What quantity is equal between two solutions that are isotonic

The amount of the solute is the quantity that would be equal between two solutions that are isotonic.

Explanation:

Tonicity can be described as the measure of osmotic pressure among solutions.

If the amount of solutes that is present in a solution is equal to the amount of solute that is present in another solution, then the two solutions would be considered as isotonic. No transfer of solutes will take place in such solutions.

Any rise or loss of solutes in either of the solutions will result in the transfer of solutes among the solutions and the solutions will no longer remain isotonic.