Background of the Question I am a high school student so maybe my understanding of this topic is quite less, so apologies in case I have asked an elementary question.
My chemistry sir taught that :-
The flow of solvent in osmosis is dependent upon the difference in vapour pressures of two solutions.
For Example , if we have a dilute solution and concentrated solution separated by a selectively permeable membrane , then apart from deciding the flow of the solvent on the basis of concentrations , we can also define it as :- The dilute solution has lesser concentration of solute , as a result of which the vapour pressure will be more ( we are considering that the solution contains non volatile solute, as a result of which its addition into the solution causes a decrease in the vapour pressure) and hence the difference in vapour pressure causes the movement of solvent particles from dilute to concentrated
And then he defined osmotic pressure on these terms and said that it will be the minimum pressure required to stop the movement of the solvent due to osmosis (to be applied on the side with less vapour pressure).
So on that basis , if we have a pure solvent on one side of SPM , and we have a solution (of the same solvent with some non volatile solute) on the other side of the SPM , and if the vapour pressure of the pure solvent is $p^0$ and that of solution is $P$ , then we can define osmotic pressure ($\pi$) as
P + $\pi$ = $p^0$
He also told me that
If two solutions have same osmotic pressure at the same temperatures , then they are isotonic solutions (by using the formula $\pi$ = $iCRT$ where i = Vant Hoff factor , C = concentration of solution , R = universal gas constant and T is the absolute temperature.) Also , there will be no movement of solvent if they are kept side by side separated by a selectively permeable membrane.
Doubt:-
Now , we define the isotonic solutions as two solutions which have the same osmotic pressures, and also , there will be no movement of solvent if they are kept side by side separated by a selectively permeable membrane
Now if we have two solutions, each of two different solvent , say A and B , but have the same osmotic pressures, i.e.
$i_1C_1RT = i_2 C_2RT$ ($i_1$ and $i_2$ are the Vant Hoff factors for both the solutes , but let us say we have the same solute)
$C_1$ = $C_2$
So we have the same concentrations of both the solutions. However,we know that the pressure of the solution can be determined by the above listed formula ,
P + π = $p^0$
$\therefore$ if $P_a$ is the pressure of the first solution(A) and $P_b$ is the pressure of the second solution(B), and the pressure of the solvents A and B (in pure states) are given by $p_a^0$ and $p_b^0$ , then :-
$P_a$ = $p_a^0$ - $\pi$
$P_b$ = $p_b^0$ - $\pi$
Now , $p_a^0$ is not equal to $p_b^0$ as we have different solvents. Therefore the pressure on the both sides of the solution will be different , and that would mean that the solvent should flow from one side to another
So Basically my question is, How do we exactly define isotonic solutions? Can any two solutions even be isotonic by definition if they consist of differen solvents?