set the osmolarity of your fixative to be isotonic with respect to the tissue. In a rough sense it is the measure of the amount of solute in a solution, but the degree of dissociation of the solute affects its osmolarity, so osmolarity is not necessarily a direct measure of molar concentration. If cells are placed in a hypertonic solution, net movement of water will be out of the cell, causing the cell to shrivel. If cells are placed in a hypotonic solution, net movement of water will be into the cell, causing the cell to swell or burst. Tonicity is useful only in reference to a particular cell or tissue.
Tonicity is a comparison of osmolarities between a cell and the solution to which it is exposed.
I hope that this does not leave readers more confused than they were before. That is ONLY true if the membrane involved is impermeable to ALL of the solutes contributing to the osmolarity. Since formaldehyde is soluble in both benzene and chloroform I expect it to be quite permeable across cell membranes. Thus it will contribute to osmolarity, but should contribute little to tonicity. The tonicity of your fixative should be adjusted with other components, and the final osmolarity of an isotonic fixative will be approx 300 mOsm PLUS the value of osmolarity contributed by the formaldehyde present.