Maharashtra BoardEnglish MediumSTD 12 ScienceChemistrySolutions4 Marks
Question
What are isotonic and hypertonic solutions?
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Answer
(1) Isotonic solutions : The solutions having the same osmotic pressure at a given temperature are called isotonic solutions.Explanation : If two solutions of substances A and B contain $n_A$ and $n_B$ moles dissolved in volume $V$ (in dm$^3$) of the solutions, then their concentrations are,
$ C _{ A }=\frac{n_{ A }}{V}\left(\text { in mol } dm ^{-3}\right) \text { and }$
$C _{ B }=\frac{n_{ B }}{V}\left(\text { in mol } dm ^{-3}\right)$
If the absolute temperature of both the solutions is T, then by the van’t Hoff equation,
$\pi _A = C_ART$ and $\pi _B = C_BRT$, where $\pi _A$ and $\pi _B$ are their osmotic pressures.
For the isotonic solutions,
$\pi _A = \pi _B$
$\therefore C_{ A }=C_{ B }$
$\therefore \frac{n_{ A }}{V}=\frac{n_{ B }}{V}$
$\therefore n_{ A }=n_{ B }$
Hence, equal volumes of the isotonic solutions at the same temperature will contain equal number of moles (hence, equal number of molecules) of the substances.
(2) Hypertonic solutions : When two solutions have different osmotic pressures, then the solution having higher osmotic pressure is said to be a hypertonic solution with respect to the other solution.
Explanation : Consider two solutions of substances A and B having osmotic pressures $\pi _A$ and $\pi _B$. If $\pi _B$ is greater than $\pi _A$, then the solution B is a hypertonic solution with respect to the solution A.
Hence, if $C_A$ and $C_B$ are their concentrations, then $C_B > C_A$. Hence, for equal volume of the solutions, $n_B > n_A$.
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