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

Question

if you put it in a 4% NaCl solution it becomes hypertonic. i think it has something to do with the concentrations and molecular weight. but im not entirely sure. please any help would greatly be appreciated.

Answer 1

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.

Answer 2

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.