why is it that when you put an elodea leaf in 5% sucrose, it becomes hypotonic, but?

Tonicity has to do with osmotic pressure, which has do with solute concentration, molecular weight is irrelevant. The molecular weight is significant with regard to the % solutions since I assume these are grams per liter. So, the answer should become evident when you convert from % to molar concentration ( using the Molecular weight to get the number of moles per liter) then you find that it all makes sense.

Some of the confusion might be the similar terms hypotonic and hypertonic. It can get confusing because when a cell is hypertonic relative to the surrounding solution, the surrounding solution is hypotonic relative to the

cell. Let's apply the -tonic terms just to the solution for clarity.

Hypertonic solution - cell less salty than surrounding solution, net water

movement out of cell

Isotonic solution - cell and surrounding solution equally salty, no net water

movement

Hypotonic solution - cell saltier than surrounding solution, net water

movement into cell

Plant cell

Hypertonic solution - Plasmolysed cell

Isotonic solution - Nonturgid or wilted cell

Hypotonic solution - Turgid cell (Usual environment)

Much better explained in the article listed.